Zebra zones

Natural History, March, 1998 by Mace A. Hack, Daniel I. Rubenstein

Of the many species of equids (horses, asses, and zebras) that dominated the world's grasslands for much of the last eight million years, only seven are left, three of them zebras. All three zebra species live on the savannas of eastern and southern Africa, all feed exclusively on grass, and all have the stripes familiar to even the youngest child just learning that "Z is for zebra." Yet just as a closer look reveals differences in stripe patterns among the species, insights gained from years of fieldwork have uncovered two very different types of zebra society, one of which is unusual for mammals.

Of the three species, the plains zebra (Equus burchelli) is the most abundant and widespread, inhabiting tropical grasslands from the vast, acacia-dotted savannas of East Africa - where we have studied it for the past ten years - down through the scrubby woodlands of the Zambezian region to the rolling, treeless veld of South Africa. The mountain zebra (E. zebra), preferring a more temperate climate, lives in the broken, mountainous regions of Namibia and southernmost South Africa, while Grevy's zebra (E. grevyi) ranges over the stony, thin grasslands of Ethiopia and northern Kenya, where it sometimes overlaps with the plains zebra.

Both plains and mountain zebras live in year-round breeding groups of one to several adult mares, their recent offspring, and a single adult male - the stallion - that defends his exclusive mating rights to the females. These groups can be visualized as a wagon wheel, with the stallion at the wheel's hub and the mares occupying the rim. The spokes of the wheel are the strong bonds between each mare and the stallion. The females form long-lasting social bonds with one another as well, adding strength to the wheel's rim.

Females of many other social mammal species, such as lions and elephants, also form bonds, but they do so almost exclusively with close kin. What makes plains and mountain zebra society unusual is that the bonds are between unrelated individuals. These zebras belong to a small set of species (which includes, among others, the mountain gorilla, the hamadryas baboon, and the greater spearnose bat) in which not only male but also female offspring ultimately leave the group in which they were born; consequently, the adult females within a group are never closely related. (The evolutionary impetus for the dispersal of young females may be avoidance of inbreeding. Since a stallion may retain a harem for many years, any daughters that stayed at home would sooner or later wind up mating with their father. Our observations in Tanzania's Ngorongoro Crater lend support to this explanation: young females there move even farther away from the group of their birth than do young males.)

Grevy's zebras are less social, with mares and their most recent offspring the only stable social unit. Although one sometimes sees a number of mares and their young feeding together, such groupings typically break apart within hours or days. Stallions live alone, defending territories near water or rich patches of grass and mating with any sexually receptive females that come to feed or drink. Mares with newborn foals must remain near water because of the physiological demands of milk production and the inability of young foals to travel long distances. Consequently, they may stay on a single stallion's territory for several months. Once her foal is old enough to travel, however, a mare leaves the stallion's territory, often joining up with other lactating mothers.

The differences in zebra society can be explained by a crucial tenet of behavioral ecology: the distribution of females is driven primarily by the distribution of the resources they need in order to reproduce. In the semiarid savannas of Kenya's Buffalo Springs and Samburu national reserves, where we and our colleagues have studied Grevy's zebras on and off for the past fifteen years, the best grass grows in widely scattered patches, forcing females to range over large areas to find sufficient food. Competition for grass, together with the differing ability of lactating and nonlactating females to travel long distances, works against the development of strong social bonds and stable groups. Plains and mountain zebras generally inhabit lusher grasslands, where both food and safe places to drink are more abundant and more evenly distributed.

Male zebras, meanwhile, respond more to the distribution of females than to the distribution of resources. But this is seldom as simple as it seems. A Grevy's stallion, for example, could follow mares around one at a time, waiting for an opportunity to mate. But such a strategy would be very time-consuming and inefficient, since females are sexually receptive for just a short time after giving birth, gestation lasts thirteen months, and foaling is not strongly seasonal. The stallion is more likely to encounter receptive females if he finds a place with the food and water females need and then stays put. If he can keep rival males away from the immediate area, he raises his chances of being the only male to copulate with receptive females.