4.2 Insulation in aquatic endotherms
Most seals and sealions (order Pinnipedia) are furred. The adult fur usually consists of short, dense stiff guard hairs that are oily from profuse secretions of the sebaceous glands. The hair probably acts like a wet suit of a human diver: a layer of water is trapped around the hair, where it is warmed by body heat and prevents much colder water from coming into direct contact with the skin. Short, oily fur dries quickly when the seals come onto land. Two genera, the northern fur seal (Callorhinus ursinus), in the north Pacific (Figure 20), and eight species of southern fur seals, Arctocephalus spp., in the Pacific and Southern Oceans, have a dense layer of underfur throughout their lives that traps small bubbles of air and keeps the skin dry.
Are there any disadvantages of such insulation for aquatic mammals?
During deep dives, the pressure of the water would compress the air bubbles, greatly reducing the insulating efficiency of the fur and increasing the possibility that the skin is wetted. The air also makes the mammal or bird more buoyant, thus hindering diving and swimming underwater.
This kind of insulation is most common in semi-aquatic mammals such as beavers, muskrats and otters, and in the feathers of ducks and penguins, which live mainly in shallow water and spend long periods exposed to very cold air. Newborn seals have a fluffy coat, quite different in both colour and texture from that of the adults, that provides good insulation in air but is ineffective in water. Seal pups cannot survive in very cold water for longer than a few minutes until they moult the neonatal coat and grow the firm, darker adult fur that provides better protection in water. Because the fur is an efficient insulator in air, adult fur seals and the juveniles of many other seal species have long been hunted for their pelts.
In view of the effectiveness of insulating fur among seals, it may seem surprising that one of the most northerly pinnipeds, the walrus (Odobenus rosmarus), and all the whales and dolphins (order Cetacea) are almost hairless, even as neonates. In these marine mammals, the skin and a specialized form of fibrous adipose tissue called blubber are the major insulators. Walrus skin is up to 5 cm thick and the blubber, although minimal or absent over the tail, flippers and parts of the head, is up to 15 cm thick over some areas of the neck and trunk (Figure 21). Measurements on dead tissues indicate that conductance of heat is about half as efficient through adipose tissue as through aqueous tissues such as muscle. However, such passive properties are probably much less important to thermal insulation than counter-current systems of blood vessels, and control of the rate of flow of blood through the tissue, that brings heat from the warm core to the surface. Blood flow through the superficial adipose tissue can be reduced almost to zero for hours without damaging the adipocytes.
Why cannot the blood flow to muscle be similarly reduced?
Muscle is much more metabolically active than adipose tissue and cannot remain functional unless supplied with sufficient blood-borne nutrients and oxygen. Deprivation of blood for longer than a few minutes causes permanent damage to most muscles.
When basking out of the water, the skin of walruses is so flushed with blood that it appears pink (Figure 21), particularly in warm weather, which is when they are most often seen by hunters, photographers and biologists. As soon as walruses enter cold water, blood vessels in the skin and outer layers of the blubber constrict, shutting off the circulation almost completely. The animals become dull grey in colour, and much less heat is lost at the surface. During strenuous exercise, or when in warmer water, perfusion can be increased, thereby adjusting accurately the rate of heat loss to internal heat production, as happens in counter-current mechanisms.
Why is the largest area of pale pink skin in Figure 21 a symmetrical patch between the walrus's eyes? Hint: compare this photograph with Figure 18a and c, and think about where you sweat most when hot!
The pale patch covers the walrus's forehead, which overlies the forebrain, a metabolically active and functionally important region of the brain that is impaired by even small changes in temperature. Blubber under this area of skin is much thinner than that on the neck and body, perhaps absent altogether. All higher mammals, including dogs (Figure 18a) and bears (Figure 18b and c) lose a lot of heat from this area of the head, protecting the brain from overheating. The human brow sweats profusely and is a good site from which to measure minor changes in overall body temperature, as in fever.
Vigorous exercise is rarely necessary for feeding, because walruses eat mainly bottom-dwelling invertebrates, especially clams and similar burrowing molluscs, which they locate with their sensitive vibrissae (Figure 21), and dig out by squirting jets of water. Being so large (up to 1.5 tonnes), adult walruses have few predators except killer whales and, occasionally, polar bears. Adipose tissue has the advantage of being almost incompressible and, although fat is less dense than water, it contributes less to buoyancy than air trapped in the pelt.
However, restricted blood flow is incompatible with certain other functions of the superficial tissues. Walruses shed the outer layers of the skin each year, possibly as a means of getting rid of external parasites. To support regrowth of the skin, blood perfusion of the superficial tissues is plentiful throughout the moult, and walruses normally spend almost the entire period basking on beaches or ice-floes. Although moulting takes place in mid-summer, walruses can die of cold if forced to spend too much time in the water during this period. Cetaceans spend their entire life in water and moult less efficiently, enabling barnacles and ectoparasites to colonize their skin.
Thick layers of superficial adipose tissue also contribute to insulation in other species of seal, although the relative importance of fur and fat probably differs greatly between the seasons and in different species. The distribution of adipose tissue in most adult seals, dolphins and the small toothed whales suggests that it is adapted to contribute to thermal insulation: the superficial blubber forms an almost continuous layer, albeit of very variable thickness, and adipose tissue is almost absent from inside the abdomen and muscles.
All kinds of seals that have been investigated have surprisingly little superficial adipose tissue at birth, the superficial depots being only 2–4 mm thick in northern fur seal pups that weigh 5–6 kg at birth. Their thick natal coats keep them warm in dry weather on land, but, although their BMR can increase to as much as 18 W kg−1, seal pups quickly become hypothermic if immersed in water or during heavy rain. Furthermore, the distribution of adipose tissue of neonates resembles that of typical terrestrial mammals: as well as several superficial depots, there are significant quantities of adipose tissue inside the abdomen, around the kidneys, and in the pericardium. Some of these internal depots contain adipocytes which appear under the electron microscope to have features in common with BAT: mitochondria are quite numerous but they lack cristae. However, although it may be thermogenic to some extent, the tissue is not true BAT in either structure or metabolism.
Birds replace old, worn feathers with new ones, usually one or twice a year, often just before breeding or migration. Moulting and replacement of the plumage impose heavy demands on the nutrient reserves because large quantities of energy and protein are used in the synthesis of new feathers. Foraging is also difficult or impossible: most large birds cannot fly (in the absence of primary wing feathers) and polar species do not swim because their insulation is so severely impaired that they would become too cold in water. The moult takes 2–5 weeks in emperor penguins and king penguins (Aptenodytes patagonica), during which time they remain on land (or on ice-floes) and fast, losing up to 45% of their body mass and up to 50% of their protein reserves.