As the days of summer pass, the antlers get longer on moose, elk and other animals in the cervid, or deer, family. The animals spend most of the summer grazing, their head weaponry sheathed in fuzzy velvet. But by the end of September, the antlers will be ready for action as male suitors gather to posture, preen and butt heads — a behavior that some people may have in mind when they use the phrase “locking horns.”
It is not always just a figure of speech. Every now and then, grisly photographs appear on the internet: A buck walking around with the head of another male tangled in its antlers. Three white-tailed deer dead in a stream, antlers twisted in a complicated tangle. Two moose, locked together in death, blocking a ditch.
Beyond the shock value, these stories provide an insight into an odd quirk of evolution: Fierce-looking antlers exist primarily for display. When animals try to use them as weapons, the consequences can be dire.
Several families of ungulates, or hoofed animals, make use of their horns in battle. But while antlers are sometimes confused for horns, they are unique to the broader deer family and anatomically distinct, said David Petersen, a nature writer and author of the book “Racks: A Natural History of Antlers and the Animals That Wear Them.” Horns are permanent cores of bone encased in sheaths of keratin, while antlers are bones that grow and are shed seasonally, starting as velvety nubs then sprouting into sharp branches.
Antlers evolved primarily as visual advertisements, Mr. Petersen said. A healthy, well-fed animal grows more points on its rack, making it more competitive during the breeding season, when males try to gather harems of females. An animal who has had a rougher year might have a less inspiring display.
“A set of antlers shows the other males and females what stage of life and health a buck is in,” Mr. Petersen said. “Bulls with the most dominant antlers tend to do most of the breeding.”
But while antlers look like dangerous weapons, Mr. Petersen said, they are seldom used that way. Cervids become markedly more aggressive and territorial during the breeding season, but they generally avoid actual fighting through a combination of posturing, testing one another and making false charges. Cervid antlers in particular are generally better at deflecting blows rather than dealing them out.
But sometimes, when passions run high, fights do happen. Some ungulate families — particularly cattle, goats and antelope — are well adapted for serious fighting, with relatively smooth horns that make fencing, battering and disengaging easy. But the branching, twisting shapes of antlers — which can vary in size and shape between animals — make getting tangled a real possibility.
Of America’s cervids, white-tailed deer are the most prone to getting themselves locked up, said Alan Cain, White-tailed Deer Program Leader of the Texas Parks and Wildlife Department. Bucks in the 3- to 5-year-old range are the most likely to be victims because of their well-developed antlers. While still a fairly rare occurrence, the relative frequency of news reports about locked deer is a result of their high population numbers, which simultaneously makes accidents more likely and evidence of them easier to find.
“Deer species aren’t well designed to butt heads,” Mr. Petersen said. “When they do, the chances of locking up horns is pretty good. Usually they manage to unlock them, but if they don’t, one or both of those animals is going to die.”
There are several avenues of death for locked cervids, and none of them are pretty, Mr. Cain said. A pair with locked antlers may struggle themselves to death through exhaustion, starvation or dying of thirst. In some cases, animals battling near water can tangle up, fall in and drown each other.
Locked up cervids also make tempting targets for predators like coyotes. If both animals are dead, that is a bonus as far as some predators are concerned — but not necessary.
“You may see locked up bucks where one is dead,” Mr. Cain said, “and the one that’s still alive is watching the dead one be eaten in front of them.”
Not all locked-up deer die, however. Deer that manage to survive this experience are sometimes left with gruesome trophies — the heads of their fallen foes locked onto their own, until they shed their antlers at the end of the rutting season and with it, the unwanted cargo.
Help can also come from an unexpected quarter.
“Sometimes a hunter or someone will come across one of these animals and free the living one,” Mr. Cain said.
Once, in South Texas, Mr. Cain was called out to look at a pair of white-tailed bucks locked up across from a business center. After securing the deer, he and his colleagues used a hacksaw to cut the tangled antlers loose. The freed males staggered away.
However often one may read about locked antlers on the internet, such occurrences remain pretty unusual, Mr. Petersen said. Used properly as display organs, antlers are remarkably effective tools. But they also make for treacherous weapons, and fighting with them is a gamble.
“Evolution doesn’t evolve systems that kill off prime breeders,” Mr. Petersen said. “When you see these things happen, it’s an evolutionary accident.”