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Article Response AssignmentCONS 472: Introduction to Animal Behavior
140 pts (7 readings; 20 pts per response post)
Responses to published literature allow you to demonstrate your understanding of a reading’s content and
critically analyze the implications of findings and ideas presented. The readings selected will also provide
you with background to understand learning content given in this course.
For this assignment, you will read the assigned article and post a response to the article in the Discussion
Board section on Blackboard, under the appropriately dated/labeled forum of “Article Responses.” Click on
the forum title, then click on “Create Thread” to start your post. Article responses need to be posted to the
Discussion Board by 10:00 AM, on the dates of Thursday 6/23, Friday 6/24, Monday 6/27, Tuesday
6/28, Wednesday 6/29, Thursday 6/30 and Friday 7/1. A total of seven (7) posts are required.
You will receive extra credit (+1 pt) if you post a meaningful response or comment to at least one of
your peers’ postings. Comments in response to classmate postings need to be posted by 9:30 AM, on the
day directly following the reading due.
Your post will include the following elements:
1) Two key ideas: Give at least two (2) key conclusions or most important overarching findings from
the article. This description should be succinct and highlight the most relevant findings or
conclusions, but they must convey complete ideas. These should be the “take-home messages” of the
article. Do not simply “copy and paste” or paraphrase the findings given in the article (and this is
plagiarism!). Dig deeper – think more broadly about the relevance or importance of the findings.
2) Critical analysis and questioning for further investigation: Give a critical evaluation of the
article or provide some topics/questions for future research or investigation. You will provide
a short analysis regarding the article, focusing on evaluation and synthesis of the ideas in the paper.
This portion should be the bulk of your post. Focus on the content of the reading, and think
about overall purpose, reasoning, and conclusions of the paper. Be logical and thoughtful in your
analysis. For example:
•
•
•
Are there any gaps in the knowledge or areas that may benefit from future growth?
Can these ideas be applied to different situations or scenarios? What are some challenges or
advantages?
What are the conservation implications of this reading? How does it address relevant and pressing
questions in conservation?
See the “Guide to Developing Critical Thinking Skills & Analysis” (in the “Resources for
Assignments” folder), to help you formulate your analysis and response.
Grading Rubric:
Each post worth: 20 pts
Key ideas (two)
10 pts
Critical analysis or further investigation
10 pts
Seven posts of 20 pts each = 140 pts total
1
Sigma Xi, The Scientific Research Honor Society
Search Strategies of Foraging Animals
Author(s): W. John O’Brien, Howard I. Browman and Barbara I. Evans
Source: American Scientist, Vol. 78, No. 2 (March-April 1990), pp. 152-160
Published by: Sigma Xi, The Scientific Research Honor Society
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Search Strategies
of Foraging Animals
W. John O’Brien
Howard I. Browman
Barbara I. Evans
Of all the activities in which animals engage, perhaps
the most important are finding and consuming food. In
addition to its urgency at the individual level, foraging?
in the broad sense of predator-prey interactions?is cru?
cial to ecosystem processes at many levels. In a very real
sense, the biosphere runs on the consumption of one
organism by another. Fabre has succinctly noted (1913)
that “from the least to the greatest in the zoological
progression, the stomach sways the world; the data
supplied by food are chief among all the documents of
life.” The when, where, how, how often, and how many
cient feeding. Thus models that instead pose the ques?
tion of how animals forage effectively may contribute to
an understanding of behavioral factors that are not
genetically fixed but vary according to local environmen?
tal conditions.
Historically, much of what is known about animal
foraging has been obtained from the analysis of stomach
contents. A more recent approach, and one that can yield
detailed information on the specific mechanisms of for?
aging behavior, breaks down the act of predation into
components (Holling 1959; O’Brien 1979, 1987). The
components of the predation cycle are similar for almost
Saltatory patterns of movement
in a wide range of animals indicate
that most foragers fall somewhere
between cruisers and ambushers
all animals: predators must first search for and locate
prey, then pursue and attack them, and finally handle
and ingest them.
If the prey are larger than the predator (as in the case
of a lion preying on gazelles), the pursuit, attack, and
of prey capture and consumption are central to studies of
individual survival, population dynamics, community
structure, nutrient cycling, and energy flow through
ecosystems.
It is not surprising, then, that foraging has been
studied extensively in virtually all animal groups by
physiologists, ethologists, ecologists, and theoreticians
(Kamil et al. 1987; Kerfoot and Sih 1987). A recent
approach to the study of feeding ecology is optimal
foraging theory, which focuses on aspects of foraging
behavior presumed to have been honed by evolution
(Schoener 1971; Pyke 1984). Theorists of optimal foraging
have produced a series of models that make quantitative
predictions concerning such aspects of predator-prey
interactions as optimizing prey choice, adopting the best
prey patch as prey are depleted, and search strategy (see,
for example, Hanson and Green 1989; Marshall et al.
1989; O’Brien et al. 1989). These models vary widely in
their accuracy when tested against field data, but serve a
valuable function in clarifying what is known and what is
not known about foraging among particular species in
particular environments.
Optimality models are perhaps least useful when
they seek to address the question of whether animals are
“optimal.” Because natural selection has left only those
species and individuals that are efficient feeders, animals
do not exhibit a gradient ranging from efficient to ineffi
152 American Scientist, Volume 78
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handling components of the cycle are of paramount
importance. In this situation, large size is an advantage
for the prey. If the prey are smaller than the predator (as
in the case of a bird feeding on small insects), the search
component of the cycle is the most important. In this
situation, small size may be an advantage for the prey.
Many predators prey on animals considerably smaller
than themselves, thereby avoiding the severe constraints
raised by the time required for handling. When prey are
smaller than their predators, they must be consumed
often and in large numbers. In this case, search time is
the factor that limits all subsequent components of the
predation cycle.
The search strategy employed by a given organism
has commonly been thought of as a trait dictated by
evolution and not open to modification under changing
environmental circumstances. Animals are typically di?
vided into two categories, based on their overall search
search, the forager moves continuously through the
environment, searching constantly for its prey at the
outer boundary of the volume being searched. Cruise
searchers include large fish that swim continuously, such
as tuna, and soaring hawks. In ambush or sit-and-wait
search, a forager remains stationary for long periods of
time, waiting for prey to cross the boundary of its strike
space. Herons and rattlesnakes fit the category of am?
bush searchers.
Clearly, the patterns of motion associated with these
two search strategies are profoundly different. It is im?
possible to conceive of a cruise searcher switching strat?
egies to ambush a particularly appealing prey item. In
fact, the kind of search behavior displayed by an organ
behavior: “cruise” searchers and “ambush” searchers
(Greene 1983). These two categories of behavior have
also been called “widely ranging” and “sit and wait”
(Huey and Pianka 1981). In cruise or widely ranging
Time
Figure 1. The strategies by which animals search for prey range
from the constant motion of cruising foragers such as the red-tailed
hawk shown at the far left to the tactics of ambushers such as the
crouching lion at the right. Most taxa, however, occupy an
intermediate position along this continuum, displaying a distinctive
pattern of stops and starts in which pauses to search for prey
alternate with moves that reposition the predator to scan new
territory. Arctic grayling and white crappie (center, top and bottom
respectively) are among the many species of fish, birds, and lizards
that have been found to use a saltatory strategy. The graph above
shows the characteristic patterns traced by the three strategies in
time and space; the velocity of the movement is indicated by the
slope of the line.
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1990 March-April 153
and at the other the ambush search?
ers, which may lie in wait for prey for
as long as several days. The vast
majority of foraging species fall in the
middle of the continuum, pausing for
brief or extended periods and mov?
? vf- : – : . . ‘~!- white c*apple “/^ ?
ing in discrete units of time and
space.
Unlike cruise and ambush
search, saltatory search is a strategy
that can potentially be varied to suit
local environmental conditions. In
our observations of planktivorous
fish, we have found variations within
species in the duration of pauses and
the speed and distance of reposition?
ing moves that appear to be related to
particular environmental conditions
(O’Brien et al. 1989). Both Arctic gray?
ling (Thymallus arcticus) and white
:o.r ;. f ‘ ‘\2 “;3;;.; – ?4:.; 5 ? e- . 7 8. ?? 9 crappie
(Pomoxis annularis) pause
more briefly and swim faster and
Figure 2. Arctic grayling and white crappie adjust both the duration of pauses and the speed
farther during repositioning moves
and length of repositioning moves according to the size of their prey. Arctic grayling feeding
when
feeding on large prey than on
on large prey (2-3 mm) employ very brief pauses interspersed with fast, long moves,
small (Fig. 2). When feeding on large
approaching the norm for cruise predators shown in the graph in Figure 1. At the other
prey, Arctic grayling pause so briefly
extreme, white crappie feeding on small prey (0.5-1.2 mm) use very long pauses and slow,
short moves, coming close to the behavior of ambush predators. In dealing with smalland
prey,
swim so rapidly that they ap?
