Sociality and reproductive skew in horses and zebras

Abstract

The outcome of competition for resources or mates often leads to individual differences in reproductive success. In populations of equids, such as those of horses and zebras, skewed distributions of reproduction emerge because a limited number of individuals achieve disproportionate gains. For both sexes, skew results fromdifferences in rank, age, and degree of social stability, although skew is generally greater formales than for females. Adultmale horses and zebras typically establish “harem” groups by bonding with a number of mature females. Although the number of females that dominants bond with can be quite variable, potentially high levels of skew are rarely reached because subordinate males adopt alternative mating tactics that exact concessions from partners, whether they are dominant stallions or other subordinates. Successful breeding females also rely on support from subordinates to minimize feeding competition by keeping group size small, and this, too, reduces skew among females. The conflict of interest between the sexes arising over differences in optimal group size, along with the tendency for females to leave groups when sexually harassed, induces, but limits, the aggression that males direct towards females. Thus female concessions can shape both female and male levels of skew, but they also can be modulated bymale behavior. Thus it appears that for equids the level of skewthat emerges depends on key phenotypic traits and how their distribution among individuals constrains reproduction, as well as on how social relationships within and between the sexes affect the ability of a small number of individuals to monopolize resources involved in reproduction. Inequality is a pervasive feature of virtually all societies, as is the attempt by the disadvantaged to attenuate it.

Publication Title

Reproductive Skew in Vertebrates: Proximate and Ultimate Causes

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