Because the bears survival and reproductive success are closely tied to their physical condition in the autumn, natural selection favors those that get the most nourishment out of the fish they eat. And bears exhibit two behaviors to this end. First, to avoid interference from other bears, they often carry captured salmon to the stream bank or into the streamside forest before eating. Bears are relatively solitary for most of their adult lives, save for a few weeks of courtship in spring and summer. When they aggregate in riverside areas to feed on salmon, they become aggressive. These confrontations can be relatively benign, resulting in one bear stealing a fish from another, or violent, ending in serious injury or death to a bear or its offspring. Carrying the carcass into the forest out of sight of other bears is a way to avoid confrontation.
The second important behavior is that the bears often eat only the most nourishing part of the salmon. If the salmon densities are high, it takes a bear less than a minute to capture a fish in a small stream, and under these conditions of plenty, the bears rarely eat the whole fish. An analysis of more than 20,000 carcasses revealed that bears consumed about 25 percent of each captured salmon, selectively eating only the parts highest in fat content, such as the eggs. In fact, it is common for bears to carry a carcass to the stream bank and not eat a bite after finding it is a male salmon or a female that has already spawned her eggs.
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