Arctic grayling shifts its strategy to one of longer pauses and slower, shorter moves, proach
whereasthe condition of cruise search.
white crappie foraging on large prey use shorter pauses and faster, longer moves.
When feeding on small prey, white
crappie pause so long and swim so
slowly that they approach ambush
search. Both species also moderate their search behavior
ism has been viewed as in part detenrdning the ecolog?
under certain conditions, tending to move toward the
ical niche of the species, defined through evolutionary
of the search continuum.
time. Although some investigators have implied midpoint
that
there may be intermediate strategies of search, to date noIf an organism’s search strategy can change depend?
ing on the species, density, and visibility of the prey,
such strategy has been explicitly described.
then a precise description of saltatory behavior is re?
In detailed studies of foraging behavior in planktiv
orous fish, we have found that the search strategies quired
they if we are to construct accurate foraging models for
species in which search is the primary factor of the cycle.
employ cannot be classified as either cruise or ambush.
Clearly,
differences in the duration of pauses and the
Rather than swirrurdng continuously while foraging
or
speed
stopping to wait for extended periods, these fish move
in and length of movement will affect net energy
gain, and how well an organism can maximize energy
a relatively rapid stop-and-go pattern. We have called
this behavior “saltatory search” (Evans and O’Brien
gain by selecting a particular saltatory search strategy
becomes crucial to relative reproductive success. Further,
1988). Since identifying this pattern of foraging in fish,
we have noted other instances of saltatory movementsthe
inrecognition of a continuum of search strategies and
foraging birds, lizards, and insects. It seems likely, the
in establishment of criteria for positioning a given
fact, that all search behavior can be placed on a “stop
forager on that continuum have obvious importance for
and-go” continuum (Fig. 1). At one extreme are an
the
understanding of the overall ecology, morphology,
cruise searchers, which appear to be in constant motion,
and behavior of a predator.
Animals for which a component of the predation
Table 1. Changes in saltatory search with size of prey
White crappie
Arctic grayling
Pause (sec)
small
prey
1.65
large
prey
0.55
small
large
prey
prey
0.3 0.07
Distance moved (cm)
5.2
9.8
6.5
12.0
Angle turned
22?
44?
25?
65?
Sources: O’Brien et al. 1986; Evans and O’Brien 1988; O’Brien et al.
1989
cycle other than search is most important or those that
search for prey distributed in patches cannot be classified
on a search-behavior continuum. However, it is possible
that other continuums, analogous to this one, can be
applied to other components of the predation cycle?for
instance, the factor of handling time. The relationship
between prey size and the costs of search, pursuit, and
handling or subduing have been discussed by Griffiths
(1980), who suggests that the relative importance of these
components can be classified along a prey-size contin?
uum. For example, animals that search for prey larger
than themselves are often sit-and-wait searchers with
high subduing costs.
154 American Scientist, Volume 78
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Saltatory search behavior
Although we first identified saltatory search through
observations of planktivorous fish, this strategy appears
to be used by a broad range of animals, all of whom feed
often and search for widely-distributed prey that are
much smaller than themselves. This general description
fits many species of fish, birds, lizards, and insects.
Many of the reports on behavior that fits this pattern
depend on qualitative or categorical descriptions, but
some quantitative data are available.
Among the most intriguing data of this sort are
those on birds. Quantitative results gathered by Cody
(1968,1971) indicate that many birds, especially ground
foraging species feeding on insects or seeds, move in a
saltatory manner; some species pause briefly and move a
short distance, whereas others pause for longer periods
and move short distances (Fig. 3). Descriptive reports are
equally suggestive in the case of thrushes and starlings,
which pause longer and move a shorter distance when
searching for hidden prey or in a more complex environ?
ment (Smith 1974a, 1974b; Brownsmith 1977). Birds that
glean insects from branches, twigs, and leaves also
exhibit a pattern of pauses and repositioning move?
ments, as do fly-catching insectivorous birds as they fly
from perch to perch, waiting briefly before either moving
to another perch or pursuing and capturing a located
prey (Davies 1977; Fitzpatrick 1981; Robinson and
Holmes 1982; Moreno 1984). Some birds that forage on
beaches or mudflats, feeding on small benthic inverte?
brates, typically move in a saltatory manner (Meyers et
al. 1980; Pienkowski 1983). Even the head bobbing of
pigeons can be interpreted as presenting a saltatory
pattern; in each cycle, the head and eye remain motion?
less for about one tenth of a second while the body
moves continuously forward, with the head rapidly
following (Frost 1978).
Quantitative data on lizards show that they, too,
move in a saltatory manner while foraging (Fig. 3). Huey
and Pianka (1981) classified the lizards in their study in
relative terms as widely ranging or sit-and-wait preda?
tors. The four species classified as widely foraging were
frequently stationary for up to 10 seconds between for?
ward movements. By contrast, the two species classified
as sit-and-wait foragers moved approximately twice a
minute and were stationary for only about 30 seconds.
Clearly, these behaviors can be classified as saltatory
movement patterns.
Several fish species other than those we studied,
such as large goldfish (Kleerekoper et al. 1970), have also
been described in terms that suggest the use of saltatory
movements. Indeed, the stop-and-go foraging of bluegill
sunfish (Lepomis macrochirus) has been labeled “hover
search” Janssen 1982; Ehlinger and Wilson 1988). Some
insects, such as wingless phorid flies searching a surface,
also exhibit stop-and-start foraging movements (Miller
1979).
For that matter, humans foraging on the written
word exhibit a type of saltatory eye movement as the eye
shifts focus from point to point along a line of type (Huey
1968). At each point of focus there is a brief pause, during
which the actual reading occurs. Humans searching for a
specific letter sequence in a vertical list four letters wide
Figure 3. Grassland birds foraging for insects or seeds (top) show
varied patterns of stops and starts resembling those of the
planktivorous fish studied. Foraging movements range from the
near-cruise behavior of Geositta cunicularia, which displays very
brief pauses and fast, long moves, to the near-ambush pattern of
Sicalis luteola, which is characterized by very long pauses and
infrequent, short moves. The classic saltatory rhythm of stops and
starts is visible in Lessonia ruf a, which combines fairly long pauses
with fairly fast, long moves. A similar spectrum of saltatory
patterns has been observed in various species of sand lizards, as
shown in the graph at the bottom. (Data from Cody 1968 and Huey
and Piartka 1981.)
1990 March-April 155
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also skip from quadruplet to quadruplet down the page
in a saltatory manner (Chase 1986).
Pausing and searching
To reach a better understanding of the saltatory search
strategy, it is necessary to examine both the “stop” and
the “go” phases of the pattern?that is, both the pauses
and the repositioning movements that alternate with
them. Our studies of feeding behavior in Arctic grayling
and white crappie have demonstrated that neither of
these fish search for prey while moving. Instead, they
search only during the stationary pauses between repo?
sitioning moves (O’Brien et al. 1986; Evans and O’Brien
1988). Janssen (1982) believes that bluegill, too, search
only during such pauses. The repositioning moves be?
tween pauses, then, serve only to move the fish into
previously unscanned space.
Studies of other predators showing stop-and-go
patterns of movement while foraging have not been
carried out in sufficient detail to determine whether they
search only during pauses. In general, however, a pred?
ator moving in this pattern and searching only while
pausing should pause frequently and moderate the du?
ration of the pause as a function of the difficulty of
locating prey. Furthermore, to maximize the efficiency of
searches, a forager should move just far enough to avoid
rescanning a previously searched area without skipping
over any area. Finally, a saltatory searcher should pursue
prey throughout the volume being searched, while cruis?
ing or ambushing predators should pursue prey near the
outer boundary of this volume.
There is the possibility that pauses may serve a
purpose other than search?for example, they might
help the predator to orient toward prey or to organize the
attack phase of the predation cycle. If either of these were
the case, all or most pauses would be followed by a
pursuit. However, it is clear that white crappie fre?
quently pause without subsequently pursuing prey
(O’Brien et al. 1986). In fact, as prey abundance de?
creased, fewer pauses were followed by pursuit and
attack of prey. Neither of these observations support the
alternate explanations for the function of stationary
pauses, but they are consistent with the hypothesis that
predators search during pauses. In addition, the dura?
tion of pauses in both white crappie and Arctic grayling
decreased when the fish were searching for large, more
easily located prey (Table 1)? a behavior also consistent
with stationary search. This phenomenon has been
noted in other species as well. Janssen (1982) found that
the duration of “hover-search” in bluegill sunfish de?
creased as apparent prey size increased. Ehlinger and
Wilson (1988) have reported that bluegill foraging among
vegetation exhibit longer pauses than those foraging in
open water, suggesting that as the difficulty of the search
increases, the pauses become more lengthy.
