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

10. The Amazing Migration and Rut of the Serengeti Wildebeest

213 The rut of the Serengeti wildebeest is like nothing else on earth. Some half a million cows are bred during three weeks of frenzied activity. Yet surprisingly little of the activity actually involves mating. Most of it involves herding, chasing, and fighting among the bulls that are currently holding territories. They herd and chase the cows and fight one another as they compete to collect and hold a group. Success depends not only on their efforts, but on the willingness of the cows to be detained. When there is a general movement, the bulls’ efforts are largely futile. They move back and forth in the passing parade in their distinctive rocking canter with heads high, calling continuously, but the horde keeps going (fig. 10.1). They’re like small boats breasting a flowing river. When an army of wildebeest numbering in the thousands stops moving , the bulls have the opportunity to round up and hold cows on their territories. The vanguard and also the tail end of the migration consists mainly of bulls. Those in the vanguard include bulls intent on staking out territories wherein they can detain passing females. Being out front enables them to meet most of the oncoming horde. How many they manage to keep, if any, depends on circumstances like the time of day, weather conditions, the location and amenities of the territory—good pasture, proximity to concealing vegetation, water, shade—and on the presence or absence of other cows. How does a lone bull acquire a herd? In the territorial network encompassing every aggregation, most bulls are alone on their property. Chapter 10 The Amazing Migration and Rut of the Serengeti Wildebeest 214 | Chapter 10 For example, of 513 territorial males I sampled during the 1980 Serengeti rut, only 116 had females on their territory = 1 out of 5. A bull canters toward an approaching cow, grunting a welcome as he tries to stop her inside his property. But blocking a cow’s path doesn’t work if she wants to move on, even if he chases her flat out. The chances of recruiting passersby improve if a group of cows and calves is already present. Having observed that cows and their accompanying offspring relay from herd to herd, despite the best efforts of the single territorial males to stop them, I eventually realized why most of the competition among territorial males is over having a herd. One needs a herd to attract passing cows, including, sooner or later, cows in estrus. You could almost liken the successful bull to a duck hunter deploying a spread of decoys to lure the desired quarry—a female in heat—to land in his spread. Aerial photos of the clustered groups typical of the rut carried out in 1963, 1965, and 1966 recorded an average of 28.4 head ± 19.8 head (standard error of the mean) in a sample of 136 groups totaling 3959 animals.1 The average of 4,032 animals in 353 Serengeti herds I counted during the 1997 rut was 11.4. The difference between 2,514 animals in 199 herds, 12.6, counted in late November 2006, was insignificant. In the Crater, during 1963–65 ruts, the average complement of 190 herds with territorial males was 17 (fig. 10.2). By comparison, herd size postrut and calving seasons of 1963–65 was 10.37 animals in 525 herds. The average number in 48 herds I counted during the 1964 Serengeti rut was 15.8, while the mean of 46 herds in the Ol Balbal in November figure 10.1. A bull cruising beside a moving aggregation in the trademark rocking canter. The Amazing Migration and Rut | 215 1964 was 18.7. The difference in the number of females and young in company with territorial bulls doesn’t look that substantial. What brings the average herd size down is the presence of many small groups: 1–3, not counting the bull. The chances of collecting and holding a herd depend importantly on the time of day. Efforts are least successful during the early morning and late afternoon feeding peaks, for obvious reasons. The best times are the transitions from feeding and moving to rest and rumination periods, from 8 or 9 a.m. to, say, 5 p.m., depending on cloud cover and ambient temperature. The animals remain active longer during cool weather. On sunny days bulls under trees have a...