Polar oceanic water can get as cold as about –2°C! Even temperate or tropical waters might drop as low as –1°C if deep enough. This makes you think, how do marine mammals survive arctic temperatures? In this post, we’ll take the artic plunge into thermoregulation in marine mammals.
Let’s Talk Ratios: Size matters!
One way that animals can reduce heat loss is by having a low surface area relative to their total body volume. Let us explain: Imagine the whale is a sphere. If you increase the size of a sphere, its volume will increase dramatically compared to its surface. Heat exchange with the environment occurs at the skin level (i.e., the surface). Larger animals typically harbor a small amount of skin (compared to their body size). They also have a large volume of body tissue. Body tissue generates the animals’ heat through biological processes.
This ratio, for marine mammals, allows them to have the smallest relative surface area that comes in contact with the water, thus reducing the amount of heat exchange. And because they are in the water, they don’t have to worry about the effects of gravity.
Shiver me, Blubber
Whales and seals rely instead on their thick blubber to insulate their bodies in cold water. Blubber is a thick layer of fat (adipose) tissue. Adipose tissue has a relatively low thermal conductivity, meaning that it does not transfer heat as other tissues do, like muscle or skin. The thickness of the blubber coat varies among species and time of year: for example, humpback whales generally have blubber layers around 6 inches thick, while after their feeding season, right whales and bowhead whales can have a blubber layer; up to 50cm thick (20 inches)! Then through conduction, the outermost skin layer is cooled to the same temperature as the surrounding water, thus further reducing heat loss.
Thicker blubber layers also make certain species more buoyant. That’s where the name “right whale” originated. Even when dead and during the whaling days, right whales would float at the surface, making it easier to catch them. So they were considered the “right” whale to kill. You can read more about the North Atlantic Right Whale in our June 2020 Whale of the month post.
Speaking of right whales and other Arctic whales, have you noticed how Arctic whales do not usually have a pronounced dorsal fin or super long flippers? It is all an excellent strategy to retain more heat, as the highly vascularized appendices tend to lose heat like crazy; smart, right?
Another way whales reduce both heat loss and drag is by internalizing their genitalia instead of being external (like most terrestrial mammals). It helps them remain more streamlined too!
Hair instead of fat?
Terrestrial mammals utilize fur to keep warm, so one might think their aquatic counterparts might as well, but they don’t! Fur works as an insulator because it traps air, thus creating an insulating layer. However, the atmospheric pressure beneath the waters’ surface causes the air to compress and lose its insulating power. However, sea otters have a trick up their sleeve: they have two layers of very thick fur- an undercoat and a longer guard hair. This double layer traps air, keeping the sea otter warm.
How to Keep your Cool…
Now, there are times when marine mammals need to release some excess heat. They utilized areas called thermal windows. They lack fat and are relatively thin and highly vascularized; these areas include flippers, dorsal fins, and flukes.
But they don’t want to lose too much heat. It could result in cold shocking the heart! Marine mammals prevent this through a system called counter-current heat exchange. Very close together, the arteries and veins in these tissues flow in different directions allowing heat to transfer across membranes. The warm blood leaving the heart will heat the cold blood headed back to the heart from the extremities. That way, the heart is continuously pumping warm-ish blood! Counter-current heat exchange also decreases heat loss in those thermal windows.