Deer rub their antlers up and down on trees and bushes once the antlers are fully grown to remove the hair-like membrane known as velvet from the outside. The antlers then dry and harden and are used as weapons in fights with other males to assert dominance during breeding season.
Normally, only male deer grow antlers. Females grow them only if they have a rare testosterone imbalance. Every year in spring, deer shed their old antlers and almost immediately begin to grow new ones. Antlers are bone growths that start at the skull base. As antlers grow, they have a high blood content and are susceptible to injuries, such as bruises and cuts. During spring and summer, antlers grow rapidly, but by late summer, growth slows as they begin to harden. If deer do not rub the velvet off when growth stops, it sheds on its own. The rubbing, along with blood residue, makes the hard antler bone brown instead of white. By winter, after breeding season, the antlers usually drop off completely.
Male deer, known as bucks, develop antlers after their first year of age. Each subsequent year, their antlers grow larger until they are several years old and antler growth maximizes. Another factor affecting antler size is nutrition, as the growing of antlers requires substantial amounts of protein and other nutrients. Deer need high-quality plant matter for optimum antler growth.