pili muscles (located between hair follicles). This white hair is almost fully concealed by the surrounding brown hairs until the fold opens up, and this is a major feature distinguishing this antelope from gazelles. Springbok differ from gazelles in several other ways; for instance, springbok have two premolars on both sides of either jaw, rather than the three observed in gazelles. This gives a total of 28 teeth in the springbok, rather than 32 of gazelles. Other points of difference include a longer, broader, and rigid bridge to the nose and more muscular cheeks in springbok, and differences in the structure of the horns. Both sexes have black horns, about long, that are straight at the base and then curve backward. In A. m. marsupialis, females have thinner horns than males; the horns of females are only 60 to 70% as long as those of males. Horns have a girth of at the base; this thins to towards the tip. In the other two subspecies, horns of both sexes are nearly similar. The spoor, narrow and sharp, is long. Ecology and behaviour Springbok are mainly active around dawn and dusk. Activity is influenced by weather; springbok can feed at night in hot weather, and at midday in colder months. They rest in the shade of trees or bushes, and often bed down in the open when weather is cooler. The social structure of the springbok is similar to that of Thomson's gazelle. Mixed-sex herds or harems have a roughly 3:1 sex ratio; bachelor individuals are also observed. In the mating season, males generally form herds and wander in search of mates. Females live with their offspring in herds, that very rarely include dominant males. Territorial males round up female herds that enter their territories and keep out the bachelors; mothers and juveniles may gather in nursery herds separate from harem and bachelor herds. After weaning, female juveniles stay with their mothers until the birth of their next calves, while males join bachelor groups. A study of vigilance behaviour of herds revealed that individuals on the borders of herds tend to be more cautious, and vigilance decreases with group size. Group size and distance from roads and bushes were found to have major influence on vigilance, more among the grazing springbok than among their browsing counterparts. Adults were found to be more vigilant than juveniles, and males more vigilant than females.