Otters and seals have a two‑layer fur system that prevents water penetration and creates an insulating layer.


Aquatic mammals such as otters and seals face a serious physical challenge: they must stay warm while spending significant periods of time in extremely cold waters. Sea lions and walruses have thick layers of blubber to keep them warm, while fur seals have much thinner layers, and otters do not have any blubber at all. Instead, fur seals and otters rely on very specialized layers of hair, the function of which is already being mimicked to design products that need to keep water out and keep heat in.

The Strategy

The pelts of all fur-bearing animals are actually made up of two different types of hair that grow in clusters together: guard hairs and underhairs. Underhairs are shorter and denser, with three or more underhairs growing in a follicle for every one guard hair. Guard hairs are longer, and usually extend out over the surrounding underhairs, creating a protective canopy for them.


Studies have shown that marine mammals relying on fur for insulation have scales on the cuticle (outer layer) of their underhair that are more elongated compared to aquatic mammals that rely more on blubber. The protruding of the scales causes the hairs to interlock, preventing the penetration of water, and also trapping air bubbles. Air is a poor conductor of heat, so the bubbles provide an insulating layer, preventing heat loss from the animal’s body.

Scientists also noticed that fur-bearing marine carnivores have significantly flatter and denser hair than carnivores found on land, such as bobcats and ermines. They performed a test of the different pelt types, putting fur samples from each in a small pressure chamber, covering them with water, and then observing how far the water penetrated into the fur when subjected to increasing pressure. The marine carnivores’ pelts were far better at blocking out water and maintaining the layer of air within the fur, providing evidence that flatter, denser hair was an that helped mammals survive in aquatic environments.

South American Fur Seal
Image: Otaria1 / Wikipedia / CC BY SA ‑ Creative Commons Attribution + ShareAlike

While some aquatic mammals rely on blubber to stay warm, fur seals and otters rely on their specialized fur.

River otter standing on its hind legs
Image: Eric Kilby / Wikipedia / CC BY SA ‑ Creative Commons Attribution + ShareAlike

Otters may inhabit waters that are as much as 60°F colder than their internal body temperature.

A river otter with wet fur
Image: Joe Mabel / Wikipedia / CC BY SA ‑ Creative Commons Attribution + ShareAlike

Air is a poor conductor of heat, so the bubbles that get trapped in the underhairs of the fur create a very effective layer of insulation.

The Potential

This two-component fur concept is being mimicked to create waterproof fabrics for outdoor clothing. While the outer layer of Nikwax’s Analogy fabric repels water trying to come in from the outside, the inner layer actively pushes sweat and condensation away from the body. Air is trapped between the layers to create insulation. It’s a great example of the potential for humans to mimic the functions of nature, even while adapting them to very different forms.

Last Updated February 21, 2021