Diet shifts in moose due to predator avoidance.

On Isle Royale, Michigan, moose cows which have calves show a different distribution and diet from solitary adults and yearling moose. Solitary adults and yearlings follow a feeding pattern predicted from the location of high nutrient plant growth. In the presence of wolves, they feed on the ridges of the main island, where they take advantage of plants that leaf early; only later in the season do they move to the small outlying islands where plant phenology is delayed by the cold water of Lake Superior. Cows with calves deviate sharply from this pattern. They remain on the wolf-free small islands throughout the growing season. While on the islands, cows with calves eat a poorer quality diet than other moose. They switch to eating high quality spring and summer foods later than other moose and they eat significantly fewer high preference shrubs and significantly more herbs and low preference shrubs than other moose.This study suggests that in order to avoid predators, cows with calves sacrifice the high quality diet available on the main island. These data also suggest that the wolves not only affect prey numbers by direct kills but may also indirectly influence prey numbers by altering the diet of their prey. In this case, the reproductive cows, those individuals that contribute most directly to growth of the population, avoid predators but frequent poor feeding areas.