Several species of birds also vary the duration of
foraging pauses as a function of the difficulty of scanning
a given space or locating a particular type of prey. For
example, plovers spend less time in scanning when
searching for larger prey than for small (Pienkowski
1983). Starlings pause for a mean of 1.7 seconds when
foraging in tall grass as opposed to 1.2 seconds in short
grass (Brownsmith 1977). Robinson and Holmes (1982)
found that red-eyed vireos paused for shorter times
when they fed in the subcanopy of a deciduous forest
than when they searched the more dense canopy. Tyrant
flycatchers also alter their search time in response to local
conditions, assessing each perch independently and
moving after varying periods that depend on the visual
characteristics of the environment (Fitzpatrick 1981).
Movement and search space
If foragers indeed move only to enter previously un?
scanned space, there should be a relationship between
Figure 4. The most efficient length for repositioning moves is
related to both the direction of the move and the shape of the
volume being searched. In the case of a pie-shaped search space
(top), a short, straight move results in the rescanning of a
considerable area, indicated by shading, while a long move leaves a
large area unscanned. By contrast, a move of intermediate length
(color) results in a relatively small overlap with space already
scanned. When a move of this same length is made at an angle
rather than straight ahead (middle), a small angle still yields some
overlap, while a large angle results in the maximum gain of
unscanned area. As the geometry of the search space shifts from
pie-shaped to semicircular to circular (bottom), a move of identical
length creates an ever-greater overlap with space already scanned.
the distance traveled in repositioning moves and the size
and geometry of the space previously scanned. If after an
unsuccessful search an animal were to move only a short
distance relative to the maximum distance at which it can
locate prey, much of the volume just searched would be
scanned again (Fig. 4). On the other hand, if the animal
were to move completely through and beyond the pre?
vious search space, it would be wasting time and energy.
In both white crappie and Arctic grayling, the di?
mensions of the search space change with the size of
prey (Table 1). Moreover, the length of the repositioning
moves is directly related to these changes. For white
crappie, the maximum distance at which small prey can
be located is 8 cm. When these fish make a repositioning
156 American Scientist, Volume 78
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move after an unsuccessful search for
small prey, the length of the move
averages 5.2 cm. White crappie can
locate large daphnid prey at about 20
cm, and moves made after an unsuc?
cessful search for prey of this size
average 9.8 cm. A simple model of
net energy gain described in more
detail below has shown that these
distances are optimal.
Other taxa behave in a similar
manner. Blackbirds, for example,
move an average of 60 cm when
searching for brown-colored prey on
a green lawn but only an average of
40 cm when searching for less obvi?
rwentageof^
ous green-colored prey of the same
Figure 5. The percentage of previously unscanned space gained by straight moves of varying
size on the same lawn (Smith 1974a).
lengths is strongly influenced by the geometry of the search space. For search spaces of 180?
Plovers move just far enough to scan or less, fairly short moves will produce a high proportion of previously unscanned space.
a totally new area after an unsuccess? Search spaces of more than 180? require the predator to move an increasingly greater
ful search for beach-dwelling inverte? percentage of the maximum distance at which prey can be located in order to obtain a high
brates (Pienkowski 1983). In the liz? percentage of new search space. A predator searching a circular space must move 200% of the
ards studied by Moermond (1979), maximum location distance to achieve the smallest possible overlap.
movements to new search sites were
directly related to the average distance traveled in cap?
turing prey. Thus a number of animals appear to modify
the length of repositioning moves after an unsuccessful
search in such a way as to minimize redundant search
efforts.
Not only the maximum distance at which a given
prey can be located but the geometry of the search space
is important in determining the most efficient distance to
move. When the horizontal dimension of the search
space is less than 180? (90? on each side, looking for?
ward), fairly short moves yield a high proportion of
unsearched space (Fig. 5). White crappie search a pie
shaped space, and on average move only 60% of the
maximum distance at which the prey in question can be
located. Because of the geometry of the space, reposi?
tioning movements of this length provide a high propor?
tion of new, unsearched territory.
As the geometry of the space being searched ap?
proaches a circle, the length of the repositioning moves
must increase to a distance greater than the maximum
distance at which prey can be located in order to obtain
a high proportion of unsearched space. Plovers search a
space of 240? (Pienkowski 1983), and consequently must
move about 90% of the maximum prey location distance
to obtain a high proportion of unsearched space. Tyrant
flycatchers and other birds that feed on flying insects
scan a nearly spherical space and move slightly more
than twice the average pursuit distance to new perches
(Fitzpatrick 1981; Robinson and Holmes 1982). This is
necessary for foragers searching a circular space.
Figure 6. The effect of turns of varying angles on the percentage of
previously unscanned space gained is shown for search spaces of
different geometries. When the search area is less than 180?, an
increase in the angle turned results in a high percentage of new
space. If the search area is greater than a hemisphere, however,
turns of greater angle will yield progressively less and less
previously unscanned space.
Repositioning moves are not always necessarily in
the same direction. Turning after pausing is another way
saltatory searchers can quickly move into unsearched
areas. By contrast, cruise searchers, which continuously
scan the outer boundary of the volume searched, would
obtain little or no benefit from turning; it would be no
more effective than simply moving straight ahead.
Both species of fish we studied turned after each
unsuccessful search. As with the length of forward
directed repositioning movements, the angle turned in?
creased with prey size (Table 1). Both white crappie and
Arctic grayling scan a volume of greater angle for large
prey than for small. Thus, to benefit from turning in
terms of yield of unsearched space, the fish must make
greater turns when feeding on large prey than when
feeding on small prey.
1990 March-April 157
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Similar data are available for other taxa. Blackbirds
make alternating left and right turns, with 67% of the
turns lying within 43? of the forward path (Smith 1974b).
Phorid flies, which search in a stop-and-go manner,
make sharp turns after each pause (Miller 1979). Plovers
also turn after search pauses (Pienkowski 1983).
Whereas the expected length of the repositioning
move is directly related to the maximum distance at
which a given prey can be located, the opposite is
expected for turns. Once the geometry of the space
and the ways in which it differs from other search
strategies are summarized in Table 2, together with the
likely attributes of prey most commonly sought through
the various search strategies.
Implications of saltatory search
It is usually assumed that animals that move frequently
while foraging employ a cruise search strategy. As we
have shown, however, many of these animals are doubt?
less using a saltatory strategy. This distinction is not
trivial; it can have important ramifications for measuring
and understanding the frequency of predator-prey en?
spherical space (Eckhardt 1979; Robinson and Holmes counters, prey choice and dietary preferences, and the
1982). In this case, turning would provide little or no efficiency of patterns of movement associated with for?
unsearched area and thus would not be expected; how?
aging, among other things.
ever, as far as we know no information is available on
The frequency or likelihood of encounter is a funda?
turning in these birds.
mental parameter in predator-prey dynamics. To deter?
A final point regarding movement concerns the mine the probability of encounter accurately, it is essen?
distribution of pursuit within the search space. Both tial to know the volume a given forager can search and
cruise and ambush searchers are expected to pursue prey
how that volume is scanned. A tool commonly used in
at the boundary of the search space. However, saltatory
estimating search volume is the average distance at
searchers are distinguished by the fact that they pursue which a predator responds to a specific prey type under
prey throughout the space being searched, as shown in
specific environmental conditions (O’Brien 1979). That
searched exceeds a hemisphere, less and less unscanned
area can be obtained through greater turns (Fig. 6). Birds
foraging in forests, for example, seem to search a nearly
Figure 7 for white crappie feeding on Daphnia pulex
(Evans and O’Brien 1986; O’Brien et al. 1986). There are
very few data of this sort for other taxa, and those that
are available present pursuit distances as mean values,
obscuring the distribution throughout the search vol?
ume. However, Eckhardt (1979), in mapping what he
terms the “foraging space” of insectivorous birds, has
shown that most of the attacks of both gleaners and
flycatchers are well within the maximum distance at
which the prey is typically located. Thus, at least in this
case, birds that forage in a saltatory manner pursue their
prey throughout the volume being searched.
The characteristics of saltatory search outlined here
2j^T
is, the predator is allowed to search for prey under
experimental conditions, and the distance at which it first
reacts to it?for example, by orienting toward it?is
recorded. Typically, many such measurements are made
and a mean is calculated.
For an animal using a cruise search strategy, and
thus scartning the outer boundary of the volume as it
moves, measurements of reaction distance will yield
accurate estimates of the maximum distance at which the
prey is located. However, for an animal using a saltatory
strategy, and thus scanning nearly the entire volume
being searched during the pause phase, measuring the
mean reaction distance is not a good method of estimat
ing the maximum distance for prey
location. For the saltatory searcher,
only the longest observed pursuit or
reaction distances are at or near the
outer boundary of the search vol?
ume. In birds feeding in forests, most
pursuit and reaction distances are
well short of the maximum distance
for locating the prey (Eckhardt 1979;
Robinson and Holmes 1982). Esti?
mates of search volume based on
measurements of reaction distance
therefore underestimate the maxi?
mum distance at which prey may be
located in saltatory search.
