The Functions of Antlers

1. What they did

• There is plenty of evidence that stags use antlers in fights: to damage opponents, to protect themselves, and in some species to grip an opponent firmly in head-to-head pushing contests (Fig. 1 shows two mature red deer stags fighting on the Isle of Rhum, Scotland).
• Losing large parts of the antler, by natural breakage or experimental removal, clearly reduces a stag’s fighting ability and dominance.
• Fights are not rare, and males are often injured. In two samples of Russian red deer, rutting accounted for 13% and 29% of all adult male deaths. On Rhum, about 6% of rutting stags are permanently injured each year and about 25% show some damage (including antler breakage) each rut. In a Polish moose population, about 4% of bulls were killed in rutting fights each year, and 19% of 21 mature mule deer bucks were wounded in one season. If 4% of bull moose die in fights each year and bulls rut for 10 years, over 30% will be killed at some point.
• There is no firm evidence that hummels (stags that never grow antlers) do better than antlered stags. Hummels are rare, about one in 100 to one in 300 Scottish stags, and the only published comparison found little difference in their body weight.
• Antler shape makes sense for combat in red deer: the many points help grip and twist an opponent and parry thrusts, protecting the head and neck as well as attacking. The palmate (flattened) antlers of fallow deer remain a puzzle; fallow lock only the top points, and the palmation may strengthen them.

2. What they found

Larger deer sometimes use antlers against predators but more often use their feet, and smaller deer usually run. Most females have no antlers, and several species shed their antlers when they are most at risk from predators.

3. What they concluded

• The only function of antlers with compelling evidence is their use as weapons in fights between males, along with occasional use against predators.
• This does not prove antlers play no part in assessment or female choice. But in red deer, the most intensively studied deer, stags seem to assess each other by other means, and there is no evidence that hinds pick stags for their antler size.
• Despite their bizarre appearance, antlers may have evolved as weapons and be kept because of their role in fighting. The same has turned out to be true of other “bizarre” weapons, such as beetle horns, babirusa tusks and narwhal tusks.
• More studies of fighting and mate choice in other species are badly needed, and they must separate the effects of antler size from those of body size and age.

4. How tracking was or was not used

• Studies claiming that antler size predicts dominance often mixed animals of very different ages, or (in one penned group) castrated and intact stags.
• Where age was taken into account, red deer studies found only weak to moderate links between antler weight or point number and dominance, and these are probably explained by body size: bigger stags grow bigger antlers and also win more fights.
• When all the antlers were removed from a group of stags, their dominance ranks did not change. On Rhum, stags did not consistently avoid rivals with larger antlers, and larger-antlered stags did not consistently win.
• Antler size stays the same through the rut while fighting ability falls. Roaring contests and parallel walks, which do change with condition, give a better measure of a rival.
• Even for mountain sheep, the best-known example of assessment by horn size, the data could be explained by age and body size.

5. Tracker or observer certification

Red deer hinds avoid mating with stags under five years old, probably because young stags cannot hold their harems. Hinds that left harems moved as often to stags with smaller antlers as to stags with larger ones.

Antlers in female reindeer and caribou

Female reindeer and caribou may carry antlers because, in winter, both sexes dig feeding craters in the snow in mixed herds, where social rank decides who feeds. Their calves are also very precocious, and among African antelope, females tend to have horns where calves are precocious.