Collective memory and survival

Migration is a calculated risk for caribou, who maximize their chances of survival by balancing the energy they need to migrate against seasonal shifts in food availability. As ELIE GURARIE and ANNA BROSE write, unpredictable weather conditions caused by climate change are making it harder for caribou to find that balance—but a phenomenon known as social memory is helping.

As far back as most people can remember, the Western Arctic caribou herd—historically the largest migratory herd in Alaska—would cross the Kobuk River at a wide bend called Paatitaaq (Onion Portage). In early October, tens of thousands would flow across the Kobuk on their southwest migration to spend the winter on the Seward Peninsula. In fact, archaeological evidence shows that both caribou and humans have returned to that site repeatedly over the last 10,000 years or more.  

From 2017 to 2020, the number of caribou crossing the river went from tens of thousands to zero. The caribou had abruptly shifted their winter range by more than 500 kilometres,

For most of those years, there was no better place to be a subsistence hunter—or a caribou biologist. Get there in late September, take a seat in the fields of wild garlic (after which the portage was named), and invariably, the caribou would come.  

Until, suddenly, they didn’t: from 2017 to 2020, the number of caribou crossing the river went from tens of thousands to zero. The caribou had abruptly shifted their winter range by more than 500 kilometres, staying north of the Kobuk River and deep in the Brooks Range mountains, considerably north and east of their historic wintering areas. 

A mysterious shift

To find out why, we analyzed GPS movement tracks from hundreds of caribou over more than a decade of monitoring and conducted an intensive evaluation of caribou deaths over that same period. Our analyses of temperature, snow depth and wind largely failed to explain the range shift. But when we started to consider the caribou as social and cognitive beings—that is, as animals that carry with them and transmit a collective knowledge of their landscape and experiences—a story emerged.  

Roughly, the story goes like this: When the caribou crossed the Kobuk River and headed southwest to the treeless but lichen-rich tundra on the Seward Peninsula, as they tended to do from 2010 to 2015, their survival rates were excellent: more than 95 per cent made it through the winter. The caribou who remained north of the Kobuk River were less likely to survive, and ended the winter in poorer health, likely due to differences in their access to lichen. 

But then, in the winters of 2016 and 2017, caribou survival on the Seward Peninsula fell by 15 per cent. These were notably warm winters. The sea ice was at record lows, and more winter precipitation came down as rain (remarkably, for the Arctic Ocean) than as snow. Perhaps that made it harder for caribou to migrate. Perhaps the freezing rain events created an impenetrable ice layer over the lichen. In any case, caribou on the Seward Peninsula fared much worse, while their cousins that had opted to winter in the north did as they usually do—which is to say not great, but not so bad.  

After this collective experience, a great number of caribou that had previously crossed the Kobuk River chose instead to join those that were toughing it out in the mountains. 

This is evidence—long reflected in Alaska Native knowledge of caribou behaviour—that these large-scale movements are informed by the herd’s collective social memory.

Collective social memory

We came to two major conclusions: First, the range shift was a smart move. Fewer caribou overall have been dying in the winter than would have if they had continued wintering south and west. Second, the choice was driven by past experiences: when collective survival was poor in the south, individuals were much less likely to head that way. This is evidence—long reflected in Alaska Native knowledge of caribou behaviour—that these large-scale movements are informed by the herd’s collective social memory.  

Caribou have survived great climate and population fluctuations in the past because they have many astonishing features and adaptations: hollow insulating hairs, wide-splayed hooves, eyes that change from blue in the winter gloom to gold in the summer sun, stomachs that glean enough from woody threads of lichen to stay fat and happy through the severest winters. But perhaps the greatest tools they have for navigating some of the harshest environments on Earth are their mobility and their sociality.  

Even as the climate in the Arctic changes faster than anywhere else on Earth, caribou are seeking solutions to the challenges posed by unpredictable weather and shifting resources by poking and prodding the landscape at vast scales.  

Since our study was published, some caribou have already resumed swimming across the Kobuk River at Paatitaaq, headed to the Seward Peninsula—having determined, perhaps, that the effort of wintering in the mountains was not worth it after all, and aiming to try their luck on the lichen tundra once more. Only time will tell whether this gambit succeeds.  

But whatever happens, the caribou will remember.