Food Density and Territory Size: An Alternative Model and a Test on the Reef Fish Eupomacentrus leucostictus

Abstract

Food maximizers are animals that can use all the food they can obtain, thereby increasing fitness. Such animals, when territorial, should not necessarily be expected to contract their territories when or where food is more abundant. The optimal territory is not simply the smallest territory that fulfills the immediate metabolic needs of the territory holder. Instead, the optimal territory size is that at which the per-area benefit of increased food most greatly exceeds the per-area (or per-perimeter) costs of travel time, territorial defense, and increased exposure to predators. A mathematical exploration of this principle indicates that when or where food is more abundant, insectivorous birds should contract their territories, female lizards and harvester ants may contract or expand their territories, and female fish should expand their territories. In contrast to female fish, the biology of male fish indicates that they are not food maximizers and therefore should not expand their territories when or where food is more abundant. Territories of 22 individuals of the West Indian reef fish Eupomacentrus leucostictus were mapped before and after artificial augmentation of food abundance. The two sexes responded differently to the food augmentation. As predicted, all of the seven females tested expanded their territories; only one of 15 males expanded its territory. Empirical data from other food-maximizer animals are also consistent with the food-maximizer model.