Ancient antlers show caribou calving grounds persist over millennia

Porcupine caribou have been going to the same place on the Arctic coast to give birth for more than 3,000 years, a new study suggests.

Researchers say the findings highlight the role played by the Coastal Plain, in Alaska’s Arctic National Wildlife Refuge, in maintaining caribou populations both today and over thousands of years.

The Porcupine herd is one of the largest caribou herds in the world, numbering roughly 218,000 as of 2017. Its range covers parts of Alaska, the Yukon and the Northwest Territories. In the spring, the animals make their way to their calving grounds throughout the Coastal Plain in Alaska and Ivvavik National Park in the Yukon.

Female caribou shed their antlers a few days after giving birth, leaving behind a record that can persist in the Arctic for millennia.

Given the size of the Porcupine herd and the fact that each female has two antlers, “that’s a huge number of antlers that get dropped onto the tundra every single year,” said Joshua Miller, assistant professor in the Department of Geosciences at the University of Cincinnati, who led the recent study. According to some estimates, shed antler concentrations in the calving grounds can reach roughly 1,000 per square kilometre.

For about a decade, Miller has been using these antlers to gain insight on how caribou use the landscape. He and his colleagues previously examined the geochemistry of shed antlers, for instance, which reflects the geology of where the antlers were grown. Through this analysis, the team was able to differentiate two caribou herds and identify changes in one herd’s summer range. The scientists also found some antlers that dated back hundreds of years, the team reported in 2021.

Most records of caribou calving geography only go back a few decades, however, which Miller said is one of the core challenges in wildlife management.

“We really haven’t been paying attention all that long,” he said, adding that looking further into the past can provide historical context to better understand the state of the population today.

In the new work, published in February in the journal Frontiers in Ecology and Evolution, Miller and his colleagues looked as far back as they could. Three antlers found on the Alaskan side of the calving grounds were roughly 1,600 to 3,000 years old or more, radiocarbon dating revealed.

“It still blows my mind that you can just walk across tundra and there are these ancient echoes of this past population,” Miller said.

The findings should reinforce respect for the caribou’s use of their landscape, said Anne Gunn, a caribou biologist who has conducted work for co-management boards and territorial governments.

“It’s another really revealing glimpse of a truly remarkable creature,” she said.

Past and future climates

Miller and his colleagues found the ancient remains while searching for antlers in dryas terraces – shrubby landscapes that provide caribou habitat. To access the areas, they travelled along rivers that bisect the calving grounds in Alaska.

“This kind of work is really just walking across the tundra and looking down,” Miller said.

The team then used radiocarbon dating to estimate when each antler was grown.

Two shed antlers dated back 1,629 and 3,157 years, the researchers found. These antlers were also recovered from an area known as the “concentrated calving grounds” in the Coastal Plain, where females give birth in greater-than-average concentrations, the team reported.

One antler – dating back 2,581 years – was found outside the calving grounds. This antler was still partially attached to a fragment of skull, however, indicating the animal died before the antler was completely shed. The caribou may have been within days or weeks of giving birth and on its way to the calving ground, the researchers reported.

The findings highlight the consistency with which caribou have used the landscape over time, Miller said. They also point to the importance of conserving the Coastal Plain as a calving ground, he said, especially given recent interest in developing the region for oil and gas extraction.

“The other thing that’s interesting is that these antlers are coming from periods of slightly different climatic settings,” Miller said. Around 2,500 and 1,600 years ago, summer temperatures were slightly higher than they have been in recent decades, according to temperature reconstructions based on cores from nearby lakes.

Although climate change is projected to cause higher levels of warming than those documented in past millennia, Miller said the findings provide some confidence that the current calving grounds will be a viable and important part of caribou biology for at least the next wave of warming.

There are important caveats to the findings, though. Miller said one is the study’s sample size. “It’s only three antlers we’re talking about,” he said.

Gunn similarly said the study’s sample size was a limitation. The small number of ancient remains makes sense, however. “Even though the sample size is low, you’d expect it to be,” Gunn said. “The chances of an antler surviving that long are pretty low.”

According to Miller, the antlers also provide only snapshots of calving activity over a long period of time rather than a continuous record. It would make sense for caribou to calve in the same place in the intervening years, he said, but scientists need to keep scouring the tundra to find out.

Miller’s goal is to continue doing just that. Although his work has largely focused on Alaska so far, he said extending the antler surveys into Canada might provide valuable historical context and insight into potential differences between calving grounds on either side of the border.