The opportunity, likelihood, and
of choosing among sev?
? 25 ^ 20 15 10 5 0 0 5 10 15 20 25^ mechanics
eral prey will also vary with the type
Prey located (%) Distance (cm)
of search strategy employed. Prey
Figure 7. The distribution of prey located and pursued by white crappie feeding on D. pulex
choice is unlikely for cruise or am?
illustrates both the distinctive saltatory strategy of foraging throughout the volume searched
bush
searchers. Both types of forag?
and the influence of the geometry of the search space. When all the events within a 180?
ers
search
the outer boundary of the
space in front of the fish are plotted as dots (right)?here conflated within 90?, with 0? as the
search
space, and presumably pur?
forward path of the fish?it is apparent that white crappie rarely located D. pulex
farther
sueatprey
almost as soon as it is
away than 18 cm or at an angle greater than 90? from the forward path. As shown
the left,
detected.
the highest percentage of prey were located between 20? and 40? of this forward
path, withUnder these conditions,
very few prey located at angles greater than 50?.
choice among several prey seems un
158 American Scientist, Volume 78
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Table 2. Characteristics of thethree search strategies and typical prey
Behavioral patterns
Cruise strategy Saltatory strategy
Ambush strategy
Search
When
Duration of pause
Variation in duration of pause
while moving
rarely pause
rarely pause
while pausing
brief
while moving
while pausing
related to prey characteristics and
environmental conditions
while pausing
long
related to prey characteristics
and environmental conditions
Prey encounter
When
Where in volume searched
outer boundary only
while pausing
outer boundary only
throughout
Forward movement after pause
Length of move
Relationship to volume searched
Relationship to prey density
Turning movement after pause
Frequency
When
Relationship to volume searched
related to size of prey
length increases with size of area searched
length decreases withnone
density of prey
move continuously
none
rarely move
uncertain
none
while moving
none
rare
after every pause
rarely turn
only after pause
none
angle increases with size of area searched
rare
Characteristics of prey
Movement
variable
large
small
little
large
extensive
Major defense strategy
speed or hiding
hiding
large size
Size relative to predator
likely, because this could occur only on the rare occasion
when two or more prey were simultaneously detected at
the boundary of the search space. By contrast, saltatory
foragers scan the entire search space while stationary,
and it is therefore far more likely that two or more prey
could be in the volume searched during any given search
episode. Many theoretical studies of prey choice and
dietary preference in animals, including many models of
optimal foraging, implicitly assume that the forager is
searching continuously except when pursuing and han?
dling prey.
As described above, not only white crappie but
several species of birds and lizards have been found to
vary the distance moved after an unsuccessful search
depending on the size of the prey. It seems reasonable to
ask if there is a specific length of repositioning move?
ment that would most efficiently balance a high propor?
tion of unscanned space against the cost of moving.
In a model of net energy gain we constructed to
general, a potential prey is more likely to be detected
while it is moving than while it is stationary (Wright and
O’Brien 1982; Howiek and O’Brien 1983). For saltatory
searchers, we would predict that vigilance for predators
would be highest during repositioning movements and
lowest during the pause to scan for prey. It is also likely
that there will be a relationship between the risk of being
preyed on and the components of the saltatory search
cycle. For example, all else being equal, in the presence
of a predator a saltatory searcher should increase its
stationary search time and decrease the speed and length
of its repositioning moves.
Obviously, some taxa do not employ saltatory
search. They may be discouraged from doing so by body
size, structure, or other considerations. One might as?
sume that insects, bats, and birds would not exhibit
saltatory search, due to the need to remain airborne.
However, insects and birds that can hover?dragonflies
agreement with those the model identified as most
and humming birds, for example?could and perhaps do
use saltatory search. Under some conditions, many birds
may be able to approach the patterns of saltatory search.
Forster’s tern appears to use cruise search at low wind
predicted that 5 to 6 cm would be the most efficient
speeds the birds have been observed to hover for some
investigate this problem in the white crappie, we found
that the distances the fish actually moved were in close
efficient (O’Brien et al. 1989). For small prey, the model
distance for repositioning moves, while for large prey it
predicted that any distance greater than 8 to 9 cm would
be most efficient. These predictions are very close to the
findings outlined in Table 1. Given the prevalence of
saltatory search and the fact that three different taxa are
known to alter the length of their repositioning move?
ments, this may prove to be an important component of
optimal foraging theory.
Foragers must divide their time between conflicting
demands?for example, vigilance for predators and
scanning for prey (Ydenberg and Dill 1986). Saltatory
search may allow a balance between these demands. In
velocities (Salt and Willard 1971), but at modest wind
W. John O’Brien is professor of systematics and ecology at the University of
Kansas, and completed his Ph.D. at,Cornell and Michigan State universities.
His research, which focuses primarily on the predator-prey interactions of
Zooplankton and fish, currently involves the classification of animal search
patterns and their ecological implications. Howard I. Browman received his
Ph.D. from the University of Kansas in 1989, and is now an E. B. Eastburn
Fellow in the Department of Biology at the University of Montreal. Barbara
I. Evans received her Ph.D. from the University of Kansas in 1986; she is
currently a NSERC Canada postdoctoral fellow at the Institute of Neuroscience
at the University of Oregon. Address for Dr. O’Brien: Department of
Systematics and Ecology, University of Kansas, Lawrence, KS 66045-2106.
1990 March-April 159
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time over a particular spot of water. This may be inter?
preted as approaching the condition of ambush search.
At even higher wind speeds the birds fly up above the
surface of the lake and then dive down directly at one
spot. This in effect keeps them briefly over one spot, and
might allow saltatory search. Soaring hawks may be able
to move their eyes in a saltatory manner. That is, they
may visually “lock in” on an area as they soar over it and
then skip ahead to lock in on another area.
Constraints on the use of saltatory movements are
varied. Large swimming animals, such as tunas and
Griffiths, D. 1980. Foraging costs and relative prey size. Am. Nat.
116:743-52.
Hanson, J., and L. Green. 1989. Foraging decisions: Prey choice by
pigeons. Anim. Behav. 37:429-43.
Holling, C. S. 1959. The components of predation as revealed by a
study of small-mammal predation of the European pine sawfly. Can.
Entomol. 91:293-320.
Howiek, G. L., and W. J. O’Brien. 1983. Piscivorous feeding behavior of
largemouth bass: An experimental analysis. Trans. Am. Fish. Soc.
112:508-16.
Huey, E. B. 1968. The Psychology and Pedagogy of Reading. MIT Press.
Huey, R. B., and E. R. Pianka. 1981. Ecological consequences of
foraging mode. Ecology 62:991-99.
sharks, may not be able to use saltatory search efficiently
due to the energy required to start and stop such a large
Janssen, J. 1982. Comparison of searching behavior for Zooplankton in
expenditure of energy for forward motion ceases (Greene
1988). The placement of the fins on the fusiform body of
salmonids and other fish makes braking with the pecto?
Kerfoot, W. C, and A. Sih, eds. 1987. Predation: Direct and Indirect
an obligate planktivore, blueback herring (Alosa aestivalis) and a
mass. On the other hand, very small aquatic animals
facultative planktivore, bluegill (Lepomis macrochius). Can. ]. Fish.
such as copepods expend no energy to stop; at low Aquat. Sei. 39:1649-54.
Reynolds numbers they begin to sink as soon as the Kamil, A. C, J. R. Krebs, and H. R. Pulliam, eds. 1987. Foraging
ral and pelvic fins difficult (Geerlink 1987), and thus
Behavior. Plenum Press.
Impacts on Aquatic Communities. University Press of New England.
Kleerekoper, H., A. M. Timms, G. F. Westlake, F. B. Davey, T. Malar,
and V. M. Anderson. 1970. An analysis of locomotor behavior of
goldfish (Carassius auratus). Anim. Behav. 18:317-30.
hinders pausing. Further, our observations indicate that
Arctic grayling can pause only briefly at the end of each
Marshall, E. A., P. L. Chesson, and R. A. Stein. 1989. Foraging in a
swimming stroke, suggesting how advantageous even a
patchy environment: Prey-encounter rate and residence time distri?
butions. Anim. Behav. 37:444-54.
brief pause can be in searching for small or hidden prey
(Evans and O’Brien 1988).
Meyers, J. P., S. L. Williams, and F. A. Pitelka. 1980. An experimental
Animals have evolved not only specific search strat?
egies that are effective in their particular situations but
the flexibility to deal efficiently with a changing environ?
analysis of prey availability for sanderlings (Aves: Scolopacidae)
feeding on sandy beach crustaceans. Can. J. Zool. 58:1564^74.
ment. It is within this ecological and evolutionary frame?
work that we propose saltatory search as a new explan?
atory and predictive tool.
