Liz Bradley, a wolf manager for Montana Fish, Wildlife and Parks has found an unusually high number of wolves killed by cougar in the Bitterroot Mountains near the Montana-Idaho border from Lolo on the north to near the Idaho border on the south. Note that the irregular state boundary jogs from n/s to e/w at the southern end of the Bitterroot Range. Her observation of this goes back to 2009 and is mostly based on radio collared wolves.
Competition between the two large carnivores is well known. Numerous stories have been reported over the years in the northern rocky mountains. Studies in the Big Creek area of central Idaho showed that the wolf packs tended to push cougar out of the prime areas for their prey into the rough, less desirable country.
The story is important because too many people believe that predation on deer and elk is strictly additive by each type of predator, but in fact the two compete. When you add bear to the mix things are even more complicated, and bear are usually present. Studies of what killed the elk in various parts of western Montana have, in fact, generally shown that wolves fall behind cougar and bear as the cause of predatory death.
Wolf predation tends to cause a bigger stir among humans because it isn’t as quick as a cougar kill. There is more blood on ground and the wolves don’t bury their prey.
Perry Backus in the Ravalli Republic (reproduced here in the Missoulian) gives the full story on the Bitterroot.