Influence of a predator on the optimal foraging behaviour of sticklebacks (Gasterosteus aculeatus L.)

Abstract

ACCORDING to the principle of natural selection, each individual animal is assumed to maximise its inclusive fitness¹. Thus, observed behaviour patterns should result from optimisation processes involving costs and benefits measured in a currency of fitness^2,3. Certain foraging strategies have been shown to maximise energy intake per unit time^3–6. Maximisation of the rate of energy intake, however, is an optimal strategy only if feeding behaviour does not conflict with other needs, such as the detection of predators. If the foraging animal runs a high risk of being preyed upon, the optimal strategy may be a compromise of both needs. A predator's influence on optimal foraging has been dealt with, so far, only in a hypothetical manner^3,5. We have investigated this problem experimentally using three-spined sticklebacks (Gasterosteus aculeatus) attacking a swarm of water fleas (Daphnia magna). We report here that after exposure to a model of an avian predator the sticklebacks' foraging behaviour changes such that they attack swarm regions of lower density which provide a lower feeding rate but should increase their ability to detect an approaching predator. This is predicted by a model using Pontryagin's principle of maximisation.