Rolf Peterson
Wolves crossing the ice during an Isle Royale winter.
No wolf pups were born on Isle Royale in 2012, according to researchers, a grim turn for a population that fell last year to just eight animals.
During the 55th annual study of wolves and moose on Isle Royale, researchers observed “no evidence of any [wolf] reproduction during the past year,” John Vucetich and Rolf Peterson write in the annual report. “This is the first year in the project’s history that we have been unable to document reproduction.”
The population declined during the year from nine to eight, “the lowest number of wolves ever observed in the population,” the researchers say.
The moose on the island, meanwhile, have boomed, from an estimated 400 animals in 2006 to 975 today. With so few wolves left, the predation rate – the percentage of moose (older than nine months old) killed by wolves – dropped to an all-time low of 2.4 percent.
“Calves comprised 21% of the moose population during winter 2013, which is one of the highest rates of recruitment ever observed in this population,” John and Rolf write in the report. The moose-to-wolf ratio, down to 15 in 2006, has risen to 122, significantly above the long-term average.
Isle Royale researchers had thought that only one female wolf remained, but DNA analysis of scat collected in 2012 showed that the population includes between three and five females. “The lack of reproduction is not due to a shortage of females,” John and Rolf write, nor is it because of a limited food supply. Moose are abundant, and the kill rate this winter was one of the highest ever observed.
Why, then, are the wolves not breeding?
“Considerable evidence suggests that the Isle Royale wolf population is highly inbred and has been impacted by genetic deterioration,” according to the report. Researchers haven’t collected a specimen without congenital spinal deformities since 1994.
Many of the surviving wolves are closely related – parent and child, or brother and sister – and “[i]t is possible that lack of courtship and mating is a manifestation of inbreeding avoidance.”
The population hasn’t had an influx of new genes since 1997, when a lone wolf crossed an ice bridge from Ontario. “The genetic input from the immigrant seemed to breathe new life into the population, allowing the population to thrive for over a decade,” John and Rolf write. But the prolific male was related to every wolf on the island within two and a half generations, and inbreeding problems resumed.
The annual study, funded by Michigan Tech, the National Science Foundation and the National Park Service, began in 1958.
Moose arrived on Isle Royale in the early 1900s, followed by wolves in the late '40s. For more on the project, the longest-running predator-prey study in the world, read Andrew Sorensen’s 2008 Lake Superior Magazine story.