Here are tidbits gleaned from these articles, together with my usual Internet sleuthing.
BMR. Basal metabolic rate is a key metric for this sea otter research. According to Wikipedia, “Basal metabolic rate (BMR) is the rate of energy expenditure per unit time by endothermic animals at rest.”
Endothermic animals, by the way, are those maintaining their own body heat, loosely, the “warm-blooded” ones.
Generally among mammals, the larger the species, the lower the BMR: As volume increases, surface area (through which body heat dissipates) increases more slowly. Thus, for instance, mice have much higher BMR than elephants.
The Sea Otter, a Skinny Outlier. Sacha Vignieri writes, “Several mammal species live in cold-water environments, enabled by adaptations such as blubber and large size. A notable exception to this rule is the sea otter, a species that is orders of magnitude smaller and skinnier than the others.”
Its Latin name, Enhydra lutris, comes from the Greek εν “in,” ύδρα “water,” and the Latin lutris, “otter.” Sea otters are the heaviest of the weasel family, but the smallest marine mammals.
As Wright, et al, note, “Their small size, combined with the high thermal conductivity of water (23 times that of air) and the cold water temperatures of their North Pacific habitat (0° to 15°C), imposes a thermoregulatory challenge to maintain a core body temperature of 37°C.”
For non-Celsius types, this water range is 32–59 degrees Fahrenheit; the sea otter’s body temperature is the same as ours: 98.6 degrees F. By the way, a handy memory aid is 20–30C= 68—86F.
The Sea Otter’s Trick. Researchers continue, “Unlike other marine mammals that have subcutaneous blubber for thermal insulation, sea otters rely on air trapped in their dense fur (the highest hair density of any mammal) for insulation. However, this is not adequate to offset heat loss, so an increase in metabolic heat production is required to maintain a stable core body temperature. As a result, sea otters have a basal metabolic rate (BMR) approximately three times that predicted for a eutherian mammal of similar size.”
Eutherian mammals are those whose offspring reach an advanced state of development prior to birth. Egg-laying platypuses and pouch-rearing kangaroos are non-eutherians.
The researchers found, “Compared with that of previously sampled mammals, thermogenic muscle leak capacity was elevated and could account for sea otter hypermetabolism.” That is, sea otters have evolved so that their muscles give off heat.
Mush, You Huskies, Mush! Curiously, another completely different species exhibits this muscular trait to an even more pronounced degree: Alaskan huskies—especially those of the Olympic sort. Wright, et al, calculated heat-generating physiology for a variety of mammals, including sea otters, Northern elephant seals, American quarter horses, Alaskan huskies, humans, mice, and rats.
The researchers observe, “We found that OXPHOS and ETS respiratory capacities [two relevant ratios] in sea otter skeletal muscle are modestly elevated compared with values measured in the muscle of other active mammals, but do not reach the extreme levels observed in some elite performance animals such as Alaskan husky Iditarod dogs.”
The researchers note, “Hence, the high BMR in sea otters is not exclusive, but it is the highest reported for any mammal with body mass >1 kg.”