Antlers Do What No Other Bones Can

Deer almost break their bodies to build antlers each year, and scientists still don’t really know why.

an illustration of a silhouette of a deer, with cut lines around its antlers
John J. Custer / The Atlantic

In the 1980s, shortly before I was born, my father killed a male white-tailed deer in the woods of Oklahoma, harvested his flesh, and mounted his head. Years later, my brothers regaled me with the tale of Tony, as they posthumously named the buck. “Dad shot him,” they told me, with glee. “And then he made us eat him.” I hated the circumstances of Tony’s death. But I was also entranced. In the corner of the living room where Tony’s head was perched, his antlers stretched from wall to wall, tines arcing toward the ceiling. The bareness of the bone, embossed with wrinkles and bumps that I could touch if I stood on my toes, made me imagine all that the antlers might have been, had Tony survived.

My younger self was right to marvel. Antlers are crowning biological achievements that do what no other tissues can. They weaponize naked bits of skeleton; they “grow faster than any other animal bone,” says Doug Emlen, an evolutionary biologist at the University of Montana. At the height of spring and summer, some big-bodied cervid species can sprout antlers at a rate of about an inch a day, surpassing the pace of fetal formation and even cancerous tissue growth. The pace is so speedy that deer must pillage minerals from other parts of their skeleton, only to cast their antlers away and sprout a new pair when the seasons turn once more.

No other mammals regularly discard and regenerate bits of bony skeleton like this. And scientists are still working to understand why deer annually jettison these “improbable appendages,” the objects of our envy and one of the greatest energetic investments the animals make. Antlers are a proclamation, majestic enough to attract the attention of deer and humans alike—enough that we may be reshaping the appendages before our knowledge is complete.

Millions of years ago, the tops of the skulls of the earliest deer were entirely bare. Males fought one another with tusks—effectively, ultra-elongated snaggle teeth—stabbing, gnashing, and slashing until their hides were freckled with punctures and their necks were striped with blood. These toothy tussles often ended “in severe injury or death,” says Nicole Lopez, an evolutionary biologist who will soon begin her graduate work at the University of Montana.

Then, as the dense forests of the early Miocene gave way to grasslands and fields, the first antlers appeared, eventually replacing most deer species’ tusks. (Antlers, by the way, aren’t the same as horns, which never fall off—except pronghorns, which are unique in other ways.) As tusks had, these new spindly structures likely manifested first as weapons, offering ancient deer fresh oomph to push and parry their rivals.

Even early on, males with heftier headgear undoubtedly had an edge—and the more open cervids’ habitats became, the more space their antlers had to balloon. The structures were showy, but safe. If large and intimidating enough, they could scare the bejesus out of other males before physical contact was made. That still seems to be the logic of antlerdom: These displays are cranial-bone-measuring contests that can keep some of the males in competition mostly wound free. “They’ll prance side by side, look at each other, posture, strut,” Emlen told me. “If they can figure it out right then, the little guy walks away.”

This motivation might help explain just how huge antlers became. Large animals are more efficient at expending energy, and as cervids grew in size, their antlers got bigger both absolutely and proportionally to their body, according to Ummat Somjee, who’s studying animal weapons at the Smithsonian Tropical Research Institute. The centimeters-long antlers of pudu, the world’s smallest deer, account for a wee 0.1 percent of total body mass, while elk haul around labyrinthine branches that can make up 3.5 percent of their weight. (One now-extinct species, the Irish elk—neither Irish nor an elk—holds the record at a whopping 10 percent, thanks to its 12-foot, 90-pound antlers.) Such disproportionate displays may especially behoove the big-bodied: Already massive moose would be hard-pressed to tell the difference between themselves and another male that’s just an inch taller at shoulder height, but a four-inch difference in antlers is far easier to spot.

Antlers are also a very honest advertisement of the resources a male harbors in excess. Cervids don’t usually dump much energy into their headgear until more vital physiological boxes have been checked; the structures are hypersensitive to a deer’s diet, stress levels, infection status, and more. This means antlers confess to the world the ways in which males are healthy and chock-full of primo DNA—or very, very much not. According to Emlen, the body size of male elks doesn’t tend to vary by more than a factor of about two—but the width of their antlers may span a range of 30-fold or more. At least some species of female deer appear to have clued into these cues, using antlers as sexual billboards that may help them home in on the season’s best sperm.

Pushed to extremes by the twin forces of male rivalry and sex appeal, modern male cervids have had little choice but to advance toward a pretty dicey goal. As Jeannine Fleegle, a wildlife biologist at the Pennsylvania Game Commission, puts it: “I want to grow the biggest set of antlers I can without it killing me.”

