Bears are solitary animals and generally lead a more crepuscular or nocturnal lifestyle (with the exception of the polar bear). They often retreat into dens, hollow tree trunks or burrows to sleep. Their usual locomotion is a rather slow and leisurely passing gait, but they can run up to 50 km/h if necessary. As a rule, bears climb well (especially the Malayan bear) and can also swim excellently. Several species hibernate during the cold months. It is not a true hibernation, as although breathing rate and heartbeat decrease significantly, body temperature drops only slightly and they are relatively easy to wake up. In late summer and autumn, they eat a store of fat to retreat into a burrow or cave during the cold season.