Female Caribou Carry Their Own Mineral Supplement to Calving Grounds - Then Eat It

Every spring, tens of thousands of caribou complete one of the most demanding migrations on Earth - a 1,500-mile round trip to calving grounds in Alaska's Arctic National Wildlife Refuge. The females arrive exhausted, give birth, and begin nursing calves in an environment where plant nutrients are limited. Within days of calving, they shed their antlers.

Biologists have puzzled for years over why female caribou have antlers at all. Among roughly 50 deer species worldwide, caribou are unique: females grow the same bony headgear as males, even though it appears to serve no obvious advantage. The leading explanations centered on social competition for food or predator deterrence. A study of shed antlers from caribou calving grounds offers a different answer.

The Evidence in Gnaw Marks

Associate Professor Joshua Miller and doctoral graduate Madison Gaetano at the University of Cincinnati examined 1,567 antlers collected during scientific expeditions to the Arctic Refuge between 2010 and 2018. In the cold, dry Arctic climate, shed antlers can persist for centuries. What struck Miller during collection was that most antlers had been chewed. The team analyzed tooth marks to identify the culprits - carnivores leave characteristic damage patterns distinctly different from marks made by herbivores or rodents.

The results were stark. Of the 1,567 antlers examined, 86% showed signs of gnawing. Of those gnaw marks, 99% were made by caribou themselves. Rodents - long assumed to be the main consumers of shed antlers - accounted for less than 4%. Carnivore gnaw marks on antlers were essentially absent.

"We knew that animals gnawed on these antlers, but everyone assumed they were mostly rodents. Now we know it's really caribou. My jaw dropped when our results started to become clear," Miller said.

The Mineral Logic

Antlers are made primarily of calcium and phosphorus - exactly the minerals nursing caribou mothers need most. Lactating females require substantial calcium and phosphorus to produce high-quality milk, but the Arctic tundra is nutrient-limited, particularly immediately after snowmelt. Shed antlers accumulated on calving grounds over decades represent a mineral cache precisely where and when nutritional demand peaks.

Female caribou drop their antlers within days of giving birth - making them available on the calving ground at the moment maternal mineral requirements are highest. Males shed their antlers in late autumn after the breeding season. This synchrony in females aligns precisely with the mineral-supplementation hypothesis. "These antlers last for centuries or longer in the Arctic and they are a source of nutrients that get revisited again and again," Miller said.

What This Means for the Predator-Defense Hypothesis

Female caribou antlers tend to be substantially smaller than males'. More decisively, female caribou shed their antlers precisely when a defensive weapon would be most needed: the days immediately surrounding calving, when calves are at their most vulnerable. "That means they are antlerless when it would be most crucial to have antlers to defend a young calf if they were a defense mechanism," Gaetano said. Reindeer herders she consulted confirmed caribou's primary defensive response is kicking and trampling - not using antlers.

As shed antlers slowly decompose, their minerals return to the soil, feeding the sedges, grasses, and lichens caribou eat. Calving grounds function partly as mineral cycling hubs: caribou bring tons of phosphorus to these sites in their antlers each year. The study, supported by the U.S. Fish and Wildlife Service, National Geographic Society, and NSF, was published in Ecology and Evolution.