Academic journal article The American Midland Naturalist

Roosevelt Elk Density and Social Segregation: Foraging Behavior and Females Avoiding Larger Groups of Males

Academic journal article The American Midland Naturalist

Roosevelt Elk Density and Social Segregation: Foraging Behavior and Females Avoiding Larger Groups of Males

Article excerpt

ABSTRACT.-

Intersexual social segregation at small spatial scales is prevalent in ruminants that are sexually dimorphic in body size. Explaining social segregation, however, from hypotheses of how intersexual size differences affects the foraging process of males and females has had mixed results. We studied whether body size influences on forage behavior, intersexual social incompatibility or both might influence social segregation in a population of Roosevelt elk (Cervus elaphus roosevelti) that declined 40% over 5 y. Most males and females in the population occurred in the same forage patches, meadows, but occupied different parts of meadows and most groups were overwhelming comprised of one sex. The extent of segregation varied slightly with changing elk density. Cropping rate, our surrogate of forage ingestion, of males in mixed-sex groups differed from males in male-only groups at high, but not low, elk density. In a prior study of intersexual social interactions it was shown that females avoided groups containing ≥6 males. Therefore, we predicted that females should avoid parts of meadows where groups of males ≥6 were prevalent. Across the 5 y of study this prediction held because ≤5% of all females were found in parts of meadows where median aggregation sizes of males were ≥6. Social segregation was coupled to body size influences on forage ingestion at high density and social incompatibility was coupled to social segregation regardless of elk density.

INTRODUCTION

Social segregation is widely reported outside the mating season in ruminants that are sexually dimorphic in body size (Mysterud, 2000; Ruckstuhl and Neuhaus, 2002). Social segregation refers to adult male and female ruminants of the same species that rarely interact and occur in groups that are either exclusively or overwhelmingly comprised of one sex (Conradt, 1999; Weckerly et al., 2001; Bon et al., 2001). An intriguing feature about social segregation is when groups of adult males occur in the same habitat with groups of adult females, often within hundreds of meters of one another (Glutton-Brock et al., 1987; McCullough et al., 1989; Ruckstuhl, 1998; Weckerly, 1998; Conradt, 1999; Bon et al., 2001).

Numerous hypotheses for why sexually dimorphic ungulates segregate have been proposed, but prevailing explanations of social segregation are founded in intersexual body size differences and social behavior. Two hypotheses are based on sexual size dimorphism: the body size-forage acquisition hypothesis, often called the body size hypothesis; and the body size-activity budget hypothesis (Main et al., 1996; Ruckstuhl and Neuhaus, 2002). Both of these hypotheses assume digestive constraints imposed by an allometric relationship between body size and metabolic rate and an isometric relationship between body size and gut capacity (Bell, 1971; Jarman, 1974; Demment and Van Soest, 1985; Barboza and Bowyer, 2000).

The body size-forage acquisition hypothesis predicts that females, with higher massspecific energy requirements, are forced to sequester high quality forage and those males, with higher absolute metabolic requirements, are more restricted by forage quantity than females (McCullough, 1979; Glutton-Brock et al., 1982; Beier, 1987). Support for the body size-forage acquisition hypothesis has been mixed. Longer retention of digesta by males has been demonstrated in a controlled lab study. However, intake rates and the degree to which lower quality forage was digested by females did not follow expectations based solely on body size. The observed disparity was attributed to adjustments in rumination behavior and gut fill due to energetic demands (Gross et αι., 1995, 1996). Glutton-Brock et al. (1982), Beier (1987), Ginnett and Demment (1997), Post et α?. (2001) and Weckerly et α?. (2001) concluded that patterns of intersexual diet quality, or forage intake, agreed with predictions of the body size-forage acquisition hypothesis. …

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