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Density Dependence and Life-History Strategies of Ungulates

Dale R. McCullough
DOI: http://dx.doi.org/10.2307/1383164 1130-1146 First published online: 6 December 1999

Abstract

Due to their large sizes and long life spans, ungulates typically exist in populations close to environmental carrying capacity (CC), and this has had a strong influence on evolution of life-history traits. I review models describing the relationship of ungulate populations to CC. The shape of the functional relationship of density dependence (DD) on population size (N) determines the response of N relative to CC and the tendency of N to come to equilibrium at CC. Environmental, and to a lesser extent demographic, stochasticity determine the realized relationship of N to CC in nature. It is difficult to distinguish DD signals from noise in environments with high stochasticity or with populations at or near CC. Life-history strategies of ungulates evolved in the context of DD and CC. Sexual selection has resulted in asymmetry of reproductive success between sexes and consequent evolution of dimorphism in body size with males being larger than females. Because males have larger bodies, greater absolute nutrient and energy costs, and higher rates of mortality than females, their reproductive fitness strategies tend to skew the population sex ratio towards females. Females are less costly to produce and maintain, so increased skew in the sex ratio of the population should, in theory, result in greater N (higher CC) relative to a given resource base. Sexual selection should lead to increasingly skewed sexual dimorphism and sex ratio up to some constraint set by the male's physical ability to sequester females or physiological limits to impregnate females. This suite of life-history traits in ungulates has led to resource partitioning between sexes and sexual segregation outside of the rut in most species. Female-female competition also complicates the relative value of sex of offspring to individual females, which varies with female age, size, and social position.

Key words
  • population models
  • density dependence
  • life histories
  • ungulates
  • sex ratios
  • sexual selection
  • sexual segregation
  • resource partitioning