For more than a century, invasive gray squirrels have bullied their way across Ireland, outcompeting and drastically reducing the native red squirrel population. But a new study finds that the red squirrel (Sciurus vulgaris, pictured) has acquired an unlikely ally: its natural predator, the pine marten. The find comes thanks to a 2007 ecological survey, which noted an unexpected decline in the number of North American eastern gray squirrels (S. carolinensis) on the island—and a concomitant rise in the number of pine martens (Martes martes), cat-sized, squirrel-eating carnivores. Researchers collected pine marten feces and hair samples from areas with and without gray squirrels. Finding places where the two species overlapped was so difficult that the researchers used a specially trained scent dog to track down pine marten scat in gray squirrel–populated areas. Where pine martens were plentiful, red squirrels thrived alongside their natural predator, the team will report next month in Biodiversity and Conservation. The feces revealed that not only had pine martens added gray squirrels to their diets, but that they were also feasting on the invasive species eight times more often than on red squirrels. The researchers believe the red squirrels acquired defenses against predation, such as better tree-climbing skills, as they coevolved with the pine marten, giving them a leg up on their non-native competition. (The gray squirrel originally came to Ireland in 1911 when a wedding gift of a dozen squirrels escaped into the wild.) As Ireland’s pine marten population continues to recover thanks to habitat protection and antihunting laws, a red squirrel resurgence may be less of a nutty idea.

See more ScienceShots.