CETACEANS
Whales, dolphins and porpoises are all known as cetaceans, from the Latin ‘cetus' (large sea animal) and the Greek ‘ketos' (a sea monster). Cetaceans are warm blooded, air breathing mammals. Just like land-dwelling mammals they give birth to live young and provide milk for nourishment, and unlike other mammals, cetaceans carry out their entire lives in a water environment.
Cetaceans range in size from the largest creature on earth, being the Blue Whale which can weigh up to 150 tonnes (sometimes more), to the smallest of the dolphins, the Franciscana and the Vaquita (actually a porpoise), which can weigh as little as 40 – 50 kg.
Classification of Cetaceans
There are seventy-eight (78) species of whales, dolphins and porpoises, all of which belong to the taxonomic order Cetacea. Within this order, modern cetaceans are divided into two suborders that can be distinguished by the type of feeding equipment found in the mouth. The smaller of the two suborders is Mysticeti, consisting of the eleven (11) species of baleen whales (toothless, filter-feeding cetaceans). The second suborder is Odontoceti, comprising sixty-seven (67) species of toothed whales and dolphins. There was once a third suborder in the order cetacea called Archaeoceti (the ancestors of modern cetaceans). These primitive cetaceans became extinct approximately 16 million years ago.
In addition to the type of feeding equipment in the mouth, you may also tell which suborder an individual belongs to by the number of blowholes (nostrils) it possesses. All species in the suborder Mysticeti (baleen whales) have two blowholes, and those in the suborder Odontoceti (toothed whales) have one blowhole.
Suborder Mysteceti
The eleven species of the suborder Mysteceti (baleen whales) may be divided into three or four families. There were three recognised families, however some taxonomists now suggest there are four.
The family Balaenopteridae contains the six species of rorqual whales such as the Blue Whale, Humpback Whale and other similarly shaped whales.
The family Eschrichtiidae is represented by one modern species, the Grey Whale.
The family Balaenidae once included all four species of Right Whales and this is still recognised by many people. However some taxonomists favour placing the Pygmy Right Whale in a family of its own, hence the fourth family – Neobalaenidae.
Suborder Odontoceti
The sixty-seven species of the suborder Odontoceti (toothed whales) comprises seven families. The Physteridae family contains the three species of Sperm Whales, however some taxonomists consider that the two smaller species (Pygmy Sperm Whale and Dwarf Sperm Whale) should be placed in their own family (Kogiidae).
The Monodontidae family is represented by three species, the Narwhal, the Beluga and the Irrawaddy Dolphin. The Irrawaddy Dolphin was once classified as belonging to the Delphinidae family (oceanic dolphins) but is now considered part of the Monodontidae family.
The Ziphiidae family covers the eighteen species of Beaked and Bottlenose Whales (not to be confused with the Bottlenose Dolphin).
The Globicephalidae family (Pilot and Killer Whales) is sometimes included, by taxonomists, as a subfamily in the Delphinidae family, though we list it here as a separate but closely related family. There are six species of Pilot and Killer Whales in the Globicephalidae family.
The Delphinidae family contains twenty-six species of oceanic dolphins, the most familiar to us being the Bottlenose Dolphin (Tursiops truncatus). There is some confusion as to the number of species in this family – there are twenty-six if the three species of Humpback Dolphins are recognised. Some taxonomists classify only two species though.
The Phocoenidae family is represented by the six species of porpoise. Many people confuse dolphins and porpoises, often due to the American custom of calling dolphins porpoises. Porpoises do belong in their own family and are quite different to a dolphin.
The final family in the suborder Odontoceti is the Platanistidae family, including five species of river dolphins. Some taxonomists say that the Ganges River Dolphin (or susu) and the Indus River Dolphin (or Indus susu) should be considered subspecies. They are generally regarded as separate species though.
Evolution of Cetaceans
The water-dwelling cetaceans that we know today were once land based animals. Scientists are unsure of the exact reason why the ancestors of whales, dolphins and porpoises took to the sea millions of years ago. Fossils show that their bodies adapted gradually to an aquatic way of life. Many changes or adaptations were required for the move from land to a purely aquatic lifestyle, including : the transition from walking to swimming, changes to the body shape, an efficient means for breathing at the waters surface and an alternative method of locating and catching prey.
The transition from walking to swimming was brought about by changes to the shape of the body. As water is a denser medium than air and therefore harder to move about in, a streamlined torpedo-shaped body was required to slip easily through the water. As cetaceans evolved they developed a broad flat blade (known as a fluke) at the end of the tail to be used as their main source of propulsion. To assist movement cetaceans also developed greater flexibility in the spine and larger blocks of muscle attached to the top and bottom of the spine in the rear third of the body (the tail) for powerful upward and downward motion. Paddle-like blades are more efficient for power and propulsion in the water than the limbs of a land based mammal. Progressively the forelimbs were modified into blades, known as pectoral fins, for steering and balance. The hind limbs disappeared altogether, the only evidence of these in today's cetaceans being in the skeleton beneath the blubber and muscle in the abdominal region. The dorsal fin was developed for balance and stability and, with its great number of blood vessels, may also aid in temperature regulation. To complete their streamlined shape the teats and sex organs were hidden beneath slits in the body wall and the external ears were reduced to a small opening flush with the skin. The loss of body hair also aided streamlining, however as hair is an insulator on land mammals, cetaceans developed a thick layer of blubber just beneath the thin layer of sensitive skin.
An efficient means for breathing at the water's surface was required for an aquatic life, as was a method for locating and catching prey. The nostril or blowhole moved from the front of the skull, as in many land mammals, to the top of the skull. The top of the skull is the first part of a cetacean to break the surface of the water and allows the animal to breath with a majority of its body remaining below the surface, particularly useful when moving at speed. Another change to the skull was in regard to the jaw (rostrum) and teeth. The jaw either became elongated or broadened depending on the species and its diet. The teeth changed from several different types of teeth, with which land mammals have different uses (e.g. grasping, cutting and chewing), to having uniformly shaped teeth for grasping only or baleen plates for filter feeding in the case of non-toothed whales. The toothed whales, dolphins and porpoises developed a structure of fatty tissue (the melon) on the rostrum that is used to focus and project sound. The melon combined with oily tissue in the lower jaw that receives sound and transfers it to the inner ear, allows cetaceans to send out sound waves. These sound waves hit objects in the surrounding area and echo back to the animal giving it a three-dimensional picture of what is around them. This is called echolocation or sonar and is used for locating and catching prey as well as navigation. |
