Dolphin teeth

Unlike most land mammals toothed whales grow only one set of teeth their "milk" teeth, these are retained throughout their lives and in some older animals they can be very well worn. Instead of having different teeth types they only have one kind - in most this is a simple peg like structure with a single root and they are generally thought to be used for no more than grasping slippery prey as dolphins don't chew their food, but swallow smaller fish whole. However, they may need to break up a large fish into smaller pieces before eating it. They achieve this by tossing the fish about, or knocking it against an object in the water.

There is a little variation in teeth of different species, which is usually due to their diet or feeding strategy. For example, river dolphins have somewhat flatter teeth for crushing shell fish and porpoises have sharper edged teeth for slicing prey that is too large to be swallowed whole. However, in beaked whales that eat almost exclusively squid, (which are slippery), the female's teeth never emerge and the males only have one pair. It is possible to estimate the age of some species by studying the series of layers (rather like rings in a tree) inside their teeth. Broadly speaking, one complete layer is equal to one year of growth.

Tooth showing 'growth rings'

Hunting

Toothed whales have needed to develop highly specialized senses including the ability to dive deep and swim fast or with agility to detect and capture their prey. As a result most toothed whales are considerably smaller than the baleen whales. For example the smallest baleen whale is the pygmy right whale which often exceeds 6 metres (20ft) in length and by contrast dolphins and porpoises seldom exceed 4.5 metres (14 f).

Dolphins are generally opportunistic feeders, taking whatever is locally available and easily caught. Dolphins for example generally favor shoaling fish such as anchovy and herring but will also eat squid, shrimp and even jelly fish. This opportunistic nature makes it difficult to define the precise diet and feeding behavior of individual species. Most of what is known about the diet of a toothed whale has been learned from the examination of their stomach contents, from those who have either been caught or stranded. As digestion is rapid in whales, this may not give the full picture.

Dolphins can, and do, hunt successfully by themselves but there are times when co-operation is the best solution. for example; If there is lots of food available, a group of dolphins can be more effective at finding prey than a single dolphin on its own. When a shoal of fish has been found, the animals work as a team to keep the fish close together and maximize the harvest. Large schools of dolphin often split up into smaller ones to be able to cover a larger area of the sea and, once a shoal of fish has been found, they can help each other catch them. They also search for fish alone, often bottom dwelling species. Feeding can also be in association with human fishing, and chasing fish into mudbanks. Common dolphins have been seen working together to herd fish into tight balls. Like many other dolphin species, the common dolphin will sometimes take advantage of human fishing activities (such as trawling), feeding on fish escaping from the nets or discarded by the fishermen.

When searching for food, the dolphins spread out in a line, and use their sonar to locate shoals of fish. Often they follow the edge of underwater escarpments where there are concentrations of plankton and shoaling fish are common. Corralling is one technique used by dolphins to catch fish. Once a likely source of fish has been identified the dolphins regroup for the attack. This strategy commonly employed by dolphins is based on a few group members swimming beneath a fish shoal and then driving it upwards towards the inescapable barrier at the surface. By circling the shoal as it rises the dolphins attempt to restrict the movement of the prey even further. Then, at some unseen signal the dolphins dart into the seeing mass of fish at will, snapping them up in their sharp-toothed jaws.

Sometimes they will employ 'fish wacking' whereby a fish is stunned (and sometimes thrown out of the water) with the fluke to make catching and eating the fish easier and some bottlenose dolphins use a technique called 'hydroplaning', which is a bit like body surfing. The dolphins chase fish into the shallows and then propel their bodies forward in water that is only a few centimetres deep in an effort to seize a fish.

Many species (including common dolphin, Pacific Whitesided & Spinner Dolphins) are believed to feed at night.

Bottlenose dolphins and Pilot whales us their teeth for catching/grasping. Their peg-like teeth serve to grasp but not to chew food. However, the mouth of the killer whale is large and well adapted for hunting. The upper and lower teeth interlock, which aids in gripping large prey and tearing it into smaller pieces for easier swallowing.

An adult bottlenose dolphin may consume 15-30 pounds (8-15 kg) of food each day and an adult pilot whale may eat up to 30 pounds per day.

