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 animals, based on simple assumptions about the characteristics of the individuals in a population, including the variation between them. Individuals are characterized by three parameters with biological relevance; a nutrient (protein) requirement, an ability to discriminate between foods of different protein contents and a need to collect information about both foods. Each animal selects perfectly a diet that avoids both a deficiency and an excess of protein, where this is possible. To construct the population two further assumptions are made. The first is that the values of each parameter are drawn from uncorrelated normal distributions subject to the values being logically possible. The second is that, for different mean values for the population, 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 individuals on each treatment, where the values of the three parameters were systematically varied. In the experiments one food was always of low protein content while the protein content of the other was the treatment variable. The quantitative effects of varying either the mean value of the parameters, or their variation, on both the mean composition of the diet selected, and on its variation, were not possible to predict without using the model. The shape of the population response was different to that for any individual. Extensions to the model may be able to increase its relevance to practical issues of diet selection. Copyright 2000 The Association for the Study of Animal Behaviour.