Odontocetes - Toothed Cetaceans

The odontocetes are members of the sub-order of cetaceans that have teeth. Other cetaceans have evolved hairy plates, which hang from their upper jaws and are used to filter plankton, krill and small fish out of the water. These plates are known as baleen and the cetaceans that have these are commonly known as baleen whales. The sub-order to which they belong is the mysticetes. The cetaceans that have teeth, the toothed cetaceans, make up the odontocete sub-order and this sub-order includes all the dolphins, all porpoises, the beaked whales, sperm whales and a few others. These are divided up into the six families listed below:

1) Delphinidae – containing oceanic dolphins (and orca) and the pilot whales.
2) Monodontidae – containing the Beluga and Narwhal
3) Phocoenidae – the porpoises
4) Physeteridae – containing the sperm whales
5) Platanistidae – the river dolphins
6) Ziphiidae – containing the beaked and bottlenose whales

12 species of odontocete are found within the Hebridean area. To find out more about these individual species click on the links:

The largest odontocete of all is the sperm whale, the male of which can grow up to 20 metres in length and can weigh 50 tonnes. The smallest is the tiny Vaquita, a porpoise which only grows up to 1.5metres long. There is a huge range of characteristics within the odontocete species but apart from the presence of teeth, all odontocetes have a single blowhole. This unique feature has many associated physical differences from the mysticete anatomy. There may only be one external blowhole but there are still two separate nasal plugs inside which close off the internal nasal passages. The odontocete nasal passages are modified for sound generation. This is extremely important for them to locate their prey, to communicate and to detect their surroundings. In water sound travels about 4 ½ times faster than through air and is used much more by cetaceans than by most other mammals.

In developing their highly specialised system of auditory sensitivity the odontocetes seem to have lost the sense of smell and have no olfactory cavity or nerve.

Odontocetes and Sound

All the mammals possess a larynx in the airway to the lungs. In humans this is very highly developed to produce a whole variety of sounds as the air passes between flaps of elastic tissue known as the vocal chords. There are similar structures found in the larynx of odontocetes but it is still unknown whether they are used to generate sound. It has been suggested that they could be used to generate the whistles produced by some species.

The soft tissues of the nasal passages of the odontocetes are adapted to produce clicks of very high frequency. These bounce off the bony structures inside their head and are picked up by a large, fatty structure known as the melon. This organ appears to focus the sound and emit it as a narrow beam. This beam travels out through the surrounding water and bounces off an object, returning as an echo.

The sound returning to the odontocetes head is picked up in a variety of ways. High frequency sound passes into the bony tissues and in particular the lower jaw where there is a fat filled canal which transmits the sounds efficiently to the inner ear. Cetaceans do not have an external ear opening so they do not need the protective ear drum found in other mammals but they do have a related bony structure which is used to help transmit sound into the inner ear. Lower frequency sound passes directly into the skull and the inner ear where it is picked up by a chain of three tiny bones as in the human ear.

All mammals possess a cochlea, a spiral shaped organ where sound is picked up and nerve receptors are stimulated to send impulses to the brain. There is one cochlea on each side of the head to allow directional hearing. In odontocetes the middle and inner ear are isolated from the skull by foam filled sacs which are thought to aid them in detecting the direction of a sound source. Air filled sinuses within the skull also isolate these delicate organs from the areas of sound production. The sounds that the odontocetes are able to detect are of a much greater range of frequencies than the audible range of humans. Many of their sounds cannot be heard by us and we have to use special electronic filters to register them. Each odontocete species has its own particular range and variety of sounds.

Very high frequency ultrasound clicks are produced and used for echolocation of their prey. These clicks are very short and repeated at a high rate so that often they sound more like a continuous creak than a series of clicks. The upper limit of a human’s hearing is 20 kHz but some odontocetes can produce and probably detect sounds up to 300 kHz in frequency. High frequency sounds are used very efficiently to locate objects in the water at a range of up to about 10metres. At a distance of about 3metres, some dolphins are able to locate objects as small as 1.5 mm! Not only can they tell where an object is but the way the sound returns to them can tell them much more than just its size, shape and position. Some dolphins can tell the difference between the material an object is made, even being able to distinguish identical shaped objects made from copper or aluminium.

Broader frequency sounds are also produced and are used to provide the odontocete with more information about their surroundings at a greater distance and with less accuracy. The differing frequencies are thought to be controlled by the melon which is able to alter the frequency of the sound passing out of it. It is controlled by muscles which alter its shape and this seems to alter the shape of the sound waves which are emitted. Lower frequency sounds are used by the larger odontocetes, such as the sperm whale, in order to communicate with other members of the species at greater distances and in particular with other members of a hunting pod. Some of the sounds produced are not only particular to a species but are also unique to animals found in certain areas or of a certain family group. It is thought that some species such as the orca can distinguish an individual by the sounds it makes.

