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1 April 1995

Do feasts favour female hyenas?

SPOTTED hyenas can drastically alter the sex ratio of their young from one litter to the next, say two American biologists. Kay Holekamp and Laura Smale of Michigan State University in East Lansing have studied a clan of hyenas in Kenya. For many years, the hyenas have produced more male offspring than females. But in 1989, when a neighbouring clan’s territory suddenly became vacant, the hyenas began to produce an excess of females.

Holekamp and Smale say that the ability of spotted hyenas to manipulate the sex ratio of their offspring makes evolutionary sense. The key, they believe, lies in the differing behaviour of male and female hyenas. Males leave their clan as soon as they grow up, and range far and wide before settling down with another clan. Females, on the other hand, hang around at home to help their relatives find food. Under crowded conditions, when food is scarce, it makes sense to produce sons whose wanderlust will take them in search of better prospects elsewhere. But in times of plenty, say the researchers, it is best to favour daughters who will stay put and help their mothers take full advantage of the agreeable local conditions.

The researchers got an unexpected opportunity to see how their hyenas would respond to changing local circumstances when Masai tribespeople poisoned a neighbouring hyena clan to protect their cattle. Swiftly, one-third of the clan moved over to the newly vacated territory. Those remaining reacted by shifting the sex ratio of their young. In the two years before the clan divided, its members had produced 28 males and 20 females. But in the three years following the split, female offspring outnumbered males two to one. The shift was most marked among the offspring of the most dominant females American Naturalist, vol 145, p 261). “It’s a remarkable finding,” says Laurence Frank, of the University of California, Berkeley, who has studied the same hyena population.

Frank suggests that hyenas manipulate the sex ratio of their offspring by altering their behaviour. Because hyenas raise their offspring in concealed dens, it is impossible in the wild to see the young until they are several months old. But when litters are observed in captivity, some of the females kill their sisters. Before the clan split, Frank suggests, young females were systematically killing one another. After the split, the adult female hyenas may have stepped in to stop these murderous contests.

Smale and Holekamp, meanwhile, think that the hyenas’ changed living conditions may have somehow altered their physiology. In other mammals, the ratio of male to female births is influenced by a number of physiological factors. The viscosity of vaginal fluid, for instance, affects the chances of a male-determining or female-determining sperm fertilising an egg. Other experiments have shown that levels of hormones circulating in a female’s bloodstream can make male or female fetuses more prone to miscarriage.