A study of these hybrid animals at Spioenkop Dam Nature Reserve in South Africa revealed that many had congenital abnormalities relating to their teeth, horns, and the Wormian bones of the skull. Another study reported an increase in the size of the hybrid as compared to either of its parents. In some hybrid animals, the auditory bullae are highly deformed, and in others, the radius and ulna are fused. Genetics and evolution The diploid number of chromosomes in the blue wildebeest is 58. Chromosomes were studied in a male and a female wildebeest. In the female, all except a pair of very large submetacentric chromosomes were found to be acrocentric. Metaphases were studied in the male's chromosomes, and very large submetacentric chromosomes were found there as well, similar to those in the female both in size and morphology. The rest were acrocentric. The X chromosome is a large acrocentric, while the Y chromosome is a minute one. This species of wildebeest seems to have evolved around 2.5 million years ago. The black wildebeest is believed to have diverged from the blue wildebeest to become a distinct species around 1 million years ago, in the Middle to Late Pleistocene. Fossil evidence suggests that blue wildebeest were quite common in the Cradle of Humankind in the past. Apart from eastern Africa, fossils are commonly found in Elandsfontein, Cornelia, and Florisbad. Description The blue wildebeest exhibits sexual dimorphism, with males being larger and darker than females. The blue wildebeest is typically in head-and-body length. The average height of the species is . Males typically weigh and females weigh . A characteristic feature is the long, black tail, which is around in length. All features and markings of this species are bilaterally symmetrical for both sexes. The average life span is 20 years in captivity. The oldest known captive individual lived for 24.3 years. The age that blue wildebeest live to in the wild is debatable. Colouration This broad-shouldered antelope has a muscular, front-heavy appearance, with a distinctive robust muzzle. Young are born tawny brown, and begin to take on their adult coloration at the age of 2 months. The adults' hues range from a deep slate or bluish-gray to light gray or even grayish-brown. The back and flanks are slightly lighter than the ventral surface and underparts. Dark brown, vertical stripes mark the area between the neck and the back of the ribcage,