Echolocation

The term echolocation refers to an ability that toothed whales (and some other marine mammals and most bats) possess that enables them essentially to "see" with their ears by listening for echoes.

The sounds used by cetaceans to navigate or to find prey differ markedly from the whistles, grunts and moaning songs they use to communicate with each other. It is rather like the system bats use to hunt in the dark and involves creating 'sound pictures' by making clicking noises and listening to the echoes that come back, but unlike bats that echolocate using extremely high-pitched calls beyond the range of most people's hearing, the 'clicks' used by cetaceans are at least partly audible.

Dolphins produce directional clicks in trains. Each click lasts about 50 to 128 microseconds. The click trains pass through the melon, which consists of lipids (fats). The melon acts as an acoustical lens to focus these sound waves into a beam, which is projected forward into water in front of the animal. Sound waves travel through water at a speed of about 1.5 km/sec (0.9 mi/sec), which is 4.5 times faster than sound travelling through air. These sound waves bounce off objects in the water and return to the dolphin in the form of an echo. High frequency sounds don't travel far in water. Because of their longer wavelength and greater energy, low frequency sounds travel farther. Echolocation is most effective at close to intermediate range, about 5 to 200 m (1 6-656 ft.) for targets 5 to 15 cm (2-6 in.) in length. The major areas of sound reception are the fat-filled cavities of the lower jaw bones. Sounds are received and conducted through the lower jaw to the middle ear, inner ear, and then to hearing centres in the brain via the auditory nerve. The brain receives the sound waves in the form of nerve impulses, which relay the messages of sound and enable the dolphin to interpret the sound's meaning.

The amount of information obtained through echolocation is astonishing. The time lapse between each click and its echo enables the whale or dolphin to estimate how far away an object is; the form of the echo helps to identify its size and shape; and the varying strength of the echo provides information on the direction in which the object is moving. Even prey hidden under the sand isn't safe because echolocation is so sophisticated that a dolphin can 'see' through the sand and tell the difference between hidden rocks and hidden fish just by the type of echo produced. Bottlenose dolphins are able to learn and later recognize the echo signatures returned by preferred prey species.

The study of dolphins at sea suggests that echolocation is used in a variety of different ways. For much of the time relatively simple, low frequency sounds are emitted to provide a dolphin with a picture of its surroundings, such as variations in the seabed and water depth. The volume of the sound determines the dolphin's range of view, from the immediate vicinity to over a kilometer. If the dolphin detects something of interest, it will emit more rapidly repeated clicks of a broader frequency. As the dolphin moves closer to its target it will focus its beam and use higher frequencies. These higher frequency clicks will provide maximum information about a subject.

A typical echolocation sequence is as follows:

1. During normal swimming with no specific target, a general low-frequency echolocation signal of fairly pure tone is used this provides an animal with information on topography of the area including water depth, changes in sea floor profile and position of coastal features. The scan range will be determined by the time between the signals (clicks) and how much energy there is in each signal. For efficient echolocation one individual click must be sent out and the echoes received before the next click is emitted.
2. Once a new echo is received, the first requirement is to determine distance and direction, and to collect more detailed information on the target itself. The dolphin emits a series of clicks with a broad frequency band, the echoes of which can give many different pieces of information. The high frequencies give the most detailed information, but they are absorbed quite quickly by the water and so are useful only at close ranges.
3. Once the bearing of the target is established, the dolphin focuses the signal on the target. This concentrates the power of the higher frequency components and gives a more detailed picture of the target. By moving its head from side to side it can also gain information about target size and movement.
4. As the range closes between dolphin and target, the dolphin can use much higher frequencies in its echolocation clicks and hence get even more detailed information. At this stage the clicks will start to come more closely together and produce what sounds like continuous 'creaking'.
5. Finally, at very close range, it may be necessary to determine texture or other fine structural information. In this case use of short-range sonar with very high frequencies would be necessary. It has been suggested that taking objects into the mouth may be associated with a close range acoustic sensory system rather than with the sense of touch.

Despite the effectiveness of echolocation, studies show that a visually-deprived dolphin takes more time to echolocate on an object than a dolphin using vision in tandem with echolocation. Many of the details of echolocation are not completely understood. Research on echolocation continues.