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Caribou migratory behavior is reflected in their genes

Caribou (known as reindeer in Europe) are large, herbivorous mammals that are famous for making extensive migrations in order to avoid severe weather and gain access to forage. They make the longest seasonal migrations of any mammal, moving up to 2,500 miles each year. But not all caribous have the same propensity to migrate.

A recent study, led by Maria Cavedon from the University of Calgary, investigated the genes that influence migratory behavior in caribou and found that an individual’s genetic background is what makes it more or less likely to undertake extreme migrations. 

The researchers followed 139 caribou (Rangifer tarandus) females using GPS tracking. Some of the animals lived in tundra habitats, while others inhabited woodlands distributed across western North America. 

In addition, DNA sequencing of the caribou was carried out to investigate the genetic factors that influence migratory behavior. Over 50 genetic mutations associated with migratory behavior were identified. The experts noted that 27 of these mutations were located on genes involved in brain activity, fat and energy metabolism, body development, or hormone production. 

Furthermore, the researchers found that the genetic sequences clustered into two distinct geographical groups – one northerly and the other southerly. These genetic subpopulations are descended from ancestral caribou that were separated from one another during the last ice age. 

Having evolved in Beringia more than two million years ago, caribou became adapted to living in the cold conditions of north eastern Russia and Alaska. But around 300,000 years ago, during a relatively warm interglacial period, caribou populations spread throughout the northern half of North America.

However, when conditions began to cool again at the start of the last ice age, glaciers formed across Canada and the caribou range was cut in two. Those to the north of the continental ice sheets, in Beringia, continued to live in cold conditions while those trapped south of the glaciers became adapted to living in woodland habitats and warmer conditions. 

These two groups of caribou remained isolated from each other, on opposite sides of the Rocky Mountains, throughout the duration of the ice age, which ended around 11,700 years ago.

The genetic and GPS analyses of the 139 caribou in the current study showed that individuals with a greater proportion of genes from northern ancestors were more likely to migrate longer distances. During the research, the more migratory individuals travelled nearly 250 kilometers on average, which was ten times further than the sedentary caribou. 

These findings, published today in the open-access journal PLOS Genetics, indicate an evolutionary legacy from the last glaciation, when northerly caribou populations continued to migrate in order to survive in harsh tundra environments, while southerly populations living in forests were more sedentary. The study is the first to investigate the genes influencing migration in an endangered terrestrial mammal. 

Seasonal migrations enable animals to follow vital food resources that change from summer to winter. But they also make these animals particularly vulnerable to habitat fragmentation. Conserving key habitats along their seasonal migration routes could help to preserve the genes underpinning the world’s longest terrestrial migration, the authors say.

“Wide-ranging animals, including migratory species, are significantly threatened by the effects of habitat fragmentation and habitat loss,” said Cavedon. “We examined migratory behavior of GPS-collared endangered caribou in western North America and carried out genomic scans for the same individuals. We detected genes associated with migratory behavior, and we determined that propensity to migrate depended upon the evolutionary history of caribou.” 

“If, as we report, migratory behavior is influenced by genes, caribou could be further impacted by the loss of the migratory trait in some isolated populations already at low numbers.”

By Alison Bosman, Staff Writer

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