great thread! Just want to mention this non-specific stuff:
Seals lost the pili muscles, which lift the hairs in furry land mammals for warmth, and make goose pimples on humans in cold. This is different than shivering muscles.
Also, furry land mammals have subcutaneous twitching muscles under the fur, this protects against biting/stinging/sucking insects. I think these are also different than shivering muscles, but the same type as those around the eyes which can close the eyelids instantly. (Maybe this is the origin of the Fast Twitch muscle fibers that lack myoglobin?) The eye region seems to have been the source of many uniquely mammalian physical traits, like fur coats derived from from eyelashes, eccrine sweat glands from saltwater processing tear glands, oil and apocrine skin glands from eyelid anti-dehydration glands.
I think seals, dolphins, sea otters and humans all lack twitching muscles. But I don't know if our primate relatives also lost them, I don't think so. Seems to me that a hairless ape in a jungle or savanna would be in big trouble without either a fly swatting tail and/or twitching muscles, but a seashore dweller living on tropical pocket beaches in warm seawater, sunny beaches and cool caves might be alright. Sand fleas might be a problem, but lemon grass apparently keeps them away. Also, Ochre mixed with wax/oil/fat was widely used as body paint, it may have protected against UV, insects, chill, sweating before clothing was first made.
AFAICT, prehistorically, shivering would typically only occur upon surfacing and breathe-up accompanied by brief all-over sweating while backfloating in the sunlit surface water, when laying on the beach sun drying off, like in seals, and perhaps at night during sleep on cool evenings.
During the actual dives, the water surface was warm, while at depth beneath the surface thermocline, the cooler water triggered a stronger (hypothermic) MDR instead of shivering(?). I guess that shivering can't happen when in MDR acidosis and/or muscles are lactic. Not sure about this though, need to test. When I dove a bit months ago, I was shivering and shaking everywhere, thanks to the damn winds that picked up once I got in the cold water. Made for a mighty short dive, but at least I got wet.
