Feature: MSU's Mara Hyena Project
Females dominate males in spotted hyena society, thus providing two MSU scientists with a fascinating glimpse of the biology of aggression.
As we drove slowly over the crest of Jackal Hill, we saw our hyenas chasing a zebra on the plain below. We immediately floored the Landrover to race downhill towards the chase. We grabbed tapedecks out of the glove compartment and began dictating our observations of the hunt without taking our eyes off the scene. Seven hyenas had separated a zebra mare from her stallion and the rest of his harem, and now the hyenas were quickly closing in on the mare. As we pulled parallel with the chase, the leading hyena bit into the mare's hind leg, and she stumbled and fell. Instantly the other six hyenas were upon her, tearing off large chunks of flesh with their powerful jaws. The mare was very heavy with a near-term pregnancy, and the burden of carrying the unborn foal had apparently made her vulnerable to such opportunistic and efficient predators as these spotted hyenas.
Contrary to the image of skulking carrion- eaters generated by popular myth and legend, spotted hyenas are actually extraordinarily good hunters, and over 80 percent of their diet consists of medium to large-sized ungulates they bring down themselves. Now we saw new hyenas racing over the plain toward the kill site. Within two minutes of the mare having fallen to earth, 18 hyenas were voraciously feeding on her carcass, and others were sill appearing on the horizon. Their rapid feeding was interrupted only by threats, sharp nips at each other, squeals of pain from the attacked hyenas, and of course the giggle-like appeasement vocalizations for which these 'laughing' hyenas are well known.
In addition to our dictated notes, by now we also had a videocamera mounted on the vehicle's window, and we used this to record feeding and aggressive interactions on videotape for subsequent slow-motion analysis in the lab.
In contrast to the relatively peaceful feeding of a lion pride on a kill, the hyenas fought constantly as they fed, with each adult hyena swallowing over two pounds of meat per minute for up to 17 minutes. Whereas all the adult females in a lion pride are members of the same family, spotted hyena societies typically contain several different families, and the feeding competition between lineages in intense. Furthermore, male lions are much larger and stronger than are females, so even though it is the females who bring down most prey, females defer to males during feeding competitions.
Among adult spotted hyenas, males and females are equally good hunters. However, females are approximately 10 percent larger than males, females are the more aggressive sex, and they are also socially dominant to adult males, so they can easily displace males from food at kills. Finally, whereas all female lions in a pride have equal access to food at kills, food access among female hyenas is strictly determined by social rank. Spotted hyena society is rigidly structured by a linear dominance hierarchy, or 'pecking order.' The highest-ranking female can displace any other hyena in the society from food until she is full, then the second-ranking female can to the same, and so forth down the hierarchical line. Since females dominate males, the lowest-ranking male has the poorest access to food of anyone in the entire society.
By half an hour after the zebra mare fell to earth, the carcass had been reduced to a skinless-skull with some vertebrae attached. All the meat had been eaten and other hyenas had run off with the separated limbs. Now the highest-ranking hyena still at the scene picked up the remaining bone assemblage, and loped off with it to the south. All that remained on the plain to suggest a live zebra had ever been present there was a small patch of blood on the grass.
We drove slowly back to our camp, which consists of five tents set up beside a river within the home range of our hyena study population. Spotted hyena social groups are called 'clans,' and we work with one large clan that lives within the Masai Mara National Reserve, in southwest Kenya. We know every individual in our study clan by its unique spots.
In addition to our tapedecks and cameras, our main research tools are binoculars and a photograph album containing left and right side 'portraits' of hyena spot patterns. We know the ages, sexes and genealogies of most hyenas in the clan, and for all individuals less that six years old, we also know their entire social history.
We began our study of the behavioral biology of the spotted hyena in early 1988, with funding from the National Science Foundation. Our original purpose was to find out how females come to dominate males in this species, with particular attention to the development of sex differences in aggression. Since males can physically dominate females in most species of mammals, including Homo sapiens, we intentionally selected for our research an animal in which sex roles were reversed from mammalian norms. Using this 'role-reversed' animal model, we were able to test a suite of new hypotheses about the biological substrates of masculine and feminine behavior patterns.
We began our study by spending thousands of hours sitting in our bush vehicles, watching the social interactions of young cubs at the clan's communal den. We watched every cub for at least an hour during every two-week interval of its life, in order to document developmental changes in behavior and rank relationships. Although we expected to see clear-cut female dominance over males even among very young cubs, to our surprise we found that infant hyenas showed no sex differences in their aggressive behavior or dominance interactions. In fact, we found that males were just as likely to dominate females as vice versa as long as they remained in their natal clan.
Spotted hyenas can live to the age of 18 or 20 in the wild. Female hyenas spend their entire lives in the clan of their birth. By contrast, when they are between two and five years old, all male hyenas disperse from their natal clan and join a new clan. This separation of males from their mothers and sisters probably functions to minimize the deleterious consequences of inbreeding. We found that it is only when males leave their natal clan that they start to behave submissively to all new hyenas they encounter. This peculiar tendency to revert to a cub-like pattern of appeasing all new hyenas they meet may be a necessary strategy for males to be accepted into a new clan, where they will eventually mate and pass on their own genes to future generations.
In recent years, we have continued to study this fascinating animal in hopes of answering a variety of new questions about their behavioral biology, their reproductive physiology, and their role in African ecosystems.
Since we joined the MSU faculty we have begun taking our undergraduate and graduate students with us to Africa, and allowing them to participate in our research there. Spotted hyenas are by far the most abundant large carnivores on the African continent, outnumbering lions by at least four to one. Given their abundance, size, appetite, and skill as social hunters, spotted hyenas are undoubtedly the most significant terrestrial predators on earth, in terms of tonnage of prey consumed. They occur in virtually all terrestrial ecosystems in Sub-Saharan Africa except in the wettest swamps and jungles. In most of these ecosystems, they appear to play the role of 'keystone' predators. Keystone species are those influencing the structure and function of an ecological community in some essential way. When spotted hyenas are removed from African ecosystems, biodiversity of prey herbivores decreases, and this in turn reduces floral biodiversity.