Assembling a Diet from Different Places

Resources are unequally distributed over the landscapes and it is only seldom that food of a herbivore at a given spot exactly matches its requirements. However, because non-sessile animals can move, they can assemble a diet from different patches that, in its total, does meet the intake requirements. Because herbivores of different sizes have different requirements for energy and nutrients, a linearprogramming model that takes into account the different satisficing requirements of herbivores of a range of body masses (or of reproductive status) yields new insights into the causality of the differential way that these animals use the same landscape. Depending on landscape configuration and extent, and especially grain size of the distribution of resources, our model predicts that lactating females are much more constrained than other animals of the same species vis-à-vis the array of patches in the landscape. We also predict that small ruminants should be much rarer than large ruminants, and conclude that small ruminants can only survive under most circumstances if they are specialised feeders or if they live in a fine-grained landscape. We further conclude that natural selection favours ruminants with a large body mass to those with a small body mass if nutrient acquirement is the dominant selection force.