vulva to determine whether a female is fertile. Males produce high-pitched whines repeatedly to initiate mating; following this display, a female may allow the male to mount; copulation can last as long as 5 minutes. Ecology and mating system characteristics Many deer species—including European fallow deer—have a social organization that can be tremendously plastic depending on their environment, meaning that group size and habitat type are closely linked to herd size. Most of the detailed research on the ecological characteristics and behaviour of European fallow deer occurs in large blocks of woodland, which means some bias may be present. European fallow deer can be found in a variety of habitats, which can range from cool and wet to hot and dry. European fallow deer seem to have a preference for older forests with dispersed areas of grass, trees, and a variety of other vegetation. The largest herd occurs right before the rutting season, while the smallest groups are females with fawns. Throughout a large portion of the year, the sexes remain separated and only congregate during the mating months, but other patterns may be described, such as bachelor groups and even mixed groups. Male European fallow deer produce low-frequency vocalizations called groans; the sound of these groans results from the consistent and complex shape of the vocal tract involving the oral and nasal cavities. Ruts are characterized by males gaining the best territory possible to increase their odds for mating, and are often characterized by the presence of females on stands. During this time, males stop feeding to defend their ruts from subordinate males. Males defending this territory often lose an average of 17% of their body weight, and the liver exhibits steatosis, which is reversible. Throughout breeding seasons, the males may obtain the same rut; in some cases, ruts can be held by more than one individual; some possibilities for this include high population density and less rut space, or more suitable habitats, which can be shared. Parental care After a female is impregnated, gestation lasts up to 245 days. Usually one fawn is born; twins are rare. The females can conceive when they are 16 months old, whereas the males can successfully breed at 16 months, but most do not breed until they are 48 months old. The females can become very cagey just before they give birth to their fawn and find secluded areas such as a