Thanks to the Caleta Cruise Club who wrote this article for us.
THE BASICS. Dolphins are mammals, not fish, which means that, like humans, they are warm-blooded, breathe air and give birth to live young whom they feed with milk. Unlike other mammals they have no fur and their skin is smooth and sleek, having neither hair follicles nor pores. They have streamlined torpedo-shaped bodies with fins. Their colouring can be any combination of black, white, grey, blue-grey, yellow, tan and pink and they may also have spots and stripes. Dolphins have large brains and are among the most intelligent creatures on earth.
Dolphins are actually small whales. Whales are known as cetaceans, which are divided into two groups: baleen whales and toothed whales. Dolphins are within the toothed whale category, of which there are about 67 species. Their closest land relatives are pigs, cows, horses and deer. Porpoises are also within this category but they are smaller than dolphins, have rounder faces and swim much faster. For those whom it might interest there is a section on the evolution of dolphins at the end of this fact sheet.
There are more than thirty-three known species of ocean dolphins and five river dolphins.
Atlantic White-Sided Dolphin, Atlantic Borneo White Dolphin, Bottlenose Dolphin, Borneo White Dolphin, Bouto Dolphin, Broadbeaked Dolphin, Cameroon Dolphin, Chinese White Dolphin, Clymene Dolphin, Commerson’s Dolphin, Common Dolphin, Dusky Dolphin, Falkland Island Dolphin, Fraser’s Dolphin, Heaviside’s Dolphin, Hector’s Dolphin, Hourglass Dolphin, Long-Snouted Spinner Dolphin, Northern Right Whale Dolphin, Pantropical Spotted Dolphin, Peale’s Dolphin, Plumbeous Dolphin, Rio de Janeiro Dolphin, Risso’s Dolphin, Rough Toothed Dolphin, Southern Right Whale Dolphin, Speckled Dolphin, Spinner Dolphin, Spotted Dolphin, Striped Dolphin, White-Beaked Dolphin, White-Bellied Dolphin and White-Sided Dolphin.
Amazon River Dolphin, Baiji Dolphin, Ganges River Dolphin, Guiana River Dolphin and Indus River Dolphin.
Viewing dolphins in the wild is an experience not forgotten. Watching a herd joyfully surf the waves; couples swim in perfect synchrony; being surrounded by a few to several hundred at a time; the sea bubbling and alive with dolphins; viewing a dolphin swiftly accelerate after a fleeing fish or witnessing a tender moment between a mother and her calf – all of these experiences tend to bring a feeling of wellbeing and give a broad grin to the faces of most onlookers. The two species of ocean dolphin regularly spotted in these situations by the Caleta Cruise Club, off the Costa del Sol, are the Bottlenose Dolphin and the Common Dolphin, for which descriptions follow.
The Bottlenose Dolphin is one of the best-known species. It is the species most likely to be seen performing at shows in aquariums and was made famous by the television series Flipper. The name ‘Bottlenose’ originated in England from the resemblance of the short rounded snout to the swollen form of an old-fashioned bottle. It is the largest of the ‘beaked’ dolphins and has a slightly hooked broad dorsal fin. The body colour varies from dark blue to brown-grey along the dorsal cape, fading to a pale grey along the flanks. The ventral surface is white-pink and the tip of the snout is normally white. Tail flukes have a distinct notch in the middle and, as with the flipper edges, are grey-black. This species is popular with people partly because of its fixed facial expression that resembles a smile. In the wild the Bottlenose lives all over the world, especially off the coast of Europe and North Africa. They can grow to between 2m/6.5 feet and 4m/13 feet, weighing between 90kg/200lb and 650kg/1400lb.
Bottlenose Dolphins are both a coastal and oceanic species, with the former preferring waters of less than 30m in depth. They occupy diverse habitats ranging from rocky reefs to calm lagoons and open waters; tending to prefer to feed on shoaling and bottom-dwelling species of fish, squid, octopi and cuttlefish. Inshore herds are mainly resident year-round, whilst offshore herds are transient undertaking seasonal migrations. Although occasionally seen individually, Bottlenose Dolphins are more usually seen in herds of between 20 and 100 inshore and several hundred offshore.
