Introduction

For my Ph.D. thesis, I studied differences between fur and blubber as insulation in mammals, with a focus on the otariids (fur seals and sea lions). The otariids represent the only mammalian family to include both types of insulation: fur seals have dense, waterproof fur and a moderate blubber layer, while sea lions rely solely on their blubber for thermoregulation in water. I examined the thermal properties of the fur and blubber of various otariid species, and comparing the physiological and behavioral responses of the California sea lion and northern fur seal to different temperatures.

Thermal properties of fur and blubber

Thermal conductivity of sculps
To examine the thermal properties of fur and blubber, I measured the thermal conductivity of sculps (fur, skin, and blubber) of various fur seal and sea lion species. Thermal conductivity is a measure of how easily a material allows heat to pass through it. The lower the thermal conductivity, the better that material serves as an insulator.

Lipid and water content of blubber
To compare the composition of the blubber layer among species, I determined the lipid and water content of the blubber. A greater lipid content and lower water content correspond with a lower thermal conductivity.

Fur density
To compare the fur among species, I determined the density of the fur. Sea lions have a lower fur density compared to fur seals, due primarily to a comparative lack of underfur. Underfur are small, fine hairs that help the animal trap air between its skin and the surroundings. Fur seals have lots of underfur, but the exact amount differs among species.

Effects of hydrostatic pressure

Masako Abney, a 2006-2007 senior thesis student, examined the effects of hydrostatic pressure on the air layer in pinniped fur. As an animal dives in the ocean, it experiences great pressure changes associated with being underwater. Masako found that true seals and sea lions are not able to keep air in their fur under pressure, while fur seals do maintain an air layer at depth. However, the air layer is compressed greatly under pressure, and this will reduce the effectiveness of the fur as insulation.

Thermal neutral zone of California sea lions and northern fur seals

Thermal neutral zone
One measure of an endotherm's thermal capability is its thermal neutral zone (TNZ). The TNZ is defined as the range of environmental temperatures in which an animal does not have to increase its metabolism above resting levels to maintain its core body temperature. Below the lower critical temperature, an animal must increase its metabolism to offset heat loss, which is energetically expensive over the long term. Above the upper critical temperature, an animal must also increase its metabolism, in order to fuel heat dissipating mechanisms.

California sea lions
I examined resting metabolic rate of five adult female California sea lions across a range of water temperatures. I found that the lower critical temperature of these animals is several degrees lower than sea surface temperatures routinely encountered by this species off coastal California. This indicates that adult female California sea lions are thermally capable in their natural environment.

Northern fur seals
I measured resting metabolic rate of three northern fur seal pups over a range of experimental water temperatures, and determined their lower critical temperature. The lower critical temperature of these pups is close to that of adult California sea lions, even though northern fur seal pups are much smaller (~75% smaller) than adult sea lions.

Jughandling behavior

Northern fur seals pups wean at four months of age, and embark on a pelagic phase for up to nine months. During these extended trips to sea, northern fur seals need to rest, and often perform a behavior called jughandling, in which the animals float with one foreflipper held between both hind flippers above the water’s surface. The purpose of this behavior is unknown, but popular hypotheses indicate opposing thermoregulatory consequences: either the animals remove the flippers from the water to avoid heat loss to cold water, or they position the flippers in the air to increase convective heat loss when warm. I measured jughandling metabolic rate, and found that this unique behavior is energetically costly to northern fur seal pups, but it may help to mitigate thermal costs below the lower critical temperature.

Huddling behavior

Otariids are highly communal pinnipeds that often congregate in large numbers on coastal rookeries. Although this behavior serves a social role, it also has the potential to change the microhabitat, and thus the local thermal conditions experienced by the animals. We would expect pinnipeds to be overinsulated on land, but they have been observed to huddle extensively on shore. I quantified huddling behavior in California sea lions, and found that there is a significant thermal benefit to huddling behavior in these animals.