Otter adaptations: How do otters remain sleek and warm

Otters cavorting in the water is a scene with which we’re all familiar. Yet, unlike many other mammals that spend a considerable amount of time in the water–polar bears, seals, dolphins, and whales–river otters do not have a thick layer of body fat to keep warm. They rely, instead, on a few unique adaptations; namely, their fur and the densely packed layer of specially adapted underhairs.

Using scanning electron microscopy and polarizing light microsopy, John W. Weisel, PhD, Professor of Cell and Developmental Biology at the University of Pennsylvania School of Medicine, and colleagues, examined the structure of these hairs for clues to their exceptional insulation abilities. (Click on the thumbnail to view full-size images). They found that the cuticle surface structure of the underhairs and base of the less-abundant guard hairs are distinctively shaped to interlock, with wedge-shaped fins or petals fitting into wedge-shaped grooves between fins of adjacent hairs. Weisel and colleagues report their findings in the Canadian Journal of Zoology.

Weisel and Research Specialist Chandrasekaran Nagaswami, MD, also in Penn’s Department of Cell and Developmental Biology, usually work on defining the physical properties of blood clots and applying this knowledge to find better treatments for heart disease. Two years ago when Weisel, an avid hiker, climber, and white-water kayaker, took a month of his sabbatical year to study wolves–a life-long interest–on Isle Royale National Park in Lake Superior, Michigan, he also collected hair samples from the island’s mammals—including wolves, moose, and otters. (The ecological studies of wolves and moose on Isle Royale, which started in 1959, are part of the longest-running animal ecology study in the world. Isle Royale has been a training ground for many ecologists, and lessons learned here have been applied to the re-introduction of wolves to Yellowstone National Park.)

Weisel examined wolf prey hair with light and electron microscopy with the idea of accurately identifying wolf diet from wolf scat. “While we have engaged molecular biologists in studies of animal genetics and isotope dynamics, John is the first structural molecular biologist that we have worked with,” says wildlife biologist Rolf Peterson from Michigan Technological University (Houghton, Mich.), who has spent the last three decades doing field research on Isle Royale. “It was a delight to learn about important basic features of animal hair that facilitate their unique lifestyles.”

“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 radio-collared wolves and collecting samples of their scat for DNA analysis.

“I discovered that it can be very enjoyable and stimulating to expand your scientific horizons beyond the familiar, and even get to take a ‘busman’s holiday’ in a beautiful place with wonderful people, enriching your scientific and personal life,” says Weisel of his experiences away from the bench. “There are still a great many new things to learn, but some approaches and ideas from one field can be useful in another.”