干旱区生态环境知识资源中心

Attempts to account for the diversity of breeding systems in ungulates have concentrated on ruminants (Jarman, 1974), and non-ruminant species, like equids, were largely neglected. Equids are basically known to have two types of breeding systems correlated with two different types of resource distribution: Species living in mesic environments like mountain zebras, Equus zebra, (Penzhorn, 1984) and plain zebras, Equus burchelli, (Klingel, 1967) or feral horses (Berger, 1986) all form small and permanent family groups, usually consisting of one male, one to two females and their offspring. Sometimes, two breeding stallions form coalitions (Berger, 1986), and small all-male groups occur in all three species. On the contrary, in species living in xeric and therefore unpredictable environments like African wild asses, Equus asinus, (Moehlman, 1974) and Grevy’s zebra, Equus grevyi, (Ginsberg, 1987), dominant males are territorial and bonds exist only between females and. recent offspring, probably to minimize group-size while foraging. According to this classification based on resource distribution, Asian wild asses, Equus hemionus, ssp. should adopt the second type of social system. However, not all of them do so. Why is this not the case?

Ressource distribution might not be the only determinant of breeding systems in equids. Apart from phylogenetic inertia mentioned for mountain and Grevy’s zebras (Berger, 1988), predators might also play an important role. The Indian wild asses, Equus hemionus khur, (Shah, pers. comm.) living in xeric environments (with small and solitary hunting predators present, Canis lupus pallipes), typically adopt type 2 breeding system. Bannikov’s (1958) descriptive study already suggested that Mongolian kulans, Equus hemionus luteus, living in a similar desert-environment (together with large and cooperatively hunting predators, Canis lupus chance), might adopt type 1 breeding system. Our own observations on group-composition of kulans carried out in the Gobi National Park, during the breeding season and in winter, support this view. Not only were kulans found to be non-territorial and to live in family- or all-male groups like horses, plain and mountain zebras, but they doubled their group size in winter, at a time when food was more dispersed but predator pressure high. This contrast suggests that understanding differences in grouping patterns between subspecies of wild asses may help to shed light on equid social systems in general.