The Gnu's World: Serengeti Wildebeest Ecology and Life History

7. Social Organization: Comparison of Migratory and Resident Populations

143 While serving as resident naturalist at a Masai Mara resort some years ago, I was often asked to explain the erratic, “stupid” behavior of wildebeest at crossing points. They can behave rather mindlessly, I have to admit. Even with no obvious deterrent, like basking crocs (often ignored and nearly stepped on, actually) or a line of cars, a file or column will walk down to the water’s edge, look ready to plunge in, only to turn suddenly and gallop back to the plain. When zebras are present, either leading or following a column of wildebeest, they are more likely to keep going. My answer: the assembled wildebeest are a crowd of strangers. Except for mothers accompanied by dependent offspring, the leaders— all bulls as a rule—are anonymous. Arriving at the water’s edge, those in front come to a halt. No one is prepared to take the plunge. “Why me?,” each says in effect. “Why don’t you go first?” Until some brave or foolhardy soul finally jumps in, there are likely to be repeated false starts. Zebras, on the other hand, are organized in cohesive family and bachelor units. When a herd comes to a crossing, the members stick together and provide mutual support. It’s as simple as that. How do I know the leading wildebeest are all bulls? By taking note of the leaders every chance I get. Whether filing over the plain, going to water, in traffic jams at river crossings and passageways through a belt of woodland, bulls are in the forefront (fig. 7.1). How come? Because males are the impetuous sex. They have no responsibilities toward other Chapter 7 Social Organization Comparison of Migratory and Resident Populations 144 | Chapter 7 wildebeest. They’re testosterone-driven to compete with other males to find and breed females in estrus. During migratory movements, they largely suspend competitive activities (except during the mating peak; see chapter 10) but only until the next stop. As soon as an aggregation spreads out and begins to forage, bulls stake out temporary territories wherein they try to detain females. In short, bulls are the most impatient and controlling class of wildebeest. Being in the vanguard, especially while simultaneously migrating and rutting, may also be an advantage. Leaders intercept more passing cows than bulls farther down the line. Females have been selected over the millennia to be careful because survival of their calves depends on maternal care and guidance. Poor mothers leave few or no descendants. So, naturally, cows are more timid and generally more alert to danger than bulls. The inherent differences in risk-taking by male and female wildebeest remind me of gender differences in our species. Take women and men drivers, for instance. It is common knowledge (reflected in insurance rates for teens) that men are more aggressive drivers than women. Women, the primary caregivers, are naturally more cautious. As both species share the same androgens and estrogens, it is not far-fetched to suppose that testosterone affects male behavior in comparable ways. figure 7.1. A moving column of wildebeest with bulls in the vanguard, as usual. Social Organization | 145 Bull leadership of movements is not inevitable. At a river crossing, sometimes a mother looking for a lost calf or a lost calf looking for its mother will take the plunge and the rest will follow like sheep. Yes, just like sheep. Wildebeest are just as sociable as the domesticated sheep. It’s instinctive, manifest at birth. The same “sociability genes” draw gregarious species to their own kind like iron filings to a magnet. Is there perhaps an actual, dedicated set of genes that predisposes species to be gregarious? I’m unaware that any such genes have been identified in any species genome, but quite conceivably herding species have inherited the same gene complex, that is, share the same genetic tool box. birds of a feather . . . From what I’ve said about the members of a mob of wildebeest being strangers, except for mothers with their offspring, it may seem contradictory to be told that subgroups based on sex, age, and reproductive status are characteristic of wildebeest aggregations. This becomes apparent when you drive through a concentration while sexing and aging the animals within 50–100 m of the vehicle. Although individuals may not be personally acquainted, there is obviously a mutual attraction between animals of the same sex and age. The first...