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160 American Scientist, Volume 78
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Sigma Xi, The Scientific Research Honor Society
Search Strategies of Foraging Animals
Author(s): W. John O’Brien, Howard I. Browman and Barbara I. Evans
Source: American Scientist, Vol. 78, No. 2 (March-April 1990), pp. 152-160
Published by: Sigma Xi, The Scientific Research Honor Society
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Search Strategies
of Foraging Animals
W. John O’Brien
Howard I. Browman
Barbara I. Evans
Of all the activities in which animals engage, perhaps
the most important are finding and consuming food. In
addition to its urgency at the individual level, foraging?
in the broad sense of predator-prey interactions?is cru?
cial to ecosystem processes at many levels. In a very real
sense, the biosphere runs on the consumption of one
organism by another. Fabre has succinctly noted (1913)
that “from the least to the greatest in the zoological
progression, the stomach sways the world; the data
supplied by food are chief among all the documents of
life.” The when, where, how, how often, and how many
cient feeding. Thus models that instead pose the ques?
tion of how animals forage effectively may contribute to
an understanding of behavioral factors that are not
genetically fixed but vary according to local environmen?
tal conditions.
Historically, much of what is known about animal
foraging has been obtained from the analysis of stomach
contents. A more recent approach, and one that can yield
detailed information on the specific mechanisms of for?
aging behavior, breaks down the act of predation into
components (Holling 1959; O’Brien 1979, 1987). The
components of the predation cycle are similar for almost
Saltatory patterns of movement
in a wide range of animals indicate
that most foragers fall somewhere
between cruisers and ambushers
all animals: predators must first search for and locate
prey, then pursue and attack them, and finally handle
and ingest them.
If the prey are larger than the predator (as in the case
of a lion preying on gazelles), the pursuit, attack, and
of prey capture and consumption are central to studies of
individual survival, population dynamics, community
structure, nutrient cycling, and energy flow through
ecosystems.
It is not surprising, then, that foraging has been
studied extensively in virtually all animal groups by
physiologists, ethologists, ecologists, and theoreticians
(Kamil et al. 1987; Kerfoot and Sih 1987). A recent
approach to the study of feeding ecology is optimal
foraging theory, which focuses on aspects of foraging
behavior presumed to have been honed by evolution
(Schoener 1971; Pyke 1984). Theorists of optimal foraging
have produced a series of models that make quantitative
predictions concerning such aspects of predator-prey
interactions as optimizing prey choice, adopting the best
prey patch as prey are depleted, and search strategy (see,
for example, Hanson and Green 1989; Marshall et al.
1989; O’Brien et al. 1989). These models vary widely in
their accuracy when tested against field data, but serve a
valuable function in clarifying what is known and what is
not known about foraging among particular species in
particular environments.
Optimality models are perhaps least useful when
they seek to address the question of whether animals are
“optimal.” Because natural selection has left only those
species and individuals that are efficient feeders, animals
do not exhibit a gradient ranging from efficient to ineffi
152 American Scientist, Volume 78
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handling components of the cycle are of paramount
importance. In this situation, large size is an advantage
for the prey. If the prey are smaller than the predator (as
in the case of a bird feeding on small insects), the search
component of the cycle is the most important. In this
situation, small size may be an advantage for the prey.
Many predators prey on animals considerably smaller
than themselves, thereby avoiding the severe constraints
raised by the time required for handling. When prey are
smaller than their predators, they must be consumed
often and in large numbers. In this case, search time is
the factor that limits all subsequent components of the
predation cycle.
The search strategy employed by a given organism
has commonly been thought of as a trait dictated by
evolution and not open to modification under changing
environmental circumstances. Animals are typically di?
vided into two categories, based on their overall search
search, the forager moves continuously through the
environment, searching constantly for its prey at the
outer boundary of the volume being searched. Cruise
searchers include large fish that swim continuously, such
as tuna, and soaring hawks. In ambush or sit-and-wait
search, a forager remains stationary for long periods of
time, waiting for prey to cross the boundary of its strike
space. Herons and rattlesnakes fit the category of am?
bush searchers.
Clearly, the patterns of motion associated with these
two search strategies are profoundly different. It is im?
possible to conceive of a cruise searcher switching strat?
egies to ambush a particularly appealing prey item. In
fact, the kind of search behavior displayed by an organ
behavior: “cruise” searchers and “ambush” searchers
(Greene 1983). These two categories of behavior have
also been called “widely ranging” and “sit and wait”
(Huey and Pianka 1981). In cruise or widely ranging
Time
Figure 1. The strategies by which animals search for prey range
from the constant motion of cruising foragers such as the red-tailed
hawk shown at the far left to the tactics of ambushers such as the
crouching lion at the right. Most taxa, however, occupy an
intermediate position along this continuum, displaying a distinctive
pattern of stops and starts in which pauses to search for prey
alternate with moves that reposition the predator to scan new
territory. Arctic grayling and white crappie (center, top and bottom
respectively) are among the many species of fish, birds, and lizards
that have been found to use a saltatory strategy. The graph above
shows the characteristic patterns traced by the three strategies in
time and space; the velocity of the movement is indicated by the
slope of the line.
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1990 March-April 153
and at the other the ambush search?
ers, which may lie in wait for prey for
as long as several days. The vast
majority of foraging species fall in the
middle of the continuum, pausing for
brief or extended periods and mov?
? vf- : – : . . ‘~!- white c*apple “/^ ?
ing in discrete units of time and
space.
Unlike cruise and ambush
search, saltatory search is a strategy
that can potentially be varied to suit
local environmental conditions. In
our observations of planktivorous
fish, we have found variations within
species in the duration of pauses and
the speed and distance of reposition?
ing moves that appear to be related to
particular environmental conditions
(O’Brien et al. 1989). Both Arctic gray?
ling (Thymallus arcticus) and white
:o.r ;. f ‘ ‘\2 “;3;;.; – ?4:.; 5 ? e- . 7 8. ?? 9 crappie
(Pomoxis annularis) pause
more briefly and swim faster and
Figure 2. Arctic grayling and white crappie adjust both the duration of pauses and the speed
farther during repositioning moves
and length of repositioning moves according to the size of their prey. Arctic grayling feeding
when
feeding on large prey than on
on large prey (2-3 mm) employ very brief pauses interspersed with fast, long moves,
small (Fig. 2). When feeding on large
approaching the norm for cruise predators shown in the graph in Figure 1. At the other
prey, Arctic grayling pause so briefly
extreme, white crappie feeding on small prey (0.5-1.2 mm) use very long pauses and slow,
short moves, coming close to the behavior of ambush predators. In dealing with smalland
prey,
swim so rapidly that they ap?
Arctic grayling shifts its strategy to one of longer pauses and slower, shorter moves, proach
whereasthe condition of cruise search.
white crappie foraging on large prey use shorter pauses and faster, longer moves.
When feeding on small prey, white
crappie pause so long and swim so
slowly that they approach ambush
search. Both species also moderate their search behavior
ism has been viewed as in part detenrdning the ecolog?
under certain conditions, tending to move toward the
ical niche of the species, defined through evolutionary
of the search continuum.
time. Although some investigators have implied midpoint
that
there may be intermediate strategies of search, to date noIf an organism’s search strategy can change depend?
ing on the species, density, and visibility of the prey,
such strategy has been explicitly described.
then a precise description of saltatory behavior is re?
In detailed studies of foraging behavior in planktiv
orous fish, we have found that the search strategies quired
they if we are to construct accurate foraging models for
species in which search is the primary factor of the cycle.
employ cannot be classified as either cruise or ambush.
Clearly,
differences in the duration of pauses and the
Rather than swirrurdng continuously while foraging
or
speed
stopping to wait for extended periods, these fish move
in and length of movement will affect net energy
gain, and how well an organism can maximize energy
a relatively rapid stop-and-go pattern. We have called
this behavior “saltatory search” (Evans and O’Brien
gain by selecting a particular saltatory search strategy
becomes crucial to relative reproductive success. Further,
1988). Since identifying this pattern of foraging in fish,
we have noted other instances of saltatory movementsthe
inrecognition of a continuum of search strategies and
foraging birds, lizards, and insects. It seems likely, the
in establishment of criteria for positioning a given
fact, that all search behavior can be placed on a “stop
forager on that continuum have obvious importance for
and-go” continuum (Fig. 1). At one extreme are an
the
understanding of the overall ecology, morphology,
cruise searchers, which appear to be in constant motion,
and behavior of a predator.
Animals for which a component of the predation
Table 1. Changes in saltatory search with size of prey
White crappie
Arctic grayling
Pause (sec)
small
prey
1.65
large
prey
0.55
small
large
prey
prey
0.3 0.07
Distance moved (cm)
5.2
9.8
6.5
12.0
Angle turned
22?
44?
25?
65?
Sources: O’Brien et al. 1986; Evans and O’Brien 1988; O’Brien et al.
