Variation between individuals and the consequences for diet selection by groups of animals

We describe a model to predict the diet selection of a population of animal s, based on simple assumptions about the characteristics of the individuals in a population, including the variation between them. Individuals are cha racterized by three parameters with biological relevance; a nutrient (prote in) requirement, an ability to discriminate between foods of different prot ein contents and a need to collect information about both foods. Each anima l selects perfectly a diet that avoids both a deficiency and an excess of p rotein, where this is possible. To construct the population two further ass umptions are made. The first is that the values of each parameter are drawn from uncorrelated normal distributions subject to the values being logical ly possible. The second is that, for different mean values for the populati on, the standard deviation is directly proportional to the mean so that the coefficient of variation is independent of the mean. The model was used to predict the outcomes of six hypothetical experiments, using 100 individual s on each treatment, where the values of the three parameters were systemat ically varied. In the experiments one food was always of low protein conten t while the protein content of the other was the treatment variable. The qu antitative effects of varying either the mean value of the parameters, or t heir variation, on both the mean composition of the diet selected, and on i ts variation, were not possible to predict without using the model. The sha pe of the population response was different to that for any individual. Ext ensions to the model may be able to increase its relevance to practical iss ues of diet selection. (C) 2000 The Association for the Study of Animal Beh aviour.