Our oceans’ most intelligent mammal — the dolphin

AS an adolescent, the word ‘dolphin’ meant one of two things to me in the harbourside town of Penzance in the far southwest of England where I was born. It meant a sea creature or ‘The Dolphin Arms’ — a public house located on the quayside. The latter I sometimes frequented on a hot summer’s day. Further along the quay a friend’s father ran a tourist boat trip business to view these magnificent creatures offshore. I suppose I became used to seeing these mammals without really much thought as they leapt out of the sea.

The word ‘dolphin’ originally came from the Greek ‘delphis’, meaning a ‘fish with a womb’. Worldwide there are essentially four families of dolphin species: the Oceanic dolphins, the Indian river dolphins, the New World river dolphins, and the Brackish dolphins amounting to 40 species in all.

The most familiar species in both tropical and temperate seas is the bottlenose dolphin, which is found around the globe apart from Arctic and Antarctic waters and is considered to be the world’s most intelligent fish which can interact with humans. My granddaughter has swum with these in an aquarium in Singapore.

Bottlenose dolphin (Tursiops truncatus)

Like all dolphin species they descended from land dwelling animals which became seaborn about 48 million years ago. Their nearest ‘landlubber’ relative is the hippopotamus. The bottlenose dolphin weighs on average at about 300kg, of which up to 20 per cent consists of blubber, and can grow up to four metres in length. Colour-wise, they vary from dark grey on their backs and lighter grey on their sides to bluish or brownish grey and may live for over 40 years.

So agile are they that they can leap up to a height of six metres through the air. Their snouts are not real noses for these are only visible when they open the blowholes on the top of their heads. They need to surface to gain air, storing as much oxygen as humans, by rising to the surface to breathe two or three times a minute yet can remain submerged for up to 20 minutes. They expel stale air whilst inhaling fresh oxygen not unlike a human snorkeler. Their flippers (pectoral fins) are used for steering whilst their tails provide forward propulsion. Usually, they swim at an average speed of 8 kph but are capable of short-lived bursts of up to 35 kph.

Literally sensational attributes

Dolphins are sound hunters, relying on inbuilt echolocation, as a sonar device locating objects by producing sound waves and then listening for the echoes. This informs them of the form, speed, size, distance, and actual location of the object. They achieve this by emitting a series of clicking sounds and then receive the echo via two small ear openings behind their eyes thus building up a sound picture of the objects ahead of them.

With very sharp eyesight and with the rapid adjustment of their pupils, their eyes can adapt quickly to underwater and surface conditions, yet their sense of smell is very poor because their blowholes are closed when underwater. Their taste buds are not affected for they have a penchant for certain species of fish.

They communicate with each other through a series of pulsed sounds, whistles, and bodily movements. These gestures and whistles keep track of other dolphins in their pod and alert each other of danger or food. Each dolphin has its own signature whistle for self-identification. A recent study by researchers at the University of Chicago has revealed that bottlenose dolphins can remember the whistles of other dolphins that they met 20 years previously. This has implied that this dolphin species has the longest memory of any known species other than humans.

Feeding

They are, indeed, voracious pelagic fishers with extreme efficiency because of their sonar, intelligence, and cooperation. Gathering in pods from a few individuals to herds of up 1,000 in size, they hunt in packs to eat shrimps, squid, molluscs, and cuttlefish, consuming up to 10kg of fish daily. Often, upon finding a shoal of fish, they will circle the shoal and trap the fish in a small whirlpool then charge the fish and drive them up onto an estuarine sandflat there to consume them at leisure. This technique only happens at low tide.

Breeding

Bottlenose dolphins start to breed between the ages of five and 15 years and during the breeding season the ‘bulls’ compete with one another for access to the ‘cows’ by fighting off other males. The gestation period is about a year long with peaks in births occurring during the warmer months. A single calf is produced at between 0.8 and 1.4 metres in length and weighs from 9kg to 30kg. Sucking the mammary glands of its mother, the calf remains with its mother in weaning for up to eight years although most calves are weaned by their fourth year. Once born, the calf swims in echelon alongside its mother. Calves are particularly prone to shark attacks, especially those of the tiger shark, great white shark, and killer whale.

Whilst living in a pod, these calves are protected to some degree by strength in numbers, as grown dolphins repeatedly charge their predators using their snouts to force them to flee or by battering the predator to death. These dolphins are polygamous.

Affiliation with humans

They have been known to protect humans from shark attacks by encircling the swimmers and driving them to shore. They follow tourist boats, putting on spectacular displays of leaping out of the sea but generally avoid fishing vessels. Often, they can be seen to drive shoals of fish landwards towards the handheld seine nets of onshore fishermen. Such is their intelligence that they can be trained to perform ‘acts’ in aquaria worldwide by recognising the hand signals of their custodians. Contrary to the beliefs of many conservationists, these dolphins are trained in military tactics to locate enemy sea mines and divers but have a particular use in mine clearance in former warzone waters.

Threats to their existence

Whilst bottlenose dolphins are not classified as endangered species on the International Union for Conservation of Nature (IUCN) Red List because of their abundance and adaptability, they are nevertheless threatened by humans. Millions of these wonderful creatures drown each year as bycatch in commercial fishing nets and especially in nets set to catch tuna.

Heavy metals and agricultural fertilisers and pesticides, found in our rivers which ultimately reach our seas, act as pollutants in dolphin systems affecting their immunity and reproduction. The once renowned species of China’s Yangtze river dolphin is now extinct because of this. Researchers in the UK and Australia have documented the by-products of industrial and agricultural uses upon the malformation of dead dolphins washed up on beaches. Commercial purse seine net tuna fishing in the USA has now been banned for its loss of dolphin lives.

On land, as a dog is recorded as ‘man’s best friend’, so in the seas, I strongly believe that bottlenose dolphins are of equal status. May they live in peace in their natural environments and long may they continue to enjoy their arc-like leaps out of the water. Their innate intelligence in the marine world is yet to be surpassed!