https://www.science.org/doi/10.1126/science.abf4557

Keeping warm when small
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.
It is
known that the sea otter's unusually thick fur helps, but Wright et al.
show that
they are also internally warmed by thermogenic leak from skeletal muscle, a
process that elevates their metabolic rate three times above that expected
for
their size. This mechanism is present even in infants with immature muscles,
providing these animals with internal warmth from birth.

Abstract
Basal metabolic rate generally scales with body mass in mammals, and
variation
from predicted levels indicates adaptive metabolic remodeling. As a
thermogenic
adaptation for living in cool water, sea otters have a basal metabolic rate
approximately three times that of the predicted rate; however, the
tissue-level
source of this hypermetabolism is unknown. Because skeletal muscle is a major
determinant of whole-body metabolism, we characterized respiratory capacity
and thermogenic leak in sea otter muscle. Compared with that of previously
sampled mammals, thermogenic muscle leak capacity was elevated and could
account for sea otter hypermetabolism. Muscle respiratory capacity was
modestly
elevated and reached adult levels in neonates. Premature metabolic
development
and high leak rate indicate that sea otter muscle metabolism is regulated by
thermogenic demand and is the source of basal hypermetabolism.