Chapter 7: Foraging Behaviour
7.1 Animals Find Food Using A Variety Of Sensory Modalities
1. Natural selection favours modalities that most efficiently and accurately provide
information about the location of food, but the efficiently of a modality can vary
with environmental conditions.
CATFISH TRACK THE WAKE OF THEIR PREY
1. Mechanoreceptors: sensory sensitive to changes in pressure.
2. Lateral line system: in fish, mechanoreceptors that provide hydrodynamic
3. Chemoreception: the process by which an animal detects chemical stimuli.
4. Olfaction: the detection of airborne chemical stimuli.
5. Gustation: The detection of dissolved chemicals, often within the mouth.
1. Some fish, such as minnows, have welldeveloped chemoreception systems
that allow them to recognize conspecific predation events and then respond
accordingly with antipredator behaviours. Other fish, such as bullhead
catfish, use the chemoreceptors on their barbelthe slender, whisker like
organs near the mouth to locate stationary food.
1. Pohlmann and colleagues investigated how nocturnal piscivorous (fish eating)
catfish fins food in the dark. The research team used guppies as prey fish because
they create a swimming wake that has been well described.
1. Pohlmann’s team aimed to determine whether catfish could use cues
provided only from the wake of individual fish to stalk and attack their prey.
2. The researchers designed an infrared video system to track the movement
path of both the predator and the prey in an environment of complete
3. They used four catfish and observed each one stalking up to ten individual
guppies. Each catfish was allowed to acclimate to the experimental tank for
one hour in the dark, after which the first guppy was added to the tank.
4. After a guppy was consumed, another was added every 20 mins to replace the
one that had been eaten. The researchers ended a trial when ten guppies had
been consumed or when no guppy had been consumed in 20 mins.
5. They used computer software to digitize the movement behaviour sequence of
both the catfish and the guppies and classified them in one of three ways.
1. Path following (the catfish followed the guppy wake)
2. Head one encounters (the catfish encountered the guppy headon without
a previous encounter)
3. Attack on a stationary guppy/
6. Found that 80% of the attacks occurred on moving guppies. In the majority of
all attacks, the catfish were following the same path as the guppy before the
attack. The researchers concluded that the catfish appeared to be following
the wake of the guppies to find and attack their prey in the dark. 7. Pohlmann’s team conducted a second experiment using the same setup. In this
experiment, they manipulated either the lateral line or the external gustation
system of the catfish and compared to their behaviour to that controls.
8. The lateral line provides hydrodynamic information and can be rendered
temporarily nonfunctional by immersing a fish in a CoCl 2cobalt chloride)
solution for six hours prior to the experiment.
9. The external gustation system, which provides chemical information, was
manipulated by surgically removing an area of the dorsal medulla oblongata
that controls this system.
10. The researchers found that individuals with a nonfunctioning lateral line
attacked guppies headon 88% of the time. Most of the gustationablated fish
also attacked guppies headon, though less often (58% of attacks); only 27%
of attacks were characterized as path following.
1. The capture success rates also differed across treatments. Control and
gustationabalted fish exhibited similar; relatively high capture success
11. Pohlmann and colleagues concluded that the lateral line is a very important
factor in catfish’s ability to track the wake of their prey; in contrast, the
external gustation system does not appear to be as important in facilitating
this mode of attack behaviour.
1. The lateral line system may be key not only because it provides feedback
to the predator about the movement of the prey but also
because it provides feedback to the predator about its
own movements and position.
GRAY MOUSE LEMURS USE MULTIPLE SENSES TO FIND FOOD
7.2 Visual Predators Find Cryptic Prey More Effectively By Learning A
1. Many predators, especially birds and mammals like the gray mouse lemur, rely
heavily on vision to find prey.
2. Evolutionary arm race: the backandfourth process of adaption in one species
favouring counter adaption in another.
CRYPTIC COLOURATION REDUCE PREDATOR EFFICIENCY IN TROUT BLUE JAYS USE A SEARCH IMAGE TO FIND PREY
3. Search image: the visual distinctive features of a single prey type that, once
learned, can enhance prey detection.
4. Pietrewicz and Kamil studied visual prey detection in blue jays, to determine
whether they use a search image to find cryptic prey.
1. They trained individual birds to search for images of moths that were
displayed as video images on tree trunks.
2. They designed a very clever apparatus to examine whether blue jays use
search images to find prey. They used two different species of barklike Catocala moths, which are active during the night but then rest during the day
on cryptic substrate.
3. To examine the jay’s use of search images, they created two treatments. In the
“run” treatment, the jays were shown eight negative images and eight
positive images of one of the moth species so that they could learn a search
image for that species. In the ”nonrun” treatment, the jays were shown a
randomly intermixed group of four positive images of one moth species, four
positive images of the other moth species, and eight negative images of no
moths. The researchers assumed that jays did not have an opportunity to learn
a search image for either species.