Bottlenose Dolphins are powerful swimmers and acrobatic in nature, often bowriding and leaping alongside vessels. Their average foraging and travelling speed is 5km per hour with a cruising speed of 10km per hour. In times of danger they will swim up to 37km per hour. In coastal waters each dive rarely lasts longer than 3-4 minutes, but the oceanic populations can dive to 600m for 15 minutes.
The Common Dolphin is one of the most colourful dolphins with black, grey, tan and white markings. They live in temperate and tropical seas, preferring temperatures of between 10 and 28º C, and are most often seen in the open ocean with depths of greater than 180 metres. Very occasionally they have been reported in Nova Scotia, Iceland and Greenland. The Common Dolphin grows to 2.5m/8 feet and 90kg/200lb. They eat anchovies, herring, lanternfish and squid.
The Common Dolphin is a fast swimmer with a maximum speed of 45km per hour. It is thought that Common Dolphins may travel up to 120km/75 miles in a day and it has been observed that they travel according to the sun. They are usually to be found travelling east to west or west to east, depending on the time of day, but never between north and south. The Common Dolphin is one of the highest and fastest jumping cetaceans. Wild specimens will often leap to 4-6 metres and trained individuals can reach 6.6 metres. They are known to dive to more than 240 metres with the deepest dives after sunset and before dawn. The longest Common Dolphin dives are approximately 4 minutes, while most are less than 1 minute.
This white marine dolphin has a hump on its back. It can grow up to 5m/16 feet and weigh as much as 1090kg/2400lb. Belugas inhabit the arctic, subarctic, Pacific and Atlantic oceans. They make a chirping noise for which they are sometimes called the ‘sea canary’.
Orca (Killer Whale)
The Orca is actually a dolphin. It is the largest of the species growing to over 9.2m/30 feet and 3636kg/8 tons. Orcas live all over the world in tropical, temperate and polar waters. They eat birds, fish, seals, sharks, other whales and dolphins. They live in groups called pods and remain in the same pod for their entire lifetimes.
Dolphin parents and young
In most dolphin species the males are slightly bigger than the females although the Orca male is much bigger than the female. Male dolphins are called bulls, females are called cows and young dolphins are termed calves. Dolphins’ reproductive organs are tucked away in slits to make swimming easier. Males have a separate anal slit, while females have a pair of mammary slits on either side of the genital slit. Dolphins court by swimming close together and touching each other with their flippers, tail fin and snout. Dolphins may have many partners over a lifetime and most mate all year round. When mating they swim belly to belly usually for about thirty seconds. The gestation period varies between nine and twelve months.
Most dolphins give birth to only one calf although twins are possible with often only one twin surviving. The size of a baby dolphin at birth varies according to the species. A baby Spotted Dolphin might be 3 feet long whereas an Orca could be 7 feet. Calves are born tail first so that they do not drown and are quickly pushed to the surface for their first breath. Dolphin calves nurse for up to 18 months although they start to eat fish at six months, gradually learning to catch their own food. Their teeth begin to erupt at about 5 weeks of age.
Different species mature at different ages. Dolphins are considered adults when they are old enough to have babies. A female Bottlenose Dolphin can have babies at between 8 and 10 years of age, whilst males mature at 10-13 years. Female dolphins are very good parents, staying with their calves for at least one year. Newborn calves stay very close to their mother’s side, swimming parallel and surfacing simultaneously. Males do not participate much in rearing the calf. Baby dolphins ‘talk’ to their mothers with a variety of sounds like squeaks, clicks and whistles. Females with calves stay together in a herd using the most productive areas of the community home range, whilst males form long-term bonds with each other and will range further afield as they age. Female calves generally remain with the group they were born into, whilst males will leave once old enough.
In the wild most species of dolphin live for at least 30 years. Some whales, including the Orca, may live to between 60 and 80 years.
Dolphins are most often thought of as friendly animals and, with some exceptions, are treated with love and respect. There are many stories, some dating back 2000 years, that tell about dolphins who befriended people. There are also reports of dolphins rescuing people who were in trouble in the water. They are playful, highly intelligent and curious, especially about people. They can be taught many different tasks.
