Dolphins

Bottlenose dolphin / Tursiops aduncus / Family - Delphinidae

Dolphins are mammals that live in the sea but they have more in common with humans, dogs and other land mammals than they do with other marine animals such as sharks and fish. Up to about fifty million years ago the ancestors of dolphins lived on land.

As mammals, dolphins are warm blooded, have mammary glands to suckle their young and have to surface to breathe air. However, many features typical of land mammals have been lost.

Dolphins possess horizontal tail fins called flukes and a fin on their back. Flukes have no skeletal support and skin outgrowths connected to the body muscles by a complex network of tendons and tough connective fibres. Powerful up- and-down strokes of the flukes propel the animal forward.

Breathing in the sea

Dolphins breathe air through their lungs and therefore must surface regularly. A small dolphin needs to surface for air about every two or three minutes. All dolphins can stay down longer when feeding or when in danger.

Dolphins have a most efficient circulation system. As well as breathing less often and taking deeper breaths than land mammals, they have more oxygen-carrying red blood cells and can regulate where their blood is distributed. During a deep dive only essential organs such as the heart and brain get oxygen-rich blood flowing through them. When dolphins are being particularly active or swimming in warm water they can cool themselves by passing more blood through their flukes and fins. The heat is lost to the surrounding water.

Sense and intelligence

The question of dolphins' intelligence often arises, but is extremely difficult to measure. An animal's ability to perform 'clever tricks' in captivity and exhibit gentleness towards humans, should not be confused with its ability to understand and reason. Like us, dolphins certainly have large brains, but the brain size varies with the different species.

Dolphins obtain much of the information about their surroundings from touch-sensitive organs in their skin, and from sounds in the environment. This highly sophisticate sense involves producing a series of clicks that travels through the water, echoes off any object in its path and returns to the sender for interpretation. The sound is focused by fat deposits in the animal's forehead and lower jaw. This method of 'seeing' with sound is similar to the sonar equipment used in ultrasound scanning and echo sounders.

Dolphins also produce sounds (high-pitched whistles and groans) for communication purposes.

Threats to survival

Dolphins belong to food chains where pesticides, heavy metals and other contaminants have accumulated. They must also cope with pollution, such as oil spills; possible entanglement in discarded fishing lines; and with other hazards from increased boat traffic and from construction activity along the coastline. Even with improved fishing methods too many dolphins are still being accidentally killed as a result of becoming entangled in tuna and anti-shark nets.

All of these threats are more difficult to control and we should all be aware of our individual responsibility to try to preserve these magnificent creatures.