The rut (from the Latin rugire, meaning "to roar") is the mating season of certain mammals, which includes ruminants such as deer, sheep, camels, goats, pronghorns, bison, giraffes and antelopes, and extends to others such as skunks and elephants. The rut is characterized in males by an increase in testosterone, exaggerated sexual dimorphisms, increased aggression, and increased interest in females. The males of the species may mark themselves with mud, undergo physiological changes or perform characteristic displays in order to make themselves more visually appealing to the females. Males also use olfaction to entice females to mate using secretions from glands and soaking in their own urine. During the rut (known as the rutting period and in domestic sheep management as tupping), males often rub their antlers or horns on trees or shrubs, fight with each other, wallow in mud or dust, self-anoint and herd estrus females together. These displays make the male conspicuous and aids in mate selection. The rut in many species is triggered by shorter day lengths. For different species, the timing of the rut depends on the length of the gestation period (pregnancy), usually occurring so the young are born in the spring. This is shortly after new green growth has appeared thereby providing food for the females, allowing them to provide milk for the young, and when the temperatures are warm enough to reduce the risk of young becoming hypothermic. Cervidae White-tailed deer The rut for white-tailed deer (Odocoileus virginianus) usually lasts three weeks in the Northern Hemisphere and may occur most of the year in tropical zones. The rut is the time when white-tail deer, especially bucks, are more active and less cautious than usual. This makes them easier to hunt, as well as more susceptible to being hit by motor vehicles. Outdoors writer Charles Alsheimer has done research demonstrating the white-tailed deer rut is also controlled by the lunar phase and that the rut peaks seven days after the second full moon during October and November (the rutting moon), while elk begin rutting during the September equinox on 21 September. A white-tail doe may be in estrus for up to 72 hours and may come into estrus up to seven times if she does not mate. Cows may come into estrus up to four or more times if they do not mate. The rut can start as early as the end of September