The common tsessebe or sassaby is a fast species of African antelope.Tsessebe are social animals. Females form herds composed of six to 10, with their young. After males turn one year of age, they are ejected from the herd and form bachelor herds that can be as large as 30 young bulls. Some unusual behaviors include the habit of sleeping tsessebe to rest their mouths on the ground with their horns sticking straight up into the air. Male tsessebe have also been observed standing in parallel ranks with their eyes closed, bobbing their heads back and forth.