Why Do Deer Shed Their Antlers? The Yearly Antler Cycle

Antlers are bone, not horn

Antlers are made of true bone and grow from two permanent bony stumps on the skull called pedicles. They are found only in the deer family. Unlike horns, which have a keratin sheath and are kept for life, antlers are dropped and regrown every year.

Antlers are among the fastest-growing tissues known in mammals. In big species such as the red deer, elk and moose, they can grow by about 2.5 cm (1 in) a day at their peak. A large set of moose antlers can weigh over 30 kg (66 lb), and all of that bone is built in roughly four to five months.

The yearly antler cycle

In temperate and Arctic deer, the cycle is driven by day length, which controls the hormones that build and release the antlers. A typical white-tailed deer buck follows this calendar:

Velvet is a living skin full of blood vessels and nerves that feeds the growing bone. Growth happens at the tips, first as cartilage that is then turned into bone. As days shorten in late summer, testosterone climbs. The bone hardens completely, blood flow to the velvet stops, and the buck scrapes the drying skin away. What is left is dead bone, hard and polished, ready for the autumn rut.

Shedding begins as testosterone drops after the breeding season. Bone-dissolving cells called osteoclasts eat away a thin layer where the antler joins the pedicle. Within days the joint becomes so weak that the antler falls off on its own, often when the deer brushes against a branch or shakes its head.

Why grow new antlers every year?

Growing a full set of bone each year costs a lot of protein, calcium and phosphorus, so biologists have asked why deer do it. Several explanations work together:

• Antlers are tools for one season. Bucks need them mainly for the rut, to fight rivals and impress females. Carrying heavy antlers all year would waste energy and make it harder to move through thick cover in winter.
• Dead bone cannot heal. Once the velvet is gone, an antler has no blood supply, so a broken tine stays broken. Regrowing a fresh set each year means every buck starts the next rut with complete antlers.
• Antlers are honest signals. Because they are expensive to grow, a large, symmetrical set shows that a buck is healthy and well fed. In red deer, stags with bigger antlers tend to win more fights and father more calves.

As long as food is good, antlers usually get larger each year until a buck reaches his prime, then may shrink in old age.

When different deer shed their antlers

Timing depends on the species, the latitude and the individual animal’s age and condition:

• White-tailed deer: bucks shed mostly between late December and March, after the November rut.
• Moose: bulls cast their antlers in early winter, often from December onward, after the September and October rut.
• Red deer: stags cast in late winter and early spring, around March and April, with older stags tending to shed first. The new set is clean by late August or September, ready for the rut.
• Roe deer: bucks shed in late autumn, around November, and grow new antlers through the winter, so they are hard by spring before their summer rut.

In tropical deer, which do not face strong seasons, antlers may be shed at almost any time of year.

Reindeer: the deer where females have antlers too

Normally only male deer grow antlers. The big exception is the reindeer, called caribou in North America, where most females grow antlers as well. Adult bulls shed theirs soon after the autumn rut, while pregnant cows keep their smaller antlers through the winter and drop them around the time they give birth in late spring. This may help the cows defend feeding spots they dig in the snow. Read more in do female reindeer have antlers?

In other species a doe very rarely grows antlers, usually because of a hormone imbalance. Such antlers are often small and may stay in velvet.

Antler myths and facts

Myth: The number of points shows a deer’s age. Point count depends on food, genetics and age together. A young buck on rich food can grow more points than an older buck in poor habitat.

Fact: Castrated deer grow strange antlers. A buck that loses his testes, through injury, does not cycle normally. Some keep their antlers permanently in velvet, and some grow lumpy, irregular masses sometimes called cactus antlers.

Fact: Shed antlers feed the forest. Rodents, porcupines and even other deer chew dropped antlers for minerals, which is why so few old antlers are found lying in the woods.