Sound is a form of energy that is absorbed by the medium through which it is travelling. The distance a sound can travel without losing all its energy is related to its frequency. Lower pitched sounds travel much further as can be demonstrated by listening to distant sounds from a city. All you can hear are the low frequency throbbing sounds of motors. High pitched sounds such as children’s voices are lost over a long distance. The same is true in water. The high pitched sounds produced by odontocetes to accurately locate their prey would not be useful across great distances. Large mysticetes such as the blue whale, which must communicate across hundreds of miles, use very low frequency sounds. They use sub-sonic sound below 5 kHz.

The frequency of a sound wave is what determines the pitch or “highness” and “lowness” of sound. The frequency of a wave is the number of vibrations it makes in one second. A high frequency sound wave is one which has a short wavelength and the shorter the wavelength, the higher the pitch of the sound. The standard “A” to which an orchestra is tuned has a frequency of 440Hz or 440 vibrations per second. The “A” an octave above this has the frequency of 880Hz and sounds much higher. Humans can usually detect sounds between 20 to 20,000Hz. (20 kHz). Dogs can detect sounds between 15 and 50,000Hz and bats can detect sounds up to 120,000Hz!

Sounds are not made up of a single continuous frequency emission. The “shape” of a sound can be changed or modulated by altering the frequency which is being emitted. Odontocetes are able to moderate their sounds rapidly and this modulation is thought to be an important carrier of information.

Odontocete Feeding

Like humans, cetaceans need to maintain their core body temperature at 37°C. They are endotherms (warm blooded) and use a variety of methods to control heat production and heat loss. They have to eat enough food to replace the energy lost in feeding, breathing, reproducing and just staying alive as well as generating heat to maintain their body temperature. They lose energy as heat to their surroundings and generally the odontocetes are smaller then many of the mysticetes. They therefore lose heat faster and have to eat more high-energy food to replace this loss. The smaller animals must consume more in order to compensate for their larger heat loss. All the odontocetes are carnivores, relying on animal prey as their food source though this can vary a great deal from species to species.

The type of teeth each species has is dependant on how they feed and what their usual prey is. Mostly their prey is very variable and depends a great deal on what is around at the time. Most odontocetes are what are known as opportunistic feeders and can eat a whole range of prey. Most eat a range of fish, squid and crustaceans. A few odontocetes will also eat larger prey such as penguins, seals and even other smaller cetaceans. These, such as the orca, have large strong pointed teeth and they are strongly rooted into short biting jaws. Dolphins that eat smaller prey have pointed teeth but often scores of them on either side of the jaw. The bottlenose dolphin for example has lots of short, sharp teeth and they grasp their prey before swallowing it whole. Since their prey is not chewed they only need one type of tooth that is designed for holding onto their prey. Mammals that must hold, tear, cut and chew their food, such as dogs, have a range of different shaped teeth with a range of functions. The odontocetes which just swallow their prey whole only need the one type of tooth so all their teeth are the same.

Porpoise teeth are very different from the dolphin teeth, being spade shaped. Porpoises feed on coastal fish, squid and crustaceans and maybe their spade-shaped teeth are an adaptation to a particular feeding technique or type of prey.

Some odontocetes are specialised feeders, feeding only on one type of prey. For example, the beaked whales feed exclusively on squid and have few or no visible teeth. Yet other species have such highly modified teeth that they are not used at all in feeding but are probably for fighting or other uses connected with selection of a mate. A good example of this is the Narwhal whose spiral tusk was once thought to have come from a unicorn. It is now known that this single tooth is primarily for display and is used to compete for females. The female does not usually show signs of this tooth, which erupts from the upper jaw of the adult male, usually on the left side.

Another interesting specialisation in odontocete teeth is found in the strap-toothed whale. This whale has a very broad tooth on either side of the lower jaw and which curve up and over the upper jaw. They very effectively “strap” the jaws together preventing the mouth opening too wide. This may have evolved as a result of feeding on a particular prey which causes damage to their jaws if they are not held together like this.

The sperm whale is the largest of the odontocetes and is well known for its strange shape and for its popularity as the victim of whaling ships. It has only vestigial teeth on the upper jaw but the lower teeth, which are large and pointed, fit into sockets in this broad upper jaw. They feed on giant squid that are found 300 to 600 metres deep in submarine canyons. However sperm whales have been known to dive to depths of up to 3,000metres. Giant squid can grow to a length of 12 metres and the whale eats them by a method known as suction feeding. This involves using the tongue and throat muscles to create a rapid inrush of water that sucks up nearby prey. It has also been suggested that sperm whale mouths often contain phosphorescent bacteria, the glow of which attracts the squid towards its mouth.

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