During the pre-rut, does release estrogen to prepare organs for breeding and birthing. They are aggressive against yearling bucks, to drive them away. During the pre-rut, a buck's level of testosterone increases, causing his antlers to mature and harden. Once the antlers harden, the velvet on the antlers begins to peel off. The peeling of velvet irritates the buck, causing it to rub antlers on trees. This rubbing smears the buck's scent on the tree. During pre-rut, a buck also achieves peak weight and experiences enlarged neck and sex organs.
Mature bucks sometimes fight each other for the doe's company. Bucks "signpost" by rubbing on trees or scrapes to leave their scent. Once a buck leaves its scent, other bucks do not. A buck may also signpost by digging paws into the ground to release glands in the feet, and then rubbing hooves onto a scrape. Sometimes bucks also urinate on scrapes to warn rivals to stay clear.
Sparring occurs between two bucks whose antlers have recently shed velvet. Sparring can be playful and improve social bonds between bucks. However, they also establish dominance of bucks in a community. Sometimes, sparring matches escalate into fights in which bucks lower heads and lock antlers. When antlers are locked, bucks try to push and shove each other so one buck flips the other. Once the dominant bucks have asserted themselves, they engage in most of the mating in the community.
Rut occurs in late autumn in northern states and early or mid-winter in southern states. Rut typically lasts between 10 days and a month. During rut, a buck and a doe isolate themselves from other deer by entering a secluded thicket. A doe urinates, and the buck analyzes the urine through a process known as vomolfaction. During this process, the buck sucks urine into an organ that is located on the roof of the mouth to determine if the doe is about to enter estrus (fertility). Vomolfaction arouses the buck and prepares him for breeding. The buck follows the doe with a lowered head and outstretched neck. The buck may follow the doe for as long as two days before the two actually mate.
Gestation in deers is typically six to seven months long. Normally, first-time mothers give birth to only one fawn. In following years, does give birth to as many as three fawns, if food is plentiful and they are well nourished. Fawns remain in the den for a few weeks. They are weaned between four and eight months of age, after which they graze for food on their own.