Standing on the berm of a small pond, I watch the resident beaver leave its lodge, a silhouetted nose moving through the water. It disappears briefly and returns with a branch in tow. The beaver clambers over the edge of its dam along a muddy path, a branch bouncing along behind. Despite the muddy trail, the muddy dam, and the overall muddiness of the pond, I’ve never seen this – or any other – beaver caked in mud.
After making similar observations of another furry animal (his dog), Andrew Dickerson, a researcher in fluid dynamics at the University of Tennessee, wondered if fur had characteristics that help it to stay clean. He was especially curious about fur that is regularly exposed to water, including the coats of semi-aquatic animals such as beavers and otters.
What Dickerson and his team found and reported in the The Royal Society Journal Interface is that hair’s ability to flex and move is an important factor in its proclivity for staying clean.
American beavers (Castor canadensis) and North American river otters (Lontra canadensis) remain active through winter and have evolved many physiological and behavioral adaptions to live through freezing weather. Beavers stockpile food and body fat for the winter, and their metabolism slows, so they don’t eat that food too fast. Otters eat a high-calorie diet of fish and other prey during winter, allowing them to maintain a comfortable body temperature and a high metabolism. One critical component of the survival strategy for both is their winter coats. Both species grow thick coats comprising a layer of long, robust guard hairs on the outside and short, dense underfur below.
The many hairs that make up beaver and otter coats are thin and flexible fibers of keratin, a multi-purpose protein that also contributes to diverse mammal features such as hooves, fingernails, and whale baleen. Keratin is tough and insoluble in water, naturally contributing to fur’s durability and water resistance.
The factors that influence how dirt accumulates on an underwater hair fiber are complex, and researchers are still working to better understand them. Dickerson’s research suggests that some characteristics of hair fibers—how curved their surface is, their cross-sectional shape, roughness, and the shear stress they experience—contribute to their ability to remain clean. Other factors that affect hair cleanliness for aquatic mammals include how long they are submerged, how dirty the water is, and how fast it is flowing.
Using various terrestrial and semi-aquatic species samples, Dickerson first tested single guard hairs’ tendency to become debris-covered. To do this, he fixed the hairs at both ends to a plastic mount and then subjected them to a flow of water containing titanium dioxide particles for 24 hours. These particles are non-toxic and so tiny that they stick very well to all kinds of surfaces.
After the hair fibers were exposed to this “dirty” water, Dickerson moved them to clean water and measured how much titanium oxide came off. He found that hairs from semi-aquatic mammals stayed clean better than terrestrial fur and synthetic fibers. Since fur has multiple, inter-related characteristics that influence how it becomes fouled, it is difficult to list exactly which characteristics most help it to stay clean. On average, however, the hairs of the semi-aquatic animals tested were thinner, smoother, and more oval in cross-section than other hairs in the study.
Next, Dickerson tested the hair fibers with one end clamped and one end hanging free, similar to how an individual hair grows out of an animal’s skin. He again exposed these hairs to water laden with titanium dioxide and then measured the accumulation of particles. He found that when hairs are anchored at one end, the free end flutters and waves in the cross current. That flutter helped the hairs stay cleaner by shaking off titanium dioxide particles.
Dickerson’s research suggests that there might be an optimal length for fluttering as a self-cleaning property of hair. Shorter hairs, whether from terrestrial or aquatic animals, didn’t flex as much as – and were dirtier than – longer hairs, but very long hairs bent and curved into line with the cross-flow of Dickerson’s experimental currents, reducing their flutter, and presumably their self-cleaning potential. He also pointed out that an animal’s hairs are packed tightly together, so they may clean each other by rubbing together.
“I want anybody who looks at my work to get a greater appreciation of how nature works and how wonderfully multi-functional fur is,” Dickerson said. Beavers, otters, and their semi-aquatic comrades rely on their fur to help them stay warm through the winter. But those coats do more than insulate against cold and wet; they’re also dirt resistant – and self-cleaning.
Rachel Sargent Mirus is a teaching artist and writer. Illustration by Adelaide Murphy Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: nhcf.org.
