Why did the moose lose its antlers?

It’s not the setup to a joke, but a question people all over the world are asking after a doorbell camera in Alaska caught the precise moment a moose wandered into view, shook off its rack, and then, seemingly frightened by the sudden event, ran off into the night.

Moose are the world’s largest deer species, with males capable of standing more than six feet at the shoulder and weighing up to 1,800 pounds, with antlers tipping the scales at 80 pounds.

“As far as being in the presence of moose, typically, it’s a little sketchy,” says Landon Magee, a wildlife biologist at the University of Montana and a member of the Blackfeet Nation. “Especially moose cows with their calves. They can be very, very aggressive.”

Though it is not often witnessed by people, antler shedding, or casting, is a normal annual process for male moose, deer, elk, and other members of the Cervidae family, commonly called cervids. The only exception is caribou, or reindeer, in which females also grow and discard antlers.

“A bull grows his first set beginning with his first birthday, in general, and they grow in size and shape each year until around 11, when growth is minimal,” says Lee Kantar, moose biologist for the Maine Department of Inland Fish and Wildlife.

Let’s dig into the biological marvel that are antlers and learn why animals bother growing them in the first place.

Horns versus antlers

Many people use the words horns and antlers interchangeably, but there’s actually a big difference between these headsets.

Horns, which adorn rams, goats, cows, and many other mammals, are part of the skull itself and never shed. Composed of keratin, a protein in our hair and fingernails, horns are dead, and simply grow slightly larger each year as new material is added onto the base. In more than a few horned species, such as yaks, oryx, and duikers, females sport cranial weaponry, too.

Contrary to lifeless horns, antlers pulse with life and are even warm to the touch while they’re growing.

“They are highly vascularized tissue that rapidly grow from early spring to near summer’s end,” says Kantar, who responded in an email from Maine as he waited out a snowstorm while out capturing moose calves. (See photos of animals with killer headgear.)

For much of the year, antlers are covered in fuzzy skin, known as velvet. And beneath that velvet are veins full of blood that carry calcium, phosphorus, and other nutrients to the growing bone beneath. To become antlers, that velvet must eventually die and get scraped off by the animal, revealing the battle-worthy bone. (Read how animals evolved horns, antlers, and other head armaments.)

Antler growth, or antlerogenesis, actually holds a Guinness World Record for being the fastest-growing tissue found in mammals. During the summer, when moose antlerogenesis is at its peak, their antlers can grow nearly an inch every single day.

What are antlers for?  

Though many mistake antlers for weapons, they’re mostly instruments of reproduction.

When vying for a female, a moose with smaller antlers may be deterred by a male with a large rack. Not only that, but a female may also perceive a male with imposing antlers to be more physically fit, and thus opt to mate with him, says Kantar.

Only when two males of equal size meet will they joust, placing their antlers together and then twisting and pushing to see which animal gains an advantage. Bulls will also gore each other in the side or the rump, sometimes causing fatal injuries or weakening an animal to the point that it succumbs to a predator or the elements. 

That’s why such duels are relatively rare: They come at a high cost. (Watch moose fight in a quiet Alaska suburb.)

Why do moose, deer, and elk shed their antlers?

Once the fall breeding season, or rut, is over, male cervids no longer have any need for their antlers. In fact, their head ornaments can become a bit of a liability.

Every year, wildlife managers and biologists get reports of cervids whose antlers have become locked together. Males in this unfortunate condition often die from their injuries, starvation, or even attacks from predators. (Two bull moose found frozen in mortal combat.)

But antlers can also pose a threat on their own, especially for animals living near people. Deer, moose, and elk frequently become caught on branches or tangled up in fencing, garbage, or even Christmas decorations.

The good news for cervids is that their head accessories are completely disposable.

As the days becomes shorter, male cervids’ bodies stop producing as much testosterone, which triggers a demineralization at the base of each antler, called the pedicle.

“So that grip to the pedicles starts to really loosen up, and then anything from jumping and landing, or shaking their head, or getting into a fight with another male can just knock it right off,” says Magee.

“And then that process starts all over again,” he says.