Territory Area as a Determinant of Mating Systems

Abstract

Territoriality is an integral part of breeding behavior in many animals. Because the reproductive success of territorial males is often limited by access to females, breeding males should behave as “area maximizers” when the basis of female choice is either the abundance of resources within the territory or territory area per se. Being reproductively more limited by energy, territorial females should be “energy maximizers.” A series of simple analytical models of territory area for such foragers is developed to explore how changes in local food production and/or local competitor density affect both the probability of a territorial male securing more than one mate (polygyny) and the probability of his and his mates' reproductive success increasing. Two cases are modeled (only males territorial vs. both sexes territorial), each for various sets of assumptions regarding interactions between food production, feeding efficiency, and competitor density. Concurrent responses in territory area, territory food reserves, net energy gain, and time budgeting provide testable sets of predictions for each scenario. Where only males are territorial (Case I), changes in food production can have different (indeed, opposite) effects upon an individual male's probability of becoming polygynous, depending upon whether the basis of female choice is the abundance of food within the territory or another factor positively correlated with territory area. Increases in competitor density usually decrease the probability of polygyny regardless of the basis of female choice. Where both sexes are territorial and territories overlap intersexually (Case II), the mating system becomes a function of the number of female territories within each male's territory, which varies with the ratio of male to female territory areas. In this case, the probability of polygyny occurring will increase if food production for both sexes increases without concurrent increases in competitor density, and will decrease if competitor density for both sexes increases without concurrent increases in feeding efficiency. Few data are presently available to test either these general predictions or numerous sets of secondary predictions tabulated in the text. Available evidence is largely consistent with the models, but mostly circumstantial. This is because the predictions of these and other models of territory area are strongly assumption dependent, and few published studies have investigated these assumptions. These analyses demonstrate that to accurately assess the mechanisms by which environmental factors affect territory area, and thus mating systems, tests of the underlying assumptions of models are essential.