The sika deer is a member of the genus cervus, a group of deers also known as the "true deers". Cervus was formerly a large genus consisting of 12 species, but recent genetic studies discovered that the species are more distantly related to each other than previously thought, and 8 species and 3genera split out. The sika deer, along with the elk, red deer, and central asian red deer, remained in the genus Cervus, with the white tailed deer of genus przewalskium being closely related.
The sika deer is one of the few species that do not lose their spots at maturity. Sikas from different areas differ in the numbers and obviousness of spots, with the ones from mainland Asia having large and obvious spots throughout the year, while the spots in Taiwanese and some Japanese populations are nearly invisible. Sika deer tend to range in color from mahogany to black. They are rarely white with very few documented cases of white as opposed to albino. They are medium sized herbivores, being 50 - 95cm tall at the shoulder, and weighing 30 - 70 kg. Males are noticeably larger than females. All sikas are compact and dainty-legged with short, trim, wedge-shaped head and a boisterous disposition. When alarmed, they will often display a distinctive flared rump much like the American elk. All sikas posses the white rump except the population of Ryukyu Island.Sika stags have stout, upright antlers with an extra buttress up from the brow tine and a very thick wall. A forward-facing intermediate tine breaks the line to the top, which is usually forked. Occasionally, sika develop some palmation. Females carry a pair of distinctive black bumps on the forehead. Antler length can range from 11 to 19 inches to better than 30 inches depending on the subspecies. Stags also sport a distinctive mane while in the rut.
In areas without human disturbance, sika deers could be active for the whole day. Lifestyles vary between individuals, with some occurring alone while others found in single-sex groups. Large herds will gather in autumn and winter. The sika deer is a highly vocal species, with over 10 individual sounds ranging from soft whistles to loud screams.
Sika males are territorial and keep haremas of females during the rut, which peaks from early September through October but may last well into the winter months. Territory size varies with type of habitat and size of the buck; strong, prime bucks may hold up to 2 ha. Territories are marked with a series of shallow pits, called "scrapes," into which the males urinate and from which emanates a strong, musky odor. Fights between rival males are sometimes fierce, long, and may even be fatal.