Aquatic Adaptations

Aquatic life requires facing up to a number of extreme situations not encountered by terrestrial life. One of these is thermoregulation. Contact with water quickly steals heat from the body, with water absorbing such heat about 27 time faster than air. Many aquatic animals such as seals and whales inhabit arctic or antarctic frigid waters, and water encountered during deep dives is only slightly above freezing. Such animals as river otters and muskrats have guard hairs over thick underfur that normally is not wetted by water, thus providing insulation. Such animals also are able to leave the water environment at will. The most extreme aquatic adaptations, of course, are those of the Cetacea, members of which cannot leave the aquatic environment and who lack fur.

Whales control their body temperature through several mechanisms. The larger ones, particularly, have a favorable volume to surface ratio. As with mammals and other endotherms in general, this ratio appears to play a large part in metabolism. Thus shrews, with their very high relative surface area must burn enormous amounts of food (for their size), whereas large endotherms require much less per body-weight unit. Also of tremendous importance is the presence of subcutaneous blubber that acts as insulation. Blubber thickness varies not only with the size of the species, but also seasonally. Large whales at high latitudes may have as much as 70% of their body mass represented by blubber. Other highly aquatic mammals, such as pinnipeds, also tend to have heavy blubber deposits.

Another feature of temperature control in whales is a well developed counter-current heat exchange system (rete mirabile). Arteries carrying blood into extremities not well padded by blubber (such as the flippers) are surrounded by a network of veins. The result is that the arterial blood heats the returning venous blood. Such counter-current heat exchange systems are quite common in endothermic vertebrates, notably in aquatic birds who may have their lower legs and feet immersed in cold water for extended periods of time.

Deep divers face the problem of pressure. Pressure increases about 1 atmosphere for every 10 m depth. Thus diving to only 30 m results in about 60 lbs/sq in and at 1000 m some 1500 lbs/sq in. The skeletal structure is relatively incollapsible, while the airways above the lungs are supported by cartilage and relatively rigid. The lungs collapse during deep dives (below ca. 30 m), with any residual air pushed to the upper, rigid portions of the respiratory system that have little or not gas exchange capabilities. This system allows whales to escape the bends. Deep diving with air in the lungs results in absorption of large amounts of nitrogen into the blood; when a body surfaces more rapidly than the nitrogen can get out of the circulatory system, bubbles form in various parts of the body, causing pain, damage, and often death.

With the lungs collapsed, how do whales remain active under water (up to 2 hours in some)? Several adaptations are involved. About 12% of the oxygen in the lungs (when the whale if ventilating on the surface) is absorbed, compared to about 4% in terrestrial mammals. Also, the density of red blood cells is about twice that of land mammals. Of great importance is the fact that the amount of myoglobin is some two to nine times as much as land animals, allowing much storage in the muscles rather than depending on "holding your breath". Other stratagems include bradycardia: slowing of the heart. Along with this, vasoconstriction reduces the amount of blood going to the extremities and other parts of the body not requiring a high level of oxygen; arterial supply is maintained to the brain, etc. The central nervous system has a high tolerance to carbon dioxide, which builds up during a dive, and the body is tolerant of high lactic acid concentrations.

Order Cetacea

The whales include what probably are the most specialized of all mammals and include the largest animals known, past or present. All whales are completely aquatic, accomplishing all roles in life in water. Common morphological features include a fusiform body, absence of sebaceous glands, almost complete lack of hair, and a thick layer of blubber. There is no clavicle. The forelimb is paddle-shaped with no external digits or claws; only the shoulder joint allows movement. The proximal segments of the forelimb are short, but the distal portion is elongated because of hyperphalangy (phalanges in excess of the normal mammalian number). The hind limbs are vestigial, not attached to the axial skeleton, and not visible externally. The notable differences between different portions of the vertebral column are mostly absent, and high neural spines on the vertebrae is the general rule. The tail is expanded horizontally to form the flukes.

In the skull, the nostrils (nares) have migrated to the top of the skull with resultant remodeling of the skull. Much of the dorsum of the skull is formed by the premaxillaries and maxillaries, while the posterior portion is formed by the occipitals and the other roofing bones, nasals, frontals, and parietals, are squeezed in between. The bones housing the middle and inner ear is not braced against the skull.

In recent years, a number of fossils have been found that show intermediate stages in the development of whales from terrestrial animals.

Two suborders divide the whales into the whalebone and baleen whales (Mysticeti) on the one hand and the toothed whales (Odontoceti) on the other. The former group lacks teeth but has sheets of horny material growing from the upper jaws that act as filters to separate plankton from water. The Odontoceti have simple, peg-like teeth, utilize echolocation, and have asymmetrical skulls in the blowhole region.

The Animal Diversity Web has numerous pictures and other information.


Last Update: 4 Feb 2007

Centennial Museum and Department of Biological Sciences, The University of Texas at El Paso