They're typically the size of a household cat, but an unusual number of Pacific marten in Haida Gwaii have started to sport some strangely plump bellies.

The reason for the weight gain is being linked to an appetite for the region's smaller Sitka blacktail deer.

"It's pretty hard to believe at first," said David Breault, a UNBC master's student in natural resources and environmental sciences, who's studying the portly predators.

"The number is growing of people who are reporting sightings of martens chasing full-grown deer and even riding them."

The marten — a larger version of a ferret — typically feasts on smaller creatures, such as woodpeckers, songbirds and squirrels.

The province conducted necropsies of a sample marten population in Haida Gwaii several years ago and found some to be in "beyond excellent condition."

"They broke the scale. There were several of them that they had to make a new designation for, which was obese," Breault said.

"Picture a rotund belly but still an elongated figure."

David Breault, a master's student at UNBC, is researching the Pacific marten in Haida Gwaii. (David Breault/Supplied)

Competing for food

Trappers have reported that Haida Gwaii's marten population has gone up over the years, but there's been no study done yet to prove it, Breault said.

Martens can help keep some pesky rodents, like rats, under control. But their growth could also be putting more pressure on species at risk, Breault said.

The region's martens, for instance, have reportedly been gobbling up the sooty grouse, which is a main food source for the goshawk.

The number of ermine — another weasel-like mammal — in the region has also reportedly shrunk.

Breault hopes his research can answer some key questions about marten ecology for wildlife managers.

"If managers can veer the populations in the right directions, they can potentially put these species at risk out of the crosshairs of martens," he said.

With files from CBC's Daybreak North

To hear more, click on the audio titled 'Obese martens devouring Haida Gwaii deer.'