Dolphins > The Odontoceti Print this page
Are the group of cetaceans that include all the Toothed Whales, Grampus, Porpoise and Dolphins. Did you think that we would never get there? They all have teeth varying in number from 1 to over 200 and are the active hunters of the marine mammal world, unlike the Baleen Whales which slowly travel along vacuuming up their food. The odontoceti are not specialist feeders and individually eat a wider range of prey.

All cetaceans have adopted a streamlined shape for travelling through the water. In the case of the smaller species of dolphins, it would appear that they can easily exceed their theoretical maximum speed by a factor of 8, giving them a top speed of about 30 to 40 mph. It is now known that this is achieved by controlling boundary layer drag, which is the effect that creates friction at the surface of an object when it moves through a liquid or a gas and accounts for the major proportion of it's resistance to movement.

This control is affected through several peculiar properties of their skin. First of all the top layer is not dead, whereas ours is. When you vacuum the carpet and then empty the bag, you find a quantity of grey dust. We're sorry to tell you but most of that is dead human skin. Compared with cetaceans, we seem to have rather a bad case of dandruff!

In comparison their skin is alive, they have control over it and are able to "shiver" off the stagnant boundary layer that would otherwise slow them down. It is also slightly porous, the first two layers absorbing water and then expelling it, further lessening boundary layer drag and helping them to travel through water with less effort. Another factor affecting speed is shape. One of the first things yo notice about these creatures, apart from those that have fins, are that there are no projections or bumps. In fact it is quite hard to tell the sexes apart, as all the reproductive and mammary organs are kept within slits on the underside of the body. The design of the nuclear submarine is a good example of man copying one of nature's forms.

Cetaceans are the largest animals that have ever lived on earth. Why is this so? They live in an environment that is gravity free for all practical purposes. This was one of the reasons that the Dinosaurs and their kind never grew any larger. They could so easily get bogged down owing to their enormous weight. Circularity problems were another factor when sleeping on a particular part of the body, as the weight would compress the blood vessels in that part, thus cutting off it's blood supply. To avoid this, even today, most large land animals either catnap or sleep standing up. Cetaceans having solved the gravity problem were free to solve the next, which was that of heat loss.

Very small animals lose heat quickly because their surface area is large in comparison with their volume. This situation changes inversely as an animal gets larger. Terrestrial animals use a large part of their food intake to generate heat and so deep themselves warm. Next to the skin, cetaceans have a layer of fat called Blubber, to insulate them from the cooler water surrounding them and because of their large size, retain their heat more efficiently. Cetaceans that live in the Arctic regions, the Blue Whale for example, can weigh up to 190 tons and 40% of this weight is made up of blubber.

Like all other mammals, cetaceans are air breathers. In order to live in the sea, several important adaptations have taken place. The most obvious is the position of the nostrils or blow hole, which are no longer found at the end of the nose but on top of the head. This enables the animal to breathe in when the blow hole is above the water, at the same time allowing to keep his eyes and ears under the surface so that he can observe his surroundings.

The next time you see dolphins, try to see if you can fill your lungs in the same time that they can, as theirs are about the same size as ours. You'll find it very difficult, especially if you're swimming in a rough sea and you don't want to inhale water. The cetacean manages this little trick by having a very sensitive area around the blow hole, the most sensitive part on it's body and this senses the approach of a wave, triggering the closure of it before the water can enter. This automatic reflex enables them to sleep on the surface without inhaling water or waking up. All we can do in a similar situation is to hold our noses shut with our fingers, not an elegant solution to the problem.

When you examine the lungs of these animals, it becomes apparent that they are much more efficient than ours. To begin with we only fill ours to a third of their capacity, the cetaceans to about 90% of their possible volume. A further difference is in the construction of the lungs. The Aveoli, the very fine tubes in the lungs that carry oxygen over into the bloodstream, are more numerous in cetaceans than in any other species of mammal, thus enabling them to transfer more oxygen into their blood. This is further helped by an adaptation of the red blood cells themselves, which are larger and more numerous than in other animals and so enable them to transport more oxygen to the organs.

