Why do sexes of polygynous ruminants segregate spatially outside the mating season? Existing hypotheses for differences in niche partitioning among species are not sufficient to explain temporal patterns of segregation and aggregation between sexes. Moreover, other hypotheses, including risk of predation, do not explain why females of some species inhabit sites with higher-quality forage while segregated from males, although competitive exclusion of males by females has been proposed. We offer a new hypothesis to explain this conundrum in sexually dimorphic deer (Cervidae) based on an allometric model of metabolic requirements, minimal food quality, and digestive retention. The model predicts that male deer consume abundant forages high in fiber because ruminal capacity prolongs retention and permits greater use of fiber for energy than in nonpregnant females. Low density of animals, high abundance of food, and adaptations of ruminal microflora keep large males on fibrous forages until quantity of food declines. Compared with males, smaller-bodied females are better suited to postruminal digestion of food, especially when intakes increase concomitantly with requirements for energy and protein during reproduction. High demands for absorption of nutrients during lactation and growth stimulate investment in intestinal and hepatic tissue in females, increasing the cost of maintenance and reinforcing differential use of habitats and forages when sexes are segregated. This new model explains sexual segregation without invoking predation or competitive exclusion of males by females.
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