Insulation to stay warm
Imagine yourself, being one of those hardy souls that accepts the challenge of a “polar bear plunge” festivity, by throwing their warm body into frigid ocean water. That is a challenge to any warm, mammalian body and just as all mammals, marine mammals need to maintain a body temperature of 98.6℉ (37℃). To be able to do that, they have evolved several anatomical and behavioral adaptations to neither overheat nor get too cold. For example, they use blubber and/or fur as insulation to be able to stay warm in frigid waters. (They also rely on body size to stay warm. See previous blog - Why are marine mammals so large?).
Most marine mammals have a layer of fat, called blubber, just under their skin to help with insulation. It works just like a blanket on land, preventing heat from escaping to the surroundings. Two groups of marine mammals, the fur seals and sea otters, have fur for insulation, just like land mammals. And just like most land mammals, including dogs and cats, their fur has two layers, guard hairs and under fur - the thicker and rougher guard hairs protect the finer underfur from wear and tear, while the underfur provides the insulation against the cold by trapping air and preventing water from reaching the skin. Compared to land mammals, however, the density of hairs on fur seals and sea otters is much higher (see the table below). This fine, dense coat of fur was very luxurious which is why these animals were almost hunted to extinction in the 19th century.
This insulation has allowed these species to inhabit the very cold waters in the arctic. However, just as with a large body size, the insulating layers can also create problems when it comes to heat loss. Marine mammals all live very dynamic lives, where the amount of body heat generated varies greatly, they can reach speeds of 30 mi/h (48 km/h) in the water, but may also rest or sleep in the water. Those that rely on blubber as insulation have special blood vessels (blood shunts), some that go through the blubber layer, moving warm blood to the skin where it can lose heat, other blood vessels keep the blood flow deeper in the blubber layer, conserving heat. These vessels can therefore be used to regulate body temperature, by leading, or shunting the blood closer to or farther away from the skin. We humans have a thin fat layer in our skin, as well as these blood shunts, which is how we blush (you can feel your face getting hot - losing heat) or get “red-faced” when exercising, or turn pale when cold.
It is a little more difficult for animals like fur seals and sea otters, that rely on fur to trap air as an insulating layer, to regulate their body temperature. Here, a tiny defect in the insulating air layer can spell disaster, as cold water suddenly can get to the skin and cool down the animal precipitously. As an example, if a sea otter gets a skin area the size of a pinhead exposed to seawater it could die due to exposure. And, since the insulating air layer is trapped in the fur, above the skin, there is no way to use the circulatory system to lose heat through the fur.
Instead, fur seals compensate by having large hairless flippers that the blood can be shunted through to lose heat. They are also known for adopting the “jug handle” position when they rest in the water, at the surface. Here they put one of their fore-flippers between their two hind flippers, to keep them all out of the cold water and stay warm. Holding their flippers in this position, not only serves to prevent heat from being lost through their flippers, but since the skin on the flippers is black, it also helps the fur seals soak up heat from the sun during the day. Sea otters, renowned for their even denser fur, lose heat through their paws in a similar way.
There is a lot more to say about the consequences of using fur vs. blubber as insulation and how it may impact the behavior of animals, but that has to be the topic for one or more future blogs.