1989
cycle other than search is most important or those that
search for prey distributed in patches cannot be classified
on a search-behavior continuum. However, it is possible
that other continuums, analogous to this one, can be
applied to other components of the predation cycle?for
instance, the factor of handling time. The relationship
between prey size and the costs of search, pursuit, and
handling or subduing have been discussed by Griffiths
(1980), who suggests that the relative importance of these
components can be classified along a prey-size contin?
uum. For example, animals that search for prey larger
than themselves are often sit-and-wait searchers with
high subduing costs.
154 American Scientist, Volume 78
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Saltatory search behavior
Although we first identified saltatory search through
observations of planktivorous fish, this strategy appears
to be used by a broad range of animals, all of whom feed
often and search for widely-distributed prey that are
much smaller than themselves. This general description
fits many species of fish, birds, lizards, and insects.
Many of the reports on behavior that fits this pattern
depend on qualitative or categorical descriptions, but
some quantitative data are available.
Among the most intriguing data of this sort are
those on birds. Quantitative results gathered by Cody
(1968,1971) indicate that many birds, especially ground
foraging species feeding on insects or seeds, move in a
saltatory manner; some species pause briefly and move a
short distance, whereas others pause for longer periods
and move short distances (Fig. 3). Descriptive reports are
equally suggestive in the case of thrushes and starlings,
which pause longer and move a shorter distance when
searching for hidden prey or in a more complex environ?
ment (Smith 1974a, 1974b; Brownsmith 1977). Birds that
glean insects from branches, twigs, and leaves also
exhibit a pattern of pauses and repositioning move?
ments, as do fly-catching insectivorous birds as they fly
from perch to perch, waiting briefly before either moving
to another perch or pursuing and capturing a located
prey (Davies 1977; Fitzpatrick 1981; Robinson and
Holmes 1982; Moreno 1984). Some birds that forage on
beaches or mudflats, feeding on small benthic inverte?
brates, typically move in a saltatory manner (Meyers et
al. 1980; Pienkowski 1983). Even the head bobbing of
pigeons can be interpreted as presenting a saltatory
pattern; in each cycle, the head and eye remain motion?
less for about one tenth of a second while the body
moves continuously forward, with the head rapidly
following (Frost 1978).
Quantitative data on lizards show that they, too,
move in a saltatory manner while foraging (Fig. 3). Huey
and Pianka (1981) classified the lizards in their study in
relative terms as widely ranging or sit-and-wait preda?
tors. The four species classified as widely foraging were
frequently stationary for up to 10 seconds between for?
ward movements. By contrast, the two species classified
as sit-and-wait foragers moved approximately twice a
minute and were stationary for only about 30 seconds.
Clearly, these behaviors can be classified as saltatory
movement patterns.
Several fish species other than those we studied,
such as large goldfish (Kleerekoper et al. 1970), have also
been described in terms that suggest the use of saltatory
movements. Indeed, the stop-and-go foraging of bluegill
sunfish (Lepomis macrochirus) has been labeled “hover
search” Janssen 1982; Ehlinger and Wilson 1988). Some
insects, such as wingless phorid flies searching a surface,
also exhibit stop-and-start foraging movements (Miller
1979).
For that matter, humans foraging on the written
word exhibit a type of saltatory eye movement as the eye
shifts focus from point to point along a line of type (Huey
1968). At each point of focus there is a brief pause, during
which the actual reading occurs. Humans searching for a
specific letter sequence in a vertical list four letters wide
Figure 3. Grassland birds foraging for insects or seeds (top) show
varied patterns of stops and starts resembling those of the
planktivorous fish studied. Foraging movements range from the
near-cruise behavior of Geositta cunicularia, which displays very
brief pauses and fast, long moves, to the near-ambush pattern of
Sicalis luteola, which is characterized by very long pauses and
infrequent, short moves. The classic saltatory rhythm of stops and
starts is visible in Lessonia ruf a, which combines fairly long pauses
with fairly fast, long moves. A similar spectrum of saltatory
patterns has been observed in various species of sand lizards, as
shown in the graph at the bottom. (Data from Cody 1968 and Huey
and Piartka 1981.)
1990 March-April 155
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also skip from quadruplet to quadruplet down the page
in a saltatory manner (Chase 1986).
Pausing and searching
To reach a better understanding of the saltatory search
strategy, it is necessary to examine both the “stop” and
the “go” phases of the pattern?that is, both the pauses
and the repositioning movements that alternate with
them. Our studies of feeding behavior in Arctic grayling
and white crappie have demonstrated that neither of
these fish search for prey while moving. Instead, they
search only during the stationary pauses between repo?
sitioning moves (O’Brien et al. 1986; Evans and O’Brien
1988). Janssen (1982) believes that bluegill, too, search
only during such pauses. The repositioning moves be?
tween pauses, then, serve only to move the fish into
previously unscanned space.
Studies of other predators showing stop-and-go
patterns of movement while foraging have not been
carried out in sufficient detail to determine whether they
search only during pauses. In general, however, a pred?
ator moving in this pattern and searching only while
pausing should pause frequently and moderate the du?
ration of the pause as a function of the difficulty of
locating prey. Furthermore, to maximize the efficiency of
searches, a forager should move just far enough to avoid
rescanning a previously searched area without skipping
over any area. Finally, a saltatory searcher should pursue
prey throughout the volume being searched, while cruis?
ing or ambushing predators should pursue prey near the
outer boundary of this volume.
There is the possibility that pauses may serve a
purpose other than search?for example, they might
help the predator to orient toward prey or to organize the
attack phase of the predation cycle. If either of these were
the case, all or most pauses would be followed by a
pursuit. However, it is clear that white crappie fre?
quently pause without subsequently pursuing prey
(O’Brien et al. 1986). In fact, as prey abundance de?
creased, fewer pauses were followed by pursuit and
attack of prey. Neither of these observations support the
alternate explanations for the function of stationary
pauses, but they are consistent with the hypothesis that
predators search during pauses. In addition, the dura?
tion of pauses in both white crappie and Arctic grayling
decreased when the fish were searching for large, more
easily located prey (Table 1)? a behavior also consistent
with stationary search. This phenomenon has been
noted in other species as well. Janssen (1982) found that
the duration of “hover-search” in bluegill sunfish de?
creased as apparent prey size increased. Ehlinger and
Wilson (1988) have reported that bluegill foraging among
vegetation exhibit longer pauses than those foraging in
open water, suggesting that as the difficulty of the search
increases, the pauses become more lengthy.
Several species of birds also vary the duration of
foraging pauses as a function of the difficulty of scanning
a given space or locating a particular type of prey. For
example, plovers spend less time in scanning when
searching for larger prey than for small (Pienkowski
1983). Starlings pause for a mean of 1.7 seconds when
foraging in tall grass as opposed to 1.2 seconds in short
grass (Brownsmith 1977). Robinson and Holmes (1982)
found that red-eyed vireos paused for shorter times
when they fed in the subcanopy of a deciduous forest
than when they searched the more dense canopy. Tyrant
flycatchers also alter their search time in response to local
conditions, assessing each perch independently and
moving after varying periods that depend on the visual
characteristics of the environment (Fitzpatrick 1981).
Movement and search space
If foragers indeed move only to enter previously un?
scanned space, there should be a relationship between
Figure 4. The most efficient length for repositioning moves is
related to both the direction of the move and the shape of the
volume being searched. In the case of a pie-shaped search space
(top), a short, straight move results in the rescanning of a
considerable area, indicated by shading, while a long move leaves a
large area unscanned. By contrast, a move of intermediate length
(color) results in a relatively small overlap with space already
scanned. When a move of this same length is made at an angle
rather than straight ahead (middle), a small angle still yields some
overlap, while a large angle results in the maximum gain of
unscanned area. As the geometry of the search space shifts from
pie-shaped to semicircular to circular (bottom), a move of identical
length creates an ever-greater overlap with space already scanned.
the distance traveled in repositioning moves and the size
and geometry of the space previously scanned. If after an
unsuccessful search an animal were to move only a short
distance relative to the maximum distance at which it can
locate prey, much of the volume just searched would be
scanned again (Fig. 4). On the other hand, if the animal
were to move completely through and beyond the pre?
vious search space, it would be wasting time and energy.
In both white crappie and Arctic grayling, the di?
mensions of the search space change with the size of
prey (Table 1). Moreover, the length of the repositioning
moves is directly related to these changes. For white
crappie, the maximum distance at which small prey can
be located is 8 cm. When these fish make a repositioning
156 American Scientist, Volume 78
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move after an unsuccessful search for
small prey, the length of the move
averages 5.2 cm. White crappie can
locate large daphnid prey at about 20
cm, and moves made after an unsuc?
cessful search for prey of this size
average 9.8 cm. A simple model of
net energy gain described in more
detail below has shown that these
distances are optimal.
