"Most hair from animals has a distinctive pattern, which is how we can distinguish one species from another," says Weisel. "But otter hair is so different that it caught my attention." The fins of one hair loosely insert into the grooves between fins of an adjacent hair, thus permitting the hairs to form a web-like pattern that keeps water from the otter's skin and decreases heat loss. Also, the grooves between fins trap air bubbles, which help increase the thermal insulation of the otter's coat. Indeed, biologists have observed otters actively blowing air bubbles into their fur while grooming, and their energetic rolling catches air in their fur. "The air insulates like a down jacket," explains Weisel.
A common otter behavior, next to their playfulness, is their constant grooming. This behavior is another important aspect of an otter's heat-sparing abilities. In addition to the interlocking structure of the underhairs, these hairs are coated with a thin layer of body oil from the otter's sebaceous glands, thus providing another barrier to water. The fins of the underhairs are also aligned away from the body, which is consistent with the direction in which otters run their paws through their hair during this self-grooming, thereby ensuring that their claws do not get caught on the fin-like projections.
Weisel is continuing these studies of mammal hair in his spare time and has returned to Isle Royale once since his sabbatical, doing radio telemetry of ra