The size of the home range is examined in mammals. It is determined, mainly, by the amount of energy expended by the species, and, therefore, the home range area may vary according to the direct and indirect influences of weather and climate on the animal. But the kind of food that is utilized will also influence home range size. Those species that must hunt for their food need larger areas for food gathering than those species that feed on the vegetation. As a result the largest hunters appear to have their food habits regulated by considerations of the efficient use of the food materials in their home range. Finally, the home range size affects population density, which in turn influences the behavior in the population.