The males leave their herds when they are over a year old, forming groups of bachelors. At about three or four years old, males set up small territories which they defend from other males, and attempt to mate with females entering their areas. Mating activity is seasonal, and is usually timed so that the majority of calves are born close to the beginning of the rainy season, when new grass is plentiful.
Although populations have increased in the Serengeti, numbers have declined in other areas, such as south-west Botswana. There, competition with livestock and destruction of crops has prompted farmers to kill wildebeest and set up long fences to prevent them from migrating to wetlands when there are seasonal droughts.