The genesis of antlers is a monstrous labor to undertake. For white-tailed deer, the process typically begins in the spring. A tangle of cartilage erupts from the skull, swaddled in fuzzy velvet—specialized skin so twined with blood vessels that it’s warm to the touch. The velvet is fragile; so are the developing antlers beneath. “Something as simple as a tree branch nicking them” can trigger deformation, says Sonja Christensen, a deer ecologist at Michigan State University. But eventually, the antlers mineralize and harden into super-solid bone. The velvet desiccates and sloughs, and the tissue beneath begins to die.

Cultivating a set of antlers “costs as much as producing and nursing two fawns all the way to weaning,” Emlen told me. The growth spurts are so swift that deer end up leaching calcium and phosphorus from other parts of their skeleton; many males enter their breeding season with their ribs riddled with osteoporosis that may not resolve for months, says Yunzhi Peter Yang, a bioengineer at Stanford University. By the end of their mating stint, the deer are “terribly exhausted, completely worn out” not just from rassling one another but from holding their headgear aloft, says Christine W. Miller, an expert in animal weaponry at the University of Florida at Gainesville. She’s seen elk “just lay their head down.”

Then, as the weather cools, cells at the base of the antlers sever the connection to the skull. Heavy and unstable, the bones pop off, leaving behind a “significant wound” that may span several inches and bleed, swell, and scab, says Katherine Brakora, a biologist at the Texas A&M College of Medicine who’s studied ruminant headgear. It’s a bizarre end to this anatomical saga: No one knows for certain why deer nearly break their body to build their antlers only to “throw ‘em away each year,” Emlen told me. “This is the biggest mystery of all.”

Maybe ditching their busted-up headgear gives deer a fresh start; maybe antlers are just too much of a pain to lug around once mating is done. “I’ve seen them tangled in bushes and carrying vegetation around,” Miller told me. And should a male find his antlers inextricably locked into another’s, “there’s no way out of that,” Fleegle told me. Generally speaking, “they’re both going to die.” (Or suffer an even worse fate.) Perhaps it suits deer best to just “regrow them when resources are plentiful,” Miller said.

But others are less sure of that evolutionary logic, especially because some deer species cling to their antlers ’til spring. Lopez, who has been investigating the biological price tags of annual antlers, told me that evolving constantly regenerating appendages may have put a drain on certain species’ life span, reproductive potential, and even brain size. “There may not be a very obvious reason” why they’re shed at all, she said. Maybe ancient deer simply picked up the habit and couldn’t shake it once it stuck.

Regardless of what kept them around, antlers now seem determined to exist. Grafted onto another part of the skull, a leg, even the head of a mouse, the tissue excised from an antler’s base can still burst into twists and tines of bone. Antlers’ irrepressible growth has helped make them irresistible to us. Herbalists have ground antlers into aphrodisiacs; medical researchers strive to mimic them for cancer treatments, wound healing, organ regeneration, osteoporosis therapies, and more. Antlers evolved to signal, in the most obvious way possible, majesty and strength. “They advertise power” and lure others in, Somjee, of the Smithsonian Tropical Research Institute, told me. “It works on us too.”

But Fleegle and others feel deeply conflicted about antlers’ sway over us. Humans have warped the structures into an almost-toxic symbol of virility, she said; they reinforce the idea that bigger is better, that males are the more stunning sex. And yet females, too, accomplish great feats: In gestating fawns, they conjure entire new skeletons from scratch. “Males get all the attention for something females are doing all the time,” Somjee said. Nor is our antler obsession without its costs. Companies hoping to spike velvet into bodybuilding supplements have surgically excised the substance off of deer heads. On captive cervid farms, farmers are breeding deer with antlers so gargantuan that they’re downright “grotesque,” Fleegle told me. And in a world where sporting spectacular headgear raises “the risk of getting shot,” Brakora told me, heavy trophy hunting may be shrinking the size of horns in certain wild populations of sheep—raising the possibility that antlers, too, could be under the gun. Those deer may end up mating a bit less—a small price to pay, perhaps, for eluding the crosshairs that get their more ostentatious colleagues killed.

In the wild, deer dispose of their antlers annually; among humans, they can be treasured for longer, sometimes for centuries. Still, our fascination also has its limits. A couple of years ago, fed up with Tony’s matted fur and decrepit state, my mother tore his head down and put it out on our curb. What evolution had offered had lasted more than three decades on our wall. But it was human whim—its own powerful force—that finally caused his magnificent antlers to fall.