Cetaceans are a group of marine mammals that include whales, dolphins and porpoises. The group is divided into mysticetes (baleen whales) and odontocetes (toothed whales, dolphins and porpoises).
Cetaceans are thought to have evolved from land based animals living in brackish waters approximately 55 million years ago, in the region that is now the Mediterranean Sea and Middle East. They are thought to share a common ancestry with artiodactyls, such as hippos, antelope and deer.
Cetaceans have a streamlined shape, even more pronounced than in pinnipeds, aided by shortening of the forelimbs and virtual loss of the hind limbs. The well-developed blubber layer of cetaceans also helps to reduce drag. Blubber consists of fat reinforced by collagen and elastic fibres and it improves the cetacean’s streamlined shape by rounding off all bony protuberances. Drag reduction is further enhanced by laminar flow over the exceptionally smooth skin of cetaceans, which is lubricated by the continual shedding of epithelial cells and exuding of oil droplets from these cells.
This is provided by the broad, flat tail fluke, which is composed of fibrous tissue (not bone) and shaped like a hydrofoil. The fluke moves vertically through the water and the rear third of the cetacean arches as the tail fluke moves up and down. The pectoral flippers, used in steering and balance, and the dorsal fin aids with stability.
Cetaceans inhale at the beginning of a dive. The nostrils are on top of the head and are known as the “blowhole(s)” (single in toothed whales, paired in baleen whales). When a cetacean surfaces, the blowhole opens and an explosive exhalation occurs. This is immediately followed by an inhalation and the blowhole closes tightly again. The blowhole remains closed during dives. The amount of air taken down is inadequate to sustain the dive, and this air is also greatly compressed at depth. The diaphragm of cetaceans is set at an oblique angle and, at depth, the abdominal contents press against the lungs, causing the lungs to collapse and pushing any air into the trachea, where no gaseous exchange can occur. This is one of the ways whales and dolphins avoiddecompression sickness (‘the bends’). Oxygen needed for diving is stored in pigments in the muscle and blood (myoglobin and haemoglobin) in large quantities. Cetaceans have a relatively large blood volume, and blood is diverted away from non-essential tissues during dives, thus ensuring that essential organs receive a rich, oxygenated blood supply. A pronounced slowing of the heart (bradycardia) also helps to conserve oxygen during dives (it may fall to 10% of the rate on the surface). On very long dives, whales can switch to anaerobic metabolism (i.e. metabolism without oxygen), although this requires a longer period of recovery on the surface between dives.
DIVING TIME VARIES GREATLY WITH THE SPECIES
Dolphins spend the majority of time within 100 metres of the surface, hence their diving time is short (e.g. bottlenose dolphins: approximately 7 minutes). Baleen whales are similar, but some of the toothed whales do dive to great depth for a long period of time, particularly sperm and northern bottlenose whales (over 1000 metres for up to 1.5 hours). The diving reflex of cetaceans becomes important when assessing an animal on the beach.
Cetaceans live in an environment with 25 times the heat conductivity of air, and thus heat conservation and thermoregulation is essential to survival. The rounded, cylindrical shape of cetaceans results in a relatively low surface area to volume ratio, which helps to restrict heat loss. Insulation is provided by the thick blubber layer of cetaceans, which accounts for up to 30 to 40% of their bodyweight. In those areas without any blubber, (the pectoral flippers, tail fluke and dorsal fin) heat is conserved by a countercurrent heat exchange system. Here, heat is exchanged between the warm blood in arteries supplying the periphery and the cold blood, returning from the skin surface, which passes through a plexus of veins surrounding each artery. In this manner, little heat is lost to the periphery. If heat needs to be dumped, e.g. after bouts of strenuous activity, the blood flow through these arteries are increased. Flow is also increased in the arteries that pass through the blubber layer to the skin, generally, allowing heat loss over the skin surface.
Several methods have evolved in cetacean feeding. Baleen whales sieve their food from the water. They take large quantities of water into their mouths and then filter it through the parallel plates of keratin (baleen) that have replaced their teeth. Their diet consists of krill and, in species with coarser baleen, small fish. Toothed whales and dolphins catch fish and squid in very much the same manner as seals (see later notes). Their eyes are well adapted to vision below and above water, as their lenses are very elastic, changing shape through the action of powerful muscles to allow for the different refractive indices of the two media. Constriction of the pupil occurs in bright light. Their hearing is acute underwater, and their ability to catch fish (and avoid predators such as sharks) is further improved by the use of a system of sonar known as ‘echolocation’. Here, high frequency clicks, thought to be emitted from the upper nasal passages below the blowhole, are focused by the melon (a fat pad in front of the skull) and, on being reflected by prey, are received probably by the lower jaw (mandible). The sound is then transmitted possibly along a fat filled canal within the bone to the middle ear.
This information has been kindly provided by the British Divers Marine Life Rescue.