Background

A foraging animal (one seeking food) faces a number of problems in finding and handling items of food.   These are general problems that apply to food as diverse as seeds, insects, or mice.   Consequently, ethologists often think of all of these items as "prey" for foraging animals.   How animals find and handle their food is clearly an important component of predator-prey (or herbivore-plant) relationships, both basic concerns of ecology, so it is not surprising that the study of foraging behavior is one of the principal subjects of study in the relatively new field of behavioral ecology.

A foraging animal must make several kinds of "decisions", such as where to forage and what items to take as food.   These choices might involve rational thought, but they do not have to.   They might result from "decisions" made by natural selection, in the sense that genes tend to persist in a population when they are associated with phenotypes that forage efficiently.

Both finding and handling prey take time (usually called search time and handling time).   Search time (the time to find an item of prey) and handling time (the time required to catch, prepare and eat an item once it is found) affect the profitability of prey.   Profitability is the energy obtained from an item divided by the handling time.   By choosing a diet that includes items with high profitability, an animal becomes an efficient forager (obtains as much energy as possible for a given amount of time spent foraging).

Search time also influences the choice of a diet.   An animal that maximizes the rate of energy intake from food must balance

the profitability of any item that it has found but not yet handled

against

the profitability of other possible kinds of food in the environment and the time it takes to find them.

Suppose two kinds of food (A and B) are available in a particular environment.   A decision to eat food A depends on the following relationship:

where e_A = the energy obtained from an item of A, h_A = the handling time for an item of A (so e_A / h_A = the profitability of A), e_B and h_B = the corresponding values for B, and s_B = the average time to find an item of B (the search time for B).

Whenever the above relationship applies to an animal, it should ignore all food of type A, because it would obtain more energy per unit of time by searching for and handling food B than by handling an item of food A once it is found.

Perceiving Prey

Search time obviously depends on how abundant the items are in the environment and how fast a searching animal moves.   It also depends on the probability that the animal will perceive these items.   How animals perceive their prey is thus a crucial issue in our understanding of foraging.

Three conditions might affect the perception of prey

1. conspicuousness
2. specific search images
3. rarity

(1) Prey that matches the background better are necessarily harder to detect or recognize.   Conspicuousness of prey, as a result of contrast with the background, is thus one condition that we can expect to influence the perception of prey.

(2) An animal's nervous system might "tune" itself to detect particular kinds of stimulation.   This process is often called forming a specific search image.   It amounts to "learning to see" specific kinds of stimulation.   Consequently, it depends on an animal's previous exposure to particular kinds of prey.   If an animal forms specific search images, types of prey encountered frequently in the past become easier to find in the future.   As a result, any type of prey becomes easier to find when it is more abundant in the environment, a situation with clear consequences for the ecology of predator-prey relationships.  

(3) There is also evidence that animals take rare prey more than expected by chance, at least when they have had no opportunity to form a specific search image.   This preference for rare prey might result from habituation to frequently encountered stimulation or from concentration on a unique target in an otherwise confusing herd or flock of prey (both physiological explanations).   In either case it might result in identifying abnormal and hence more profitable prey (an evolutionary explanation).

In this exercise, you will investigate some of these influences on perceiving prey.   The foragers in this case will be you -- and the prey will be jellybeans!   Yet the principles apply just as well to a bird hunting insects, a mouse hunting seeds, or a lion hunting antelope.

Procedures

For this exercise, we will use a grassy location outdoors.   A circle of rope will define the area in which you will search for prey (jellybeans).   Note that we are eliminating the problem of where to forage.   The object is to "catch" as many jellybeans as possible.

A group of three students will constitute the three essential features of a foraging animal:   the mouth, the stomach, and the anus.   The three students in a team cannot change roles during or between trials, unless your Teaching Assistant gives instructions otherwise.   Each team member has a different task, as follows:

"Mouth"

find a jellybean, pick it up (one at a time) with a spoon, carry it to the edge of the arena, and place it in a cup held by the Stomach

"Stomach"

carry the jellybean in the cup around the arena to the anus on the opposite side, then return for another jellybean from your Mouth.   The Stomach never enters the arena and must receive jellybeans from the Mouth on the opposite side of the arena from the Anus.

"Anus"

record the color of the jellybean and toss it back into the arena (anywhere in the arena, not necessarily the center).   The Anus cannot deposit the jellybean where any Mouth is currently searching.

Nobody touches a jellybean except with a spoon or a cup.

After several trials arranged by your Teaching Assistant, everyone will return to the lab to analyze and to discuss the results.

References

Dawkins, M.   1971.   Perceptual changes in chicks: another look at the 'search image' concept.   Animal Behaviour 19:   566-74.

Endler, J.A.   1991.   Interactions between predators and prey.   In: Behavioural Ecology, An Evolutionary Approach, 3rd edition (eds. J.R. Krebs and N.B. Davies). Blackwell Scientific Publications, Oxford.   Pp. 169-196.

Feltmate and Williams.   1989.   A test of crypsis and predator avoidance in the stonefly Paragnetina media (Plecoptera: Perlidae).   Animal Behaviour 37:   992-999.

Kono, H., Reid, P.J., and Kamil, A.C.   1998.   The effect of background cueing on prey detection.   Animal Behaviour 56:   963-972.

Pietrewicz, A.T., and Kamil, A.C.   1979.   Search image formation in the Blue Jay (Cyanocitta cristata).   Science 204: 1332-1333.