During deep dives, the circulatory system is modified by a slowing down of the heart rate and the re-routing of the blood supply from non-essential organs to the brain and locomotive muscles. The whole process allows some species to stay underwater for periods of up to 1 hour and to descend to depths of 10,000 ft. At this depth the pressure on their bodies is 2.5 tons over each square inch of their surface area.

You may wonder why they don't get crushed to death at these great depths or at least suffer the dreaded "divers bends". If you dive in sub aqua gear to more than about 30 ft underwater using compressed air to breathe, you cannot come straight back to the surface without stopping to decompress. The effect becomes more noticeable the farther down you go, until the time you have to spend coming up is far longer than the time you spent on the bottom. This is because under pressure, the nitrogen in the air you are breathing becomes dissolved in the blood. Conversely, on the way up, unless the time of ascent is long enough, the dissolved nitrogen in the bloodstream re-forms as bubbles before it can be re-processed in the lungs. These bubbles lodge in the joints and heart causing excruciating pain and in some cases death. Many of the diving deaths and injuries in the North Sea oil industry have been due to this cause. Cetaceans, because of their superb efficiency in breathing, do not have this problem. They need only one lungful of air to last them for a long period and there is not enough nitrogen in that lungful to cause this situation.

We are often asked, "if they are not breathing compressed air, why don't they get squeezed to death at pressures of up to 4500 lbs per sq inch?" Water is incompressible. All living creatures are chiefly composed of water, 90% in fact; the water pressure inside each cell of their bodies is equal to that outside.

"Surely if they open their mouths to feed at these depths, the water will rush in and drown them?" Yet again, it can't. The pressure is the same inside and outside and in any case, the respiratory system is separated from the mouth and digestive tract. Finally, there is one further modification to the respiratory system. When the lungs are full of air they are not able to withstand the enormous pressure upon them. The ribcage not being rigid, allows the ribs of the chest to be pushed in by the pressure until the lungs are completely collapsed, so reaching a state of equilibrium. During the ascend, with the release of pressure, the lungs return to normal.

Cetaceans evolved from a terrestrial creature and some remnants are left from that existence on land. First of all the front limbs are still apparent, but much modified. These flippers or lateral fins are not used for propelling the animal, but are used for balancing and inducing turns. They have an internal bone structure rather similar to our own hands with the exception of the thumb and this omission may well have played a vital part in the competition for dominance of the planet. The hind limbs have almost completely disappeared and are only represented in some species by two small bones unattached to the main skeleton.

Most cetaceans have good eye sight. You only have to go to a Dolphinarium to realise this when they are leaping though hoops, catching balls and performing tricks that call for a high level of co-ordination of body and eye. This was one of the senses that was developed on land and has not been impaired by living in the water. In fact the eyes have learned to adapt to the problems of refraction and distortion found in the two different mediums. Eyesight is not much help however when the visibility is no more than 60 or 60 feet, which it si in most places in the ocean. How do these animals find their food and each other? They all have an incredible sense of hearing. We now know that some of the larger cetaceans, the Humpback Whales for instance, can hear each other over distances well in excess of a thousand miles. The hearing range extends from about 20Hz to over 150kHz. The most expensive Hi-Fi has the performance of a Crystal set in comparison. The cetaceans have external ear channels about the size of a pin hole, but as these are usually blocked with wax they are not used to carry sounds to the inner ears. This is achieved by conduction along the lower jaws and is efficient enough to detect a pin dropped into a large pool.

The sense of touch is well developed. The skin of most cetaceans is very sensitive and you have only to ee the way they constantly touch each other to realise that this is so. Human beings receive a lot of information from this sense at an early age, by using the lips and then later on the tips of the fingers. The cetaceans, not having fingers, cannot receive information about texture and form by this method. It has been suggested that the male cetacean sometimes uses his penis for this purpose. In the case of the larger Baleen Whales the finger would be nearly 10ft long! The testes of the same species also weigh 1 ton each.

Of the other senses, it would appear that cetaceans do not have a sense of smell as the olfactory organs are not developed.