Other taxa behave in a similar
manner. Blackbirds, for example,
move an average of 60 cm when
searching for brown-colored prey on
a green lawn but only an average of
40 cm when searching for less obvi?
rwentageof^
ous green-colored prey of the same
Figure 5. The percentage of previously unscanned space gained by straight moves of varying
size on the same lawn (Smith 1974a).
lengths is strongly influenced by the geometry of the search space. For search spaces of 180?
Plovers move just far enough to scan or less, fairly short moves will produce a high proportion of previously unscanned space.
a totally new area after an unsuccess? Search spaces of more than 180? require the predator to move an increasingly greater
ful search for beach-dwelling inverte? percentage of the maximum distance at which prey can be located in order to obtain a high
brates (Pienkowski 1983). In the liz? percentage of new search space. A predator searching a circular space must move 200% of the
ards studied by Moermond (1979), maximum location distance to achieve the smallest possible overlap.
movements to new search sites were
directly related to the average distance traveled in cap?
turing prey. Thus a number of animals appear to modify
the length of repositioning moves after an unsuccessful
search in such a way as to minimize redundant search
efforts.
Not only the maximum distance at which a given
prey can be located but the geometry of the search space
is important in determining the most efficient distance to
move. When the horizontal dimension of the search
space is less than 180? (90? on each side, looking for?
ward), fairly short moves yield a high proportion of
unsearched space (Fig. 5). White crappie search a pie
shaped space, and on average move only 60% of the
maximum distance at which the prey in question can be
located. Because of the geometry of the space, reposi?
tioning movements of this length provide a high propor?
tion of new, unsearched territory.
As the geometry of the space being searched ap?
proaches a circle, the length of the repositioning moves
must increase to a distance greater than the maximum
distance at which prey can be located in order to obtain
a high proportion of unsearched space. Plovers search a
space of 240? (Pienkowski 1983), and consequently must
move about 90% of the maximum prey location distance
to obtain a high proportion of unsearched space. Tyrant
flycatchers and other birds that feed on flying insects
scan a nearly spherical space and move slightly more
than twice the average pursuit distance to new perches
(Fitzpatrick 1981; Robinson and Holmes 1982). This is
necessary for foragers searching a circular space.
Figure 6. The effect of turns of varying angles on the percentage of
previously unscanned space gained is shown for search spaces of
different geometries. When the search area is less than 180?, an
increase in the angle turned results in a high percentage of new
space. If the search area is greater than a hemisphere, however,
turns of greater angle will yield progressively less and less
previously unscanned space.
Repositioning moves are not always necessarily in
the same direction. Turning after pausing is another way
saltatory searchers can quickly move into unsearched
areas. By contrast, cruise searchers, which continuously
scan the outer boundary of the volume searched, would
obtain little or no benefit from turning; it would be no
more effective than simply moving straight ahead.
Both species of fish we studied turned after each
unsuccessful search. As with the length of forward
directed repositioning movements, the angle turned in?
creased with prey size (Table 1). Both white crappie and
Arctic grayling scan a volume of greater angle for large
prey than for small. Thus, to benefit from turning in
terms of yield of unsearched space, the fish must make
greater turns when feeding on large prey than when
feeding on small prey.
1990 March-April 157
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Similar data are available for other taxa. Blackbirds
make alternating left and right turns, with 67% of the
turns lying within 43? of the forward path (Smith 1974b).
Phorid flies, which search in a stop-and-go manner,
make sharp turns after each pause (Miller 1979). Plovers
also turn after search pauses (Pienkowski 1983).
Whereas the expected length of the repositioning
move is directly related to the maximum distance at
which a given prey can be located, the opposite is
expected for turns. Once the geometry of the space
and the ways in which it differs from other search
strategies are summarized in Table 2, together with the
likely attributes of prey most commonly sought through
the various search strategies.
Implications of saltatory search
It is usually assumed that animals that move frequently
while foraging employ a cruise search strategy. As we
have shown, however, many of these animals are doubt?
less using a saltatory strategy. This distinction is not
trivial; it can have important ramifications for measuring
and understanding the frequency of predator-prey en?
spherical space (Eckhardt 1979; Robinson and Holmes counters, prey choice and dietary preferences, and the
1982). In this case, turning would provide little or no efficiency of patterns of movement associated with for?
unsearched area and thus would not be expected; how?
aging, among other things.
ever, as far as we know no information is available on
The frequency or likelihood of encounter is a funda?
turning in these birds.
mental parameter in predator-prey dynamics. To deter?
A final point regarding movement concerns the mine the probability of encounter accurately, it is essen?
distribution of pursuit within the search space. Both tial to know the volume a given forager can search and
cruise and ambush searchers are expected to pursue prey
how that volume is scanned. A tool commonly used in
at the boundary of the search space. However, saltatory
estimating search volume is the average distance at
searchers are distinguished by the fact that they pursue which a predator responds to a specific prey type under
prey throughout the space being searched, as shown in
specific environmental conditions (O’Brien 1979). That
searched exceeds a hemisphere, less and less unscanned
area can be obtained through greater turns (Fig. 6). Birds
foraging in forests, for example, seem to search a nearly
Figure 7 for white crappie feeding on Daphnia pulex
(Evans and O’Brien 1986; O’Brien et al. 1986). There are
very few data of this sort for other taxa, and those that
are available present pursuit distances as mean values,
obscuring the distribution throughout the search vol?
ume. However, Eckhardt (1979), in mapping what he
terms the “foraging space” of insectivorous birds, has
shown that most of the attacks of both gleaners and
flycatchers are well within the maximum distance at
which the prey is typically located. Thus, at least in this
case, birds that forage in a saltatory manner pursue their
prey throughout the volume being searched.
The characteristics of saltatory search outlined here
2j^T
is, the predator is allowed to search for prey under
experimental conditions, and the distance at which it first
reacts to it?for example, by orienting toward it?is
recorded. Typically, many such measurements are made
and a mean is calculated.
For an animal using a cruise search strategy, and
thus scartning the outer boundary of the volume as it
moves, measurements of reaction distance will yield
accurate estimates of the maximum distance at which the
prey is located. However, for an animal using a saltatory
strategy, and thus scanning nearly the entire volume
being searched during the pause phase, measuring the
mean reaction distance is not a good method of estimat
ing the maximum distance for prey
location. For the saltatory searcher,
only the longest observed pursuit or
reaction distances are at or near the
outer boundary of the search vol?
ume. In birds feeding in forests, most
pursuit and reaction distances are
well short of the maximum distance
for locating the prey (Eckhardt 1979;
Robinson and Holmes 1982). Esti?
mates of search volume based on
measurements of reaction distance
therefore underestimate the maxi?
mum distance at which prey may be
located in saltatory search.
The opportunity, likelihood, and
of choosing among sev?
? 25 ^ 20 15 10 5 0 0 5 10 15 20 25^ mechanics
eral prey will also vary with the type
Prey located (%) Distance (cm)
of search strategy employed. Prey
Figure 7. The distribution of prey located and pursued by white crappie feeding on D. pulex
choice is unlikely for cruise or am?
illustrates both the distinctive saltatory strategy of foraging throughout the volume searched
bush
searchers. Both types of forag?
and the influence of the geometry of the search space. When all the events within a 180?
ers
search
the outer boundary of the
space in front of the fish are plotted as dots (right)?here conflated within 90?, with 0? as the
search
space, and presumably pur?
forward path of the fish?it is apparent that white crappie rarely located D. pulex
farther
sueatprey
almost as soon as it is
away than 18 cm or at an angle greater than 90? from the forward path. As shown
the left,
detected.
the highest percentage of prey were located between 20? and 40? of this forward
path, withUnder these conditions,
very few prey located at angles greater than 50?.
choice among several prey seems un
158 American Scientist, Volume 78
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Table 2. Characteristics of thethree search strategies and typical prey
Behavioral patterns
Cruise strategy Saltatory strategy
Ambush strategy
Search
When
Duration of pause
Variation in duration of pause
while moving
rarely pause
rarely pause
while pausing
brief
while moving
while pausing
related to prey characteristics and
environmental conditions
while pausing
long
related to prey characteristics
and environmental conditions
Prey encounter
When
Where in volume searched
outer boundary only
while pausing
outer boundary only
throughout
Forward movement after pause
Length of move
Relationship to volume searched
Relationship to prey density
Turning movement after pause
Frequency
When
Relationship to volume searched
related to size of prey
length increases with size of area searched
length decreases withnone
density of prey
move continuously
none
rarely move
uncertain
none
while moving
none
rare
after every pause
rarely turn
only after pause
none
angle increases with size of area searched
rare
Characteristics of prey
Movement
variable
large
small
little
large
extensive
Major defense strategy
speed or hiding
hiding
large size
Size relative to predator
likely, because this could occur only on the rare occasion
when two or more prey were simultaneously detected at
the boundary of the search space. By contrast, saltatory
foragers scan the entire search space while stationary,
and it is therefore far more likely that two or more prey
could be in the volume searched during any given search
episode. Many theoretical studies of prey choice and
dietary preference in animals, including many models of
optimal foraging, implicitly assume that the forager is
searching continuously except when pursuing and han?
dling prey.
