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What is a Cetacean?

Whales, dolphins and porpoises all belong to a single group of marine mammals called the cetaceans. A cetacean is any of the members of the Order Cetacea. This is defined as being the order of placental mammals having no hind limbs and a blowhole for breathing. The word is pronounced see-tay-shun and comes from the Latin “cetus” and the ancient Greek “ketos” which mean a sea monster.
Classification of cetaceans Living things can be divided up into at least 5 Kingdoms: Animals, Plants, Fungi, Protists and Bacteria.

Plants and Animals are both multi-celled organisms with complex cells. Plants have rigid cell walls made up of cellulose and can utilize the sun’s energy in order to build up carbohydrates using carbon dioxide and water. Animals cannot utilize the sun’s energy in this way and do not have rigid cell walls so they are free to change shape. This has allowed them to evolve muscle cells and therefore movement. Since they cannot use the sun’s energy they have to obtain their energy from other organisms, such as plants. They are therefore referred to as heterotrophs – meaning feeding from others as opposed to plants which are autotrophs or self-feeders.

Fungi are made up of organisms which, like animals, need to absorb their energy from other organisms, but have rigid cell walls. They often look like plants but they have a completely different way of life. Good examples of a fungus are mushrooms and toadstools but it is also a fungus which causes the condition known as Athlete’s foot. The fungus is “eating” the skin of the foot!

Protists are basically organisms which don’t fit in anywhere else. They are all single-celled but their cells are quite complex unlike the fungi. A well-known example of a protist is an Amoeba.

Bacteria are very tiny single-celled organisms which have a cell structure very different to all the other living organisms. Some bacteria can use the sun’s energy in the same way that plants can. Because of their cell structure they are referred to as Prokaryotes. All the other living organisms are referred to as Eukaryotes.

Minke Whale; Copyright IFAW; www.ifaw.orgWithin the Kingdom of Animals there are many different groups. Most of the world’s animals are invertebrates meaning that they have no backbone. The invertebrates are made up of more than 30 groups or phyla. These include the Arthropod phylum which contains the insects, spiders and crustaceans, the Molluscs such as snails, the Cnidarians such as jellyfish and sea anemones, the Echinoderms such as starfish and sea urchins and the Annelids which include the earthworm.

Vertebrates all have a backbone (vertebral column) and a central nervous system. Their skeleton is inside their body, unlike the insects or crustaceans and they have keen senses and generally a large brain. This forms part of the well-developed nervous system which is needed to process the information from their sense organs. Reptiles, birds, amphibians and fish are all vertebrates as are the mammals, which is the class to which humans and cetaceans belong.

Cetaceans and humans both belong to a sub-class of mammals called the Placental Mammals (the sub-class Eutheria). These all have the following features:

  • skin with hairs
  • pentadactyl limbs (made up of an upper limb, a lower limb and a hand/foot)
  • a muscular diaphragm which divides the thorax from the abdomen
  • they are endotherms, maintaining their body temperature at 37*C
  • fertilization of the ovum takes place within the female’s body
  • the embryo completes its early development within the uterus
  • when the young are born they are fed milk produced form the mother’s mammary glands.

All living organisms are related and have similarities to others. In order to work out which groups they should be placed into it must be remembered that superficial features may lead us astray. Dolphins and sharks look very similar because they have both adapted to a similar way of life. They both live under water and are streamlined to allow them to move smoothly through the water. However sharks obtain their oxygen from the water which passes over their gills, whereas dolphins have lungs and exchange gases with the air by breathing. Inside a dolphin’s flipper are little bones arranged in rows as in a human hand. A shark’s fin may look very similar on the outside but inside it has no bones, only long stiff rods.