4. Over eight sessions, the percentage of correct responses increased in the run
condition but not in the nonrun condition. In fact, the increase occurred quite
rapidly. They concluded that jays were using a search image of the moth
species, which resulted in the increased success rate observed in the run
5. This test provided the first experimental evidence of animals using a search
image. It also showed that shortterm changes in the behaviour of a predator
can result in intense predation.
7.3 The Optimal Diet Model Predicts The Food Types An Animal Should
Include In Its Diet
5. Optimal Foraging Theory (OFT): an approach to studying feeding behaviour
that assumes that natural favour has favoured feeding behaviours that maximize
6. Optimal behaviour: the behaviour that maximizes fitness in an optimality model.
1. OFT models are widely used because they produce testable predictions and
can be applied to many species and behaviours. Two classic OFT models
examine diet and food patch use.
THE DIET MODEL
7. Handling Time: the amount of time to manipulate a food item so that it is ready
8. The optimal model is based on three assumptions:
1. Foragers maximize fitness by maximizing energy intake rate.
2. Food items are encountered one at a time and in proportion to their
3. All food items in the environment can be ranked according to their
9. Profitability: for a food item, the energy it contains divided by its handling time.
1. For example, consider a squirrel feeding on a sunflower seed and a walnut.
Sunflower seeds have soft shells and small handling times but also contain
relatively little food, or energy, compared to that found in a walnut. The
walnut has a hard seed coat, and thus a much longer handling time but
contains more food, or energy. The sunflower seed has higher profitability
and so is a higher ranked food item. A GRAPHICAL SOLUTION
10. Generalist: a forager that consumes a wide variety of items in its diet.
11. Specialist: a forager that has a narrow diet.
7.4 The Optimal PatchUse Model Predicts How Long A Forager Should
Exploit A Food Patch
12. Diminishing returns: a decline in instantaneous harvest rate as a food patch is
13. Marginal benefit: In foraging, the benefit obtained by feeding for one more
14. When a forager enters a patch with abundant food, the marginal benefit of
feeding high, because the instantaneous rate is high. Because of the diminishing
returns, the marginal benefit declines as the path becomes depleted. When the
patch contains little food, the marginal benefit of feeding is very low, because it
takes a long time to find the next food item.
THE OPTIMAL PATCH USE MODEL
15. Charnov created an OFT model to determine how long a forager should stay in a
food patch to maximize its fitness, the optimal patchuse model. Charnov’s model
is based on four assumptions:
1. Foragers attempt to maximize energy intake rate.
2. All patches are identical (contain the same kind and amount of food).
3. Travel time between patches is constant.
4. As a forager depletes a patch, its instantaneous harvest rate declinesthat is,
it experiences diminishing returns.
16. Marginal Value Theorem: an optimal foraging model that predicts how long an
individual should exploit a food patch. The average energy intake rate from the
environment is calculated by dividing the total energy acquired from all parches
by the total time to travel to and then exploit the patches (travel times plus patch
1. In Charnov’s original model, the travel time between patches is held constant,
and all patches are identical.
2. Initially, there is abundant food in a patch, so the cumulative gain curve
begins with a steep slope. However, the longer the time spent in a patch, the
less food is left (i.e. diminishing returns), and so the slope of the curves
becomes shallower as more time passes.
3. Next, we need to include the amount of time). ent traveling to a patch (T) in t
addition to time spent in the patch (T Thp energy intake rate can now be
calculated for any T : pt is energy accumulated, divided by the T plus t p.
1. We can solve for this relationship graphically, because the energy intake
rate for any T ps the slope of the line that runs from the start of the travel
time to the cumulative gain curve at each T The p. ch time that
maximizes energy intake rate is the TP where this line is tangent to (i.e.
where it just touches) the cumulate energy gain curve. 4. We can see that the model predicts the amount of time a forager should spend
in each patch for any fixed travel time. This number depends on the travel
time: as travel time increases, foragers should spend longer in each patch.
PATCH USE BY RUDDY DUCKS
17. Tome tested the predictions of the marginal value theorem with ruddy ducks.
Ruddy ducks feed on aquatic invertebrates and vegetarian on muddy lake
1. They created a large concrete and glass aquarium. The bottom of the
aquarium contained 16 wooden trays filled with sand, and the side had glass
windows for observation.
2. Each tray constituted a potential food patch: Tome buried wheat grains in
some of the trays during his experiments and trained birds to search the sand
for food. The model assumes that ducks would experience diminishing harvest
rates while feeding on wheat grains in the artificial food patches.
3. To test this assumption, Tome buried 150