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Predators

A variety of predators prey heavily on reindeer, including overhunting by people in some areas, which contributes to the decline of populations.

Golden eagles prey on calves and are the most prolific hunter on the calving grounds. Wolverines will take newborn calves or birthing cows, as well as (less commonly) infirm adults.

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

In 2020, scientists on Svalbard witnessed, and were able to film for the first time, a polar bear attack reindeer, driving one into the ocean, where the polar bear caught up with and killed it. The same bear successfully repeated this hunting technique the next day. On Svalbard, reindeer remains account for 27.3% in polar bear scats, suggesting that they "may be a significant part of the polar bear's diet in that area".

Additionally, as carrion, reindeer may be scavenged opportunistically by red and Arctic foxes, various species of eagles, hawks and falcons, and common ravens.

Bloodsucking insects, such as mosquitoes, black flies, and especially the reindeer warble fly or reindeer botfly (Hypoderma tarandi) and the reindeer nose botfly (Cephenemyia trompe), are a plague to reindeer during the summer and can cause enough stress to inhibit feeding and calving behaviors. An adult reindeer will lose perhaps about 1 L (0.22 imp gal; 0.26 US gal) of blood to biting insects for every week it spends in the tundra. The population numbers of some of these predators is influenced by the migration of reindeer. Tormenting insects keep caribou on the move, searching for windy areas like hilltops and mountain ridges, rock reefs, lakeshore and forest openings, or snow patches that offer respite from the buzzing horde. Gathering in large herds is another strategy that caribou use to block insects.

Reindeer are good swimmers and, in one case, the entire body of a reindeer was found in the stomach of a Greenland shark (Somniosus microcephalus), a species found in the far North Atlantic.