Brown bears and polar bears prey on reindeer of all ages, but like the wolverines they are most likely to attack weaker animals, such as calves and sick reindeer, since healthy adult reindeer can usually outpace a bear. The grey wolf is the most effective natural predator of adult reindeer and sometimes takes large numbers, especially during the winter. Some wolf packs as well as individual grizzly bears in Canada may follow and live off of a particular reindeer herd year round.