Springbok passing through bushes tend to be more vulnerable to predator attacks as they can not be easily alerted, and predators usually conceal themselves in bushes. Another study calculated that the time spent in vigilance by springbok on the edges of herds is roughly double that spent by those in the centre and the open. Springbok were found to be more cautious in the late morning than at dawn or in the afternoon, and more at night than in the daytime. Rates and methods of vigilance were found to vary with the aim of lowering risk from predators. During the rut, males establish territories, ranging from , which they mark by urinating and depositing large piles of dung. Males in neighbouring territories frequently fight for access to females, which they do by twisting and levering at each other with their horns, interspersed with stabbing attacks. Females roam the territories of different males. Outside of the rut, mixed-sex herds can range from as few as three to as many as 180 individuals, while all-male bachelor herds are of typically no more than 50 individuals. Harem and nursery herds are much smaller, typically including no more than 10 individuals. In earlier times, when large populations of springbok roamed the Kalahari desert and Karoo, millions of migrating springbok formed herds hundreds of kilometres long that could take several days to pass a town. These mass treks, known as trekbokking in Afrikaans, took place during long periods of drought. Herds could efficiently retrace their paths to their territories after long migrations. Trekbokking is still observed occasionally in Botswana, though on a much smaller scale than earlier. Springbok often go into bouts of repeated high leaps of up to into the air – a practice known as pronking (derived from the Afrikaans pronk, "to show off") or stotting. In pronking, the springbok performs multiple leaps into the air in a stiff-legged posture, with the back bowed and the white flap lifted. When the male shows off his strength to attract a mate, or to ward off predators, he starts off in a stiff-legged trot, leaping into the air with an arched back every few paces and lifting the flap along his back. Lifting the flap causes the long white hairs under the tail to stand up in a conspicuous fan shape, which in turn emits a strong scent of sweat. Although the exact cause of