With no grizzly bears or wolves to keep them on their toes, the moose around southern Yellowstone National Park have let their guard down. According to findings published today in the journal Science, the moose failed to recognize their predators, which after a 50-year hiatus have recently returned to the area. First-time encounters thus resulted in initially high levels of predation.

Joel Berger of the University of Nevada and his colleagues studied moose populations in Yellowstone and in Scandanavia, where grizzlies and wolves have only recently recolonized. They compared them with Alaskan moose, whose predators have maintained their presence. When they exposed the moose to various auditory and olfactory predator cues, they got two very different reactions. Moose in the predator-free areas were six times less likely to recognize these cues than were those in Alaska, which would often stop feeding or adopt a defensive posture.

Still, the naive moose learn quickly. "Wyoming moose that have lost even one of their offspring to predators may become as savvy as their Alaskan cousins within a single generation," Berger observes, "which indicates that mechanisms for predator avoidance are already in place and that fears of imminent extinction may be unwarranted."