New research carried out in the Kruger National Park (KNP) has revealed that buffalo bulls are ready, willing and able to take their chances with lions in order to get into tip-top condition so that they stand a better chance of successfully mating with a buffalo cow.
In Kruger, a male buffalo hanging out with ‘the boys' in a bachelor herd is five to seven times more likely to fall victim to an ambush from a lion than he is if he spends the same amount of time in a breeding herd with the females.
However, the benefits of eating in different places and on better quality food with the bachelor group seem to outweigh the increased risk of predation, as the bulls can gain condition and stand a better chance against other bulls when it comes to competing for a cow's favours.
The cows, on the other hand, stick together in large groups where there are lots of eyes to keep a lookout for lions, and prefer to feed in open areas where lions are less likely to successfully attack. The price of this behaviour is that it may mean they end up compromising on food quality, despite the fact that pregnancy and suckling a calf require a good deal of energy.
During the winter period, when the grass is dry and least nutritious, it's more important for cows to protect their calves from predators than to seek out the remaining lush, succulent vegetation that fattens up the bulls. It's all about passing on your genes, and even within a species males and females may have different strategies to achieve this.
Buffalo are known to biologists as a ‘sexually dimorphic' species – the males are different from the females. There are a number of theories about the strategies that these species employ to pass on their genes, but many of the theories were tested in areas where there are few natural predators.
The Kruger National Park (KNP) offers a perfect place to test the theories in the presence of predators, and master's student Craig Hay spent several years watching the section of Kruger's buffalo population that lives between Lower Sabie and Satara.
Craig's aim was to try and find an explanation of why buffalo behave as they do – why do the females spend their time in large herds year-round, while the males split off into bachelor groups for most of the year and then rejoin the females in the breeding season?
His work forms part of a series of interlinked projects that are looking at Kruger's buffalo, especially in the light of the spread of bovine tuberculosis, and is supervised by Dr Paul Funston from the Tshwane University of Technology and Dr Paul Cross from Montana State University.
Craig's study involved both field observations of hundreds of individually identifiable buffalo and the fitting of radio-collars to a total of 166 buffalo of both sexes over the course of five years. Along the way he also picked up a lot of buffalo dung, which in the case of bulls was analysed for the male sex hormone, testosterone.
Nitrogen content was also established for all the dung, as the amount of nitrogen in the dung can be directly related to the quality of the food that the buffalo eat. The average size of a breeding herd in the central area of Kruger is about 230 animals, although larger and smaller herds existed. Breeding herds consisted of cows, bulls and immature animals.
Bachelor groups contain only bulls, and range in size from one to 50 animals. In Kruger, the buffalo breeding season is between March and May, with most calves being born from January to April. Craig found that young bulls (up to about the age of eight) spent nearly the entire year in breeding herds, and these bulls consistently had low testosterone levels.
Older bulls spent most of their time in bachelor herds, only rejoining the breeding herd during the breeding season. The older bulls displayed a range of testosterone levels, with some bulls having exceptionally high levels, especially when they were associated with the breeding herds.
Those bulls with high testosterone are thought to be in the upper levels of the buffalo's social structure, and are more likely to dominate when it comes to mating. From the data obtained from the radiocollars, Craig was able to calculate that buffalo in bachelor herds move around in search of food 30 percent less than the bulls in the breeding herds do.
The bachelor bulls tend to feed in and around watercourses, where the vegetation is more dense, but also more nutritious, especially during the dry season. However, these areas also offer good cover for stalking lions, and the research showed that adult male buffalo are at least five times more likely to die in a bachelor herd than in a breeding herd.
The breeding herds play it safe, and stick to areas where there is better visibility but potentially poorer forage. From the data that Craig has gathered, it seems that the reasons why buffalo split into male and female groups and forage in different places is strongly influenced by the presence of predators.
This is contrast to herbivores in other areas where there are no predators, where scientists believe that the sexes separate because their different body sizes means that they have different dietary needs. This produces differences in the way in which males and females spend time feeding and resting, and results in them forming separate groups. In fact, females of some predator-free species may spend more time in areas of high quality food, in contrast to what the buffalo research has shown.
In a predator-free environment, unlike with buffalo in Kruger, it seems that other factors may explain why males and females separate. Craig is currently preparing his research for publication in a scientific journal while working as the training manager for the Skills Development and Projects department at the Southern African Wildlife College just outside Orpen Gate.