A native predator of the red squirrel appears to be an unlikely ally in its battle with the grey squirrel.
Scientists from Queen's University Belfast discovered that, while the pine marten preys on both species, the greys are much more vulnerable to attack.
The key seems to be in the reds' innate ability to "sniff out" the danger posed by the pine marten.
Previous research has shown that pine martens had a beneficial impact on red squirrel numbers and caused declines in the greys, but the reasons were not fully understood.
So Joshua Twining from Queen's University Belfast used pine marten scent to investigate.
When the researchers applied the scent to squirrel feeding stations across Northern Ireland, they found that only the red squirrels responded - showing much more vigilance.
Grey squirrels, on the other hand, seemed to ignore the scent and carry on regardless.
This lack of a behavioural response, researchers say, means greys are much more vulnerable. And with more than two million grey squirrels in the UK and just 150 thousand native reds, they certainly need the help to compete.