Northern sea otters, unlike other marine mammals, lack blubber. Instead, they rely solely on their dense and specialized fur to keep warm. In the frigid waters of Alaska that can reach around 32°F, that’s one incredible coat! Let’s take a close look at how this impressive fur works.
Before we dive in, let’s get familiar with some fur terms:
- Guard hair: tougher, typically longer, hair that grows longer than the guard hair
- Under hair/under coat: soft, typically wavy, hair that grows shorter than the under hair
- Density: how many hairs are in a given area
- Length: how long a hair is
- Loft: distance between the skin and the top of the hair follicle
- Cuticle: outermost layer of a hair follicle
- Cuticle scale: little plates that make up the cuticle
- Insulation: Material that hinders the transfer of heat
Otter coats work by trapping a layer of air against the skin, which keeps the heat in and the water out. This is achieved through specialized guard hairs and a very dense undercoat.
Guard Hair
The guard hairs have oval-shaped cuticle scales that help the hairs interlock with one another. These interlocking guard hairs are the first line of defense against water and heat loss. Take a look at the photo above to see the interlocking hairs in action on a sea otter that just hauled out of the water.
Undercoat
Under the “roof” of guard hairs is the extremely dense undercoat. The undercoat of a sea otter has a reported fur density of about 850,000 to 1,000,000 hairs packed into one square inch. These hairs are soft, wavy, and full of air pockets.
These air pockets act as insulation since heat doesn’t travel through air very well. Due to the surface tension of water and how concentrated the layer of air in the undercoat is, these air pockets help push water away from the skin, thus keeping the otter dry. Otter hair is also oily, which further helps repel water.
Pup vs. Adult Coats
You might have noticed that otter pups are a lot fluffier and lighter colored than adults. They also tend to float like little cotton balls on the water. Why is this?
Wild sea otter mother and pup in Homer, Alaska.
While the same basic idea described previously still applies, otter pup coats are structured a bit differently. Both adults and pups have the same number of guard hairs, but pup guard hairs are a lot thinner and longer than adult guard hairs, and they clump into a point instead of lying flat when wet. Pups also have a less dense undercoat than adults. Despite these differences, pup fur does almost as good a job at keeping the otter pup warm as an adult coat. These differences also make the pup more buoyant, so they can float on the surface of the water while their parents search for food. Check out this video of rehabilitated sea otter pup Cali floating on the top of the water when she had her pup coat.
But how does that work? Wouldn’t having less hair mean that the pups get cold faster? Not necessarily. Pups make up for the lack of coat density with greater loft. When wet, the longer hairs don’t bend as far, resulting in a higher loft, which is the distance between the skin and the top of the hair. This high loft means less compressed air space in the undercoat. Instead of having lots of little air pockets to slow the flow of heat, they have a few big air pockets which have the same effect. These bigger air pockets also act like balloons, helping the pup stay afloat
Otter pups can’t keep their fluffy coats forever. As they get older and start hunting their own food, they need a coat that lets them dive down deep. Between the ages of 10 and 14 weeks, pups shed their long guard hairs in exchange for the shorter adult hairs and their undercoat gets more dense.
Research is still ongoing on how otter hair works. For example, how the natural oils otters produce help with waterproofing their fur, or the role of intermediate hairs in insulation. But for now, this article serves as an overview of the basic mechanics of otter hair.
Stay curious,
Anna R, ASLC Design & Communications Apprentice
Sources
Riordan, Kate, et al. “Review for ‘ontogenetic changes in southern sea otter (enhydra lutris nereis) fur morphology.’” Journal of Morphology, vol. 284, 1 June 2023, https://doi.org/10.1002/jmor.21624/v1/review2.
Riordan, Kate, Nicole M Thometz, et al. “Effects of Ontogeny and Oiling on the Thermal Function of Southern Sea Otter (Enhydra lutris nereis) Fur.” Conservation Physiology, vol. 11, 2023, https://doi.org/10.1093/conphys/coad095.
Zagrebelny, Sergey V. “MORPHOLOGICAL CHARACTERISTICS OF SEA OTTER Enhydra Lutris L. (CARNIVORA, MUSTELIDAE) PELAGE AND FIRST AGE MOULT.” IUCN OTTER SPECIALIST GROUP BULLETIN, vol. 15, no. 2, 1998, pp. 87–92.
