The ecology of female social behavior in horses, zebras, and asses

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Societies are the outcome of the reproductive strivings of their members, their interactions and their relationships. The precise nature of social relationships is shaped by features of the physical and social environment. Knowledge of the resources available to, and resource needs of, society members should contribute to an understanding of the form of sociality exhibited.

This chapter examines the female social behaviour of four equid species (horse, Plains Zebra, Grevy's Zebra, and Onager) which, despite biological similarities, differ in their types of sociality. Female horses and Plains Zebras form long-term bonds within stable groups accompanied by a single male; female Grevy's Zebras and Onagers change associates frequently and form unstable groups with brief association with each of several territorial males. The differences relate to the former two species occurring in more mesic habitats, the latter in arid climates.

Females of all the species compete little while foraging. Rates of agonistic interactions, bite rate, and time spent feeding are generally unaffected by group size. By living in groups females reduce the amount of male harassment that they experience, without sacrificing their feeding time or freedom of movement. Horse females with dominant males gain up to 6 minutes per hour advantage in feeding time, which consequently enhances their reproductive success. Mares may change group from one with a high-to one with a lower-ranking male, but only when the latter has recently improved his rank, demonstrating his long-term potential for rank-holding.

Not all equid females live in persistent groups, because intensity of female competition increases as resources become patchy, or because the physiological needs of females sometimes diverge. Females in early lactation need to drink more than others, and stay nearer to water to do so. Thus groups of dissimilar females cannot persist.

Social flexibility is common in equids. Being species limited by time available for foraging, their females associate to gain foraging time; even their choice of male may be made for that reason. Fission-fusion society is a mere variant upon the theme of a society based on female association.