Compare their classification:

Classification Dolphin Shark
Common name Bottlenose dolphin Shortfin Mako
Kingdom Animalia Animalia
Phylum Chordata Chordata
Subphylum Vertebrata Vertebrata
Class Mammalia
(cartilaginous fish)
Subclass Elasmobranchii
Order Cetacea Lamniformae
Suborder Odontocetes
Family Delphinidae Lamnidae
Genus Tursiops Isurus
Species Truncates Oxyrinchus
Scientific name Tursiops truncates Isurus oxyrinchus

As can be seen from the table above, the bottlenose dolphin is a member of the sub-order Odontocetes. Cetaceans are divided into two sub-orders:

Odontocetes – toothed cetaceans containing about 68 species of whales, dolphins and porpoises all of which have teeth. These can be placed into 6 families of which the largest is the Delphinidae. This contains over 30 species of dolphins including the Orca or Killer Whale and the pilot whales.

Mysticetes – baleen cetaceans or moustached cetaceans containing 10 or 11 species, all of which have baleen plates growing from their upper jaw and lack any teeth. These are placed into 4 families. The enormous blue whale is a member of the Balaenopteridae, the family of rorqual whales.

To find out more about the specific features of these two sub-orders please click on the photos below:

Cetacean Reproduction

Being mammals thBottlenose Dolphin; Copyright IFAW; www.ifaw.orge cetaceans give birth to live young which are suckled by their mother. They also show very complex patterns of reproductive and parental care. In order to maintain their streamlined shape the genitals and mammary glands are hidden within the body. The anus and genital openings are separate and the females have “mammary slits” either side of the urogenital opening within which the nipples are found.

Sexual maturity occurs from 2 to 20 years depending on the species and the gender. Orca females reach sexual maturity at about 7 years whereas the male Orca begin to show growth of their dorsal fin at that time but do not become sexually mature till they are between 10 or 12 years of age. They do not attain their adult size until they are about 14 years old.

Some species of cetacean have developed very complex mating rituals including sounds and movements. For instance, male humpback whales produce a distinct variety of songs when they are seeking a mate. Some species, such as the sperm whale, have schools of females with a single bull whereas other species live within small family groups or pods. Sexual activity in many species is not restricted to reproductive cycles but occurs frequently and through all seasons suggesting that it may have a wider social meaning. In migratory species such as the minke whale, hormonal activity rises as they reach the breeding areas. Other non-migratory species such as the bottlenose dolphin may breed throughout the year since their environment changes very little with the seasons. Fertilisation takes place internally with the male penis being inserted within the female’s urogenital opening. Mating occurs belly to belly and due to the difficulties of manoeuvring underwater courtship rituals play an important role.

Competition between the males of some species can be quite fierce as they fight for the right to mate. Narwhal males use their tusks to fight and this can lead to fatal injuries.

Pregnancy varies from 8 to 16 months. The bottlenose dolphin for example has a gestation period of 12 months. A single calf is born usually tail first and must be lifted to the surface for the first breath. In some species the mother and calf are assisted by other females who will lift the calf with their own bodies. Heat loss is a danger with all small animals because of the surface to volume ratio so cetacean calves are generally quite large at birth. Their growth rate is extremely rapid and is supported by very nutritious milk. The fat content can be as high as 50% compared to human milk which is only about 3% fat. Newborn blue whales consume more than 100 litres of milk per day and grow from being about 7 metres at birth to 15 metres at 7 months of age.

The young are suckled for at least 4 months and in some species a calf may feed from more than one mother within the group. They do not become independent for at least a year and this can be much longer for Odontocetes. Old sperm whale cows have been observed still suckling calves of 10 years old. Calves stay close to their mothers whilst young and “bow ride” beside her in order to keep up with the adults. However they do not join their mothers when they dive deeply to feed and remain with the others in the group at this time. As with most mammals the adults will defend their young and some mothers can be very aggressive. Sperm whales in particular will attack boats which threaten their young.

Cetaceans have low reproductive rates with only one calf being born at a time and a long time between calving. The smaller cetaceans such as the porpoises can have a calf every year but in other larger species it may be as long as 4 years before the next calf is born. Cetaceans are the only mammal other than humans known to have a post-reproductive period when ovulation stops and this has only been observed in a few species.

This low reproductive rate is offset by the fact that they are generally long lived and can therefore produce many calves in one lifetime. The average life span of a cetacean ranges from 20 to 40 years with some species being considered old when they are 15 compared to others which can live for 80 years.