Date of Award


Degree Name

MS in Biological Sciences


Biological Sciences


College of Agriculture, Food, and Environmental Sciences


Heather Liwanag

Advisor Department

Biological Sciences

Advisor College

College of Science and Mathematics


Sea otters (Enhydra lutris) have the densest fur of any animal, and the hairs function to trap a layer of air that is used for insulation. When a sea otter is born, it has a natal pelage (baby fur), and sea otters eventually molt that fur and replace it with a pelt resembling the adult fur. Sea otter fur is composed of 3 types of hairs: underhairs, intermediate hairs, and guard hairs. Sea otters are more susceptible to the negative effects of oiling from oil spills compared to other marine mammal species because they solely rely on fur for insulation. Though numerous studies have investigated the functional morphology of adult sea otter fur and how it is affected by oil, not much is known about the characteristics and insulative abilities of sea otter natal fur.

To better understand the structure and function of sea otter fur, I examined the hair morphology and related it to the thermal function and buoyancy of southern sea otter (Enhydra lutris nereis) pelts across different age classes. Guard hair length, hair circularity, guard hair and underhair cuticular scales, and hair density of sea otter pelts were measured across six age classes: neonate, small pup, large pup, juvenile, subadult, and adult. Thermal conductivity and thermal resistance of sea otter pelts were measured under three treatment conditions: in air, in water, and oiled. Pelt buoyant force in water was also measured under three treatment conditions: control (normal), oiled, and washed (with Dawn® dish soap). To scale up to the whole animal and examine differences related to body size, I used these measurements to estimate total heat loss, whole-animal buoyancy force, and mass-specific buoyant force.

I confirmed the timing of the transition between the natal fur and adult fur, which occurs some time in between the small pup and large pup age classes. I observed a consistent pattern in the hair circularity and shape of guard hairs across all age classes. It is important for all sea otters to have the ellipsoid shape of the guard hair so that the hairs can lie flat against the air layer to protect it in water. Comparisons of hair density indicated that sea otters with natal fur (neonate, small pups, and some large pups) have approximately 25-53% lower hair density than older age classes with the adult-type pelage. This thinner hair density may explain why young sea otters are always on their mother’s belly to stay out of the cold water.

When I examined the thermal function of the fur in air, I found that pups with natal fur have less efficient but equally effective thermal insulation. However, when I scaled up the thermal findings using body morphometrics and created a volume-based heat loss model, it was clear that younger sea otters have substantially more heat loss for their body size, compared to older age classes. Values for oiled heat loss demonstrated the vulnerability associated with sea otters relying on fur for insulation, and the large surface area to volume ratio associated with a small body size. It is already a high energy requirement for sea otters to maintain mammalian body temperate in cold water, and when the fur becomes oiled, the severe reduction of their insulation can cost them even more energy.

Across age classes, there was no difference in the buoyant force of the fur when I focused on the pelt samples from the laboratory-based experiments. When I scaled up the pelt buoyant force values to the whole animal using body mass, total body length, and surface area, it became clear that buoyant force varied across sea otter age classes. The smaller body size present in younger sea otter age classes allows for a higher mass-specific fur buoyant force. The larger size and higher hair density in adult sea otters explains the higher whole-animal fur buoyant force, as more individual hairs allow for more air to be trapped within the fur. Oiling of the fur reduces the overall buoyancy, and washing the pelts with Dawn® did not consistently restore the air layer in the fur.

In summary, the results of these studies indicate that the success of sea otters is due to the overall shape and structure of the individual hairs, along with the incredibly high hair density. The different hair types function together to trap the air layer in the fur to provide sufficient insulation. Ultimately, all ages of sea otters are vulnerable to the effects of oiling, with younger sea otters being the most susceptible.