As described above, not only white crappie but
several species of birds and lizards have been found to
vary the distance moved after an unsuccessful search
depending on the size of the prey. It seems reasonable to
ask if there is a specific length of repositioning move?
ment that would most efficiently balance a high propor?
tion of unscanned space against the cost of moving.
In a model of net energy gain we constructed to
general, a potential prey is more likely to be detected
while it is moving than while it is stationary (Wright and
O’Brien 1982; Howiek and O’Brien 1983). For saltatory
searchers, we would predict that vigilance for predators
would be highest during repositioning movements and
lowest during the pause to scan for prey. It is also likely
that there will be a relationship between the risk of being
preyed on and the components of the saltatory search
cycle. For example, all else being equal, in the presence
of a predator a saltatory searcher should increase its
stationary search time and decrease the speed and length
of its repositioning moves.
Obviously, some taxa do not employ saltatory
search. They may be discouraged from doing so by body
size, structure, or other considerations. One might as?
sume that insects, bats, and birds would not exhibit
saltatory search, due to the need to remain airborne.
However, insects and birds that can hover?dragonflies
agreement with those the model identified as most
and humming birds, for example?could and perhaps do
use saltatory search. Under some conditions, many birds
may be able to approach the patterns of saltatory search.
Forster’s tern appears to use cruise search at low wind
predicted that 5 to 6 cm would be the most efficient
speeds the birds have been observed to hover for some
investigate this problem in the white crappie, we found
that the distances the fish actually moved were in close
efficient (O’Brien et al. 1989). For small prey, the model
distance for repositioning moves, while for large prey it
predicted that any distance greater than 8 to 9 cm would
be most efficient. These predictions are very close to the
findings outlined in Table 1. Given the prevalence of
saltatory search and the fact that three different taxa are
known to alter the length of their repositioning move?
ments, this may prove to be an important component of
optimal foraging theory.
Foragers must divide their time between conflicting
demands?for example, vigilance for predators and
scanning for prey (Ydenberg and Dill 1986). Saltatory
search may allow a balance between these demands. In
velocities (Salt and Willard 1971), but at modest wind
W. John O’Brien is professor of systematics and ecology at the University of
Kansas, and completed his Ph.D. at,Cornell and Michigan State universities.
His research, which focuses primarily on the predator-prey interactions of
Zooplankton and fish, currently involves the classification of animal search
patterns and their ecological implications. Howard I. Browman received his
Ph.D. from the University of Kansas in 1989, and is now an E. B. Eastburn
Fellow in the Department of Biology at the University of Montreal. Barbara
I. Evans received her Ph.D. from the University of Kansas in 1986; she is
currently a NSERC Canada postdoctoral fellow at the Institute of Neuroscience
at the University of Oregon. Address for Dr. O’Brien: Department of
Systematics and Ecology, University of Kansas, Lawrence, KS 66045-2106.
1990 March-April 159
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time over a particular spot of water. This may be inter?
preted as approaching the condition of ambush search.
At even higher wind speeds the birds fly up above the
surface of the lake and then dive down directly at one
spot. This in effect keeps them briefly over one spot, and
might allow saltatory search. Soaring hawks may be able
to move their eyes in a saltatory manner. That is, they
may visually “lock in” on an area as they soar over it and
then skip ahead to lock in on another area.
Constraints on the use of saltatory movements are
varied. Large swimming animals, such as tunas and
Griffiths, D. 1980. Foraging costs and relative prey size. Am. Nat.
116:743-52.
Hanson, J., and L. Green. 1989. Foraging decisions: Prey choice by
pigeons. Anim. Behav. 37:429-43.
Holling, C. S. 1959. The components of predation as revealed by a
study of small-mammal predation of the European pine sawfly. Can.
Entomol. 91:293-320.
Howiek, G. L., and W. J. O’Brien. 1983. Piscivorous feeding behavior of
largemouth bass: An experimental analysis. Trans. Am. Fish. Soc.
112:508-16.
Huey, E. B. 1968. The Psychology and Pedagogy of Reading. MIT Press.
Huey, R. B., and E. R. Pianka. 1981. Ecological consequences of
foraging mode. Ecology 62:991-99.
sharks, may not be able to use saltatory search efficiently
due to the energy required to start and stop such a large
Janssen, J. 1982. Comparison of searching behavior for Zooplankton in
expenditure of energy for forward motion ceases (Greene
1988). The placement of the fins on the fusiform body of
salmonids and other fish makes braking with the pecto?
Kerfoot, W. C, and A. Sih, eds. 1987. Predation: Direct and Indirect
an obligate planktivore, blueback herring (Alosa aestivalis) and a
mass. On the other hand, very small aquatic animals
facultative planktivore, bluegill (Lepomis macrochius). Can. ]. Fish.
such as copepods expend no energy to stop; at low Aquat. Sei. 39:1649-54.
Reynolds numbers they begin to sink as soon as the Kamil, A. C, J. R. Krebs, and H. R. Pulliam, eds. 1987. Foraging
ral and pelvic fins difficult (Geerlink 1987), and thus
Behavior. Plenum Press.
Impacts on Aquatic Communities. University Press of New England.
Kleerekoper, H., A. M. Timms, G. F. Westlake, F. B. Davey, T. Malar,
and V. M. Anderson. 1970. An analysis of locomotor behavior of
goldfish (Carassius auratus). Anim. Behav. 18:317-30.
hinders pausing. Further, our observations indicate that
Arctic grayling can pause only briefly at the end of each
Marshall, E. A., P. L. Chesson, and R. A. Stein. 1989. Foraging in a
swimming stroke, suggesting how advantageous even a
patchy environment: Prey-encounter rate and residence time distri?
butions. Anim. Behav. 37:444-54.
brief pause can be in searching for small or hidden prey
(Evans and O’Brien 1988).
Meyers, J. P., S. L. Williams, and F. A. Pitelka. 1980. An experimental
Animals have evolved not only specific search strat?
egies that are effective in their particular situations but
the flexibility to deal efficiently with a changing environ?
analysis of prey availability for sanderlings (Aves: Scolopacidae)
feeding on sandy beach crustaceans. Can. J. Zool. 58:1564^74.
ment. It is within this ecological and evolutionary frame?
work that we propose saltatory search as a new explan?
atory and predictive tool.
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Article Response Assignment
CONS 472: Introduction to Animal Behavior
140 pts (7 readings; 20 pts per response post)
Responses to published literature allow you to demonstrate your understanding of a reading’s content and
critically analyze the implications of findings and ideas presented. The readings selected will also provide
you with background to understand learning content given in this course.
For this assignment, you will read the assigned article and post a response to the article in the Discussion
Board section on Blackboard, under the appropriately dated/labeled forum of “Article Responses.” Click on
the forum title, then click on “Create Thread” to start your post. Article responses need to be posted to the
Discussion Board by 10:00 AM, on the dates of Thursday 6/23, Friday 6/24, Monday 6/27, Tuesday
6/28, Wednesday 6/29, Thursday 6/30 and Friday 7/1. A total of seven (7) posts are required.
You will receive extra credit (+1 pt) if you post a meaningful response or comment to at least one of
your peers’ postings. Comments in response to classmate postings need to be posted by 9:30 AM, on the
day directly following the reading due.
Your post will include the following elements:
1) Two key ideas: Give at least two (2) key conclusions or most important overarching findings from
the article. This description should be succinct and highlight the most relevant findings or
conclusions, but they must convey complete ideas. These should be the “take-home messages” of the
article. Do not simply “copy and paste” or paraphrase the findings given in the article (and this is
plagiarism!). Dig deeper – think more broadly about the relevance or importance of the findings.
2) Critical analysis and questioning for further investigation: Give a critical evaluation of the
article or provide some topics/questions for future research or investigation. You will provide
a short analysis regarding the article, focusing on evaluation and synthesis of the ideas in the paper.
This portion should be the bulk of your post. Focus on the content of the reading, and think
about overall purpose, reasoning, and conclusions of the paper. Be logical and thoughtful in your
analysis. For example:
•
•
•
Are there any gaps in the knowledge or areas that may benefit from future growth?
Can these ideas be applied to different situations or scenarios? What are some challenges or
advantages?
What are the conservation implications of this reading? How does it address relevant and pressing
questions in conservation?
See the “Guide to Developing Critical Thinking Skills & Analysis” (in the “Resources for
Assignments” folder), to help you formulate your analysis and response.
Grading Rubric:
Each post worth: 20 pts
Key ideas (two)
10 pts
Critical analysis or further investigation
10 pts
Seven posts of 20 pts each = 140 pts total
1
