A study shows that otters, beavers and other semi-aquatic mammals stay clean underwater thanks to their flexible fur.

Underwater surfaces can become dirty as they accumulate dirt, algae and bacteria, a process scientists call “fouling.” But fur-bearing mammals like beavers and otters, which spend most of their lives wet, manage to avoid making their fur slimy. These antifouling abilities come in part from one of fur’s unique properties: Each hair can bend and flex as the animal moves.

I am a mechanical engineer who studies fluid dynamics or the behavior of liquids. My team recently published a study showing that fur that was allowed to move back and forth in a current of dirty water accumulated less than half the amount of dirt as fur held stiff at both ends.

While many animals have fur that seems to clean itself, semi-aquatic mammals have the most dirt-resistant, or “fouling-proof” fur.

Our recent study, published in the Journal of the Royal Society Interface, compared fur fibers from beavers, otters, springbok, coyotes and more using a stream of water containing titanium dioxide, a common additive in cosmetics. Titanium dioxide attaches easily to surfaces like skin. Our team pumped the dirty water over individual fibers in a closed loop for 24 hours, then cleaned the fibers to measure how much titanium dioxide they had accumulated.

My colleagues and I then used mathematical techniques to combine all of the fur’s properties into a single number that predicts its antifouling behavior. We took into account each strand of fur’s ability to bend, how fluid flows over it, and other unique characteristics of each species.

We found that bending ability was essential to keeping the pet’s fur clean.

Contamination can damage the affected surface. When the fur becomes dirty, the arrangement of individual strands on the animal’s skin is disrupted and the animal may have difficulty staying warm or dry.

Industrial repellent methods used to protect the bottom of ships and the interior of pipes often use harmful chemicals and consume energy and materials, unlike naturally grown solutions.

Understanding how fur stays clean naturally could lead to more environmentally friendly solutions for repelling dirt in water supplies, in marine environments and even in the medical field. Solutions could include surfaces that have parts that can flex and move or that have small hairs on a surface.

Fur research also reveals more about how these mammals evolved to survive in various environments.

Both animal fur and the fouling process are complex, so we still don’t fully understand how all of the complex properties of fur, from texture and length to cross-sectional shape and environmental conditions, contribute to cleanliness .

The strands of fur hair do not always move individually. On an animal, the hairs are close together and they probably clean each other by rubbing when their host moves. We cannot yet say whether friction and movement affect the cleanliness of the host animal.

My colleagues and I have just scratched the surface of the mystery of keeping furry mammals clean, and there are many more things we can test. Future work could expose fur to biological agents such as bacteria and algae, or examine the role fur areas play in cleanliness.

The only known mammal that succumbs to dirt is the sloth: algae grows on its fur.

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