Seals are marine mammals that spend their lives in environments where water temperatures are often near freezing. The internal body temperature of a seal typically remains around 38°C, a significant difference from the surrounding ocean water, which can be 0°C or colder. This large temperature gradient constantly pulls heat away from the animal’s body, a process that would quickly lead to hypothermia in a human. Seals are highly adapted to counter this heat loss through a combination of passive insulation and sophisticated biological controls.
Specialized Layers for Insulation
The first line of defense against the cold ocean water is a thick layer of subcutaneous fat known as blubber. This dense layer acts as a highly effective, passive thermal barrier, significantly slowing the rate at which body heat transfers to the cold water. In seals living in polar regions, this blubber can reach a thickness of between 3 and 5 centimeters, and it also serves as an important energy reserve for periods of fasting or migration.
True seals, or phocids, depend almost entirely on their thick blubber layer for insulation while in the water. Conversely, eared seals, such as fur seals, possess a dense underfur composed of thousands of hairs per square centimeter, which traps a layer of air against the skin. This trapped air is the primary insulator for fur seals; their blubber layer is often thinner and used more for energy storage than thermal protection.
Managing Heat Through Circulation
While blubber protects the main body mass, seals must employ active physiological strategies to manage heat loss from their flippers and tails, which lack a thick insulating layer. One primary mechanism is vasoconstriction, which involves the narrowing of peripheral blood vessels in the skin and extremities. This action restricts the flow of warm blood to the body’s surface, effectively creating a cooler outer shell and keeping the heat concentrated in the core organs.
The most sophisticated regulation occurs through a system called countercurrent heat exchange, which is particularly active in the flippers. In this arrangement, the arteries carrying warm blood away from the core are bundled closely alongside the veins returning cooler blood from the flippers. Heat flows directly from the warm arterial blood to the adjacent cool venous blood, pre-warming the returning blood and ensuring minimal heat is lost at the extremity. By controlling the amount of blood directed through this exchange versus the surface vessels, seals can precisely regulate how much heat is retained or dissipated. This allows them to prevent overheating when hauled out on land or during intense physical activity.
The Vulnerability of Seal Pups
Seals are most susceptible to cold during the pup stage, despite the robust adaptations of adults. Newborn pups initially lack the extensive blubber layer that provides adult insulation. Many species are born with a specialized coat of fine, dense, white fur called lanugo, which offers thermal protection while the pup remains on land. However, lanugo is not effective once thoroughly wetted, making young seals highly vulnerable to hypothermia if they enter the water prematurely. Pups must rely on a higher metabolic rate to generate internal heat and shiver, racing to develop the insulating fat layer before weaning. Once the blubber layer is sufficiently developed, the pup’s thermoregulatory capabilities are comparable to those of an adult, allowing them to better withstand the cold ocean temperatures.