Sexual Segregation in Alaskan Moose

Miquelle, Dale; Peek, James; Van Ballenberghe, Victor

Sexual segregation patterns of Alaskan moose (Alces alces gigas) were documented in Denali National Park and Preserve (DNPP). Data on group composition, spatial distribution, habitat selection, social behavior, activity patterns, foraging ecology, and forage quality and quantity were collected to assess 9 hypotheses that have been proposed to explain sexual segregation in ungulates. Predictions for each hypothesis were compared to empirical observations of 4 sex-age classes (small males, large males, adult females without calves, adult females with calves). A factorial energy budget model was developed as an aid to assessing the importance of size dimorphism and variation in allocation of energy for reproduction. There were clear differences in spatial distribution of females with calves versus other sex-age classes in summer. Segregation in early summer appeared to be due to dimorphic predator avoidance strategies: female moose with calves remained solitary and preferred forested habitats whereas males selected habitats with high forage biomass. Solitary behavior of females and sexual segregation diminished through the summer as females lost calves. Aggregations of males in summer did not appear to be associated with assessment of dominance. Segregation was greatest in winter, but varied between the 2 winters of observation. Spatial segregation in the 1985 winter resulted from changes in spatial distribution of both sexes, but in 1986 it was due primarily to movements by males out of areas used by females. Male moose did not employ a female mimicry strategy in the postrut or winter seasons. There was no evidence that habitat selection by large males was dependent on availability of thermal cover. Rates of social interaction of males were low when in mixed groups in winter, suggesting that sociality and association with females imposed little cost to males. Segregation due to competition for resources appeared unlikely: the hypothesized differential competitive ability based on allometric relationships between body size and bite size was not supported for the data for moose, and availability of forage was relatively high, suggesting that competition for a limited resource was not occurring. Males may have segregated to lower the risk of predation, but patterns in spatial distribution, activity budgets, and foraging ecology appeared to be primarily the result of ecological and body size dimorphism. Results of the energy budget simulation model suggested that large males incurred greater energy costs than small males or females, due primarily to their greater size and lower fat reserves. Due to a greater energy deficit, large males faced a greater risk of starvation before the end of winter. During the severe winter in 1985, large males appeared to reduce energy expenditure by reducing activity levels, decreasing travel distances, increasing bite size, and selecting areas of high forage biomass. The seasonal pattern of energy expenditure associated with the life history strategy of mature male moose likely puts them at greater risk of winter mortality than other sex-age classes, which may partially explain the disparate sex ratio in DNPP. A generalized model to explain sexual segregation patterns based on ecological and body size dimorphism is presented.

Sexual Segregation in Alaskan Moose

Series