Abstract: Animal's behaviour can be reiewed as having eosts and benefits, and animals should be designed by natural selection to maximize net benefit. This economic idea can be used as a basis for formulating optimality models in behavioural ecology. When shore crabs are given a choice of different sized mussels, they prefer the size which gives them the highest rate of energy return. In order to calculate exactly how many different sizes should be eaten, it's need to develop an optimality model. Territorial behaviour should be favoured by selection whenever the benefits are greater than the costs. Optimal territorial size can be also predicted by building an economic model. The basic principle that animals minimize the risk of starvation can be applied to foraging decisions. Downy woodpeckers can use the gleaned information while foraging in order to maximize their food-intaking rate.

Key words: Analysis of costs and benefits, Optimality models, Risk of starvation, Sampling behaviour