Dolphins live and travel together in groups, looking after each other. The groups are variously called herds, schools, pods or shoals. The size of a herd depends on where the dolphins live. In the open ocean herds are very large, up to several thousand, whilst along the coast herds are smaller. As with baby and parent dolphins, communication with each other is via an assortment of squeaks, whistles and clicks. It is thought that they are warning each other of danger or food, and continual experiments are conducted in the hope that one day scientists may be able to ‘translate’. Sharks and killer whales eat dolphins but by staying together they have a better chance of avoiding trouble. Some dolphins work together to herd fish into tight shoals near the surface and then charge into the shoal to eat. Orcas hunt either alone or in packs and can tip ice flows to grab penguins and seals when they fall into the water. Dolphins can travel hundreds of miles with the seasons following the fish that they eat.
Strong social bonds can be seen between individuals of either and mixed gender. Dolphin friends swim face to face touching flippers. They swim in synchrony, twisting, turning, jumping and diving in perfect harmony.
Dolphins communicate with two types of sounds: vocalisations and echolocation. Vocalisations are the many sounds emitted from the blowhole. Echolocation, or sonar, is used to navigate and locate objects in water. A series of intense, short, broadband pulses of ultrasonic sound, called clicks, which echo and bounce back, to determine the size, shape and location of an object are sent out. Up to 1,200 clicks a second can be transmitted ahead of a dolphin. These clicks come from the rounded forehead of the dolphin, called the melon. The melon and lower jaw are filled with a jelly-like substance used to simplify sound waves. By listening to the reflected sound they can distinguish a shark from a boat etc. Dolphins do not have outer ears but they do have inner ears and can hear very well. The inner ear is adapted for hearing ultrasonic frequencies far beyond the range of human hearing, well over 100kHz (greater than bats), which are too high-pitched for the human ear, as well as those sounds within human range.
In order for dolphins to sleep, whilst living underwater and needing to breathe air, their brains are divided in two. The dolphin takes a series of short naps whilst one half of the brain is asleep alternating with the other half which stays awake to get the dolphin to the surface for air through the blowhole. A dolphin may empty and refill its lungs in less than a fifth of a second. As the dolphin breathes, air leaves the blowhole at speeds of over 100mph. Complex nerve endings around the blowhole sense pressure changes so that the dolphin knows exactly when the blowhole is in the air and can be opened. Water in a dolphin’s blowhole could drown it, so powerful muscles close the blowhole as the dolphin dives again.
Dolphins have eyelids which can close when they sleep or to protect the eyes from debris in the water. Elasticised lenses allow expansion and contraction for focusing clearly above and below the water in dim and bright light, and special glands protect eyes from salt water. The dolphin eye is very good at detecting moving objects. It has evolved to give a faster projection of a larger image, and large nerve fibres rapidly send information to the brain. The Bottlenose Dolphin has a visual range of 180º forwards, backwards and to the side although it cannot see upwards. The Bottlenose species can also move each eye independently of the other. Aerial vision is important for foraging and it is common to see dolphins rise vertically to look around above the water surface when they rise to breathe.
Dolphins have no sense of smell because over many millions of years the blowhole has moved from the snout to the top of the head. Dolphins and other whales evolved from land mammals and fossil remains have been found of dolphin-like animals that lived in the sea more than 45 million years ago. More of the dolphin evolution in the end section.
Dolphins have a sense of taste that they use to tell if a fish is fresh. They will not eat spoiled fish. They need to eat large quantities to keep themselves warm and to have enough energy to swim long distances. They don’t need to eat constantly because they can store energy in their fat. Dolphins do not need to drink water as this is obtained from the fish and food that they eat. Different species have different eating habits. For example the Bottlenose eats fish, squid and jellyfish; River dolphins eat fish, crabs and clams whilst Orcas also eat seals, sea lions, penguins, sharks, sea turtles and other dolphins and whales. The number of teeth a dolphin might have depends on the species. Most have between 100 and 160 teeth, although some may have as many as 250 or as few as 14. However dolphins do not chew their food whole as their teeth are designed only for catching.
Dolphins have very sensitive skin, which allows them to detect very small changes in pressure around their bodies. The skin is completely smooth allowing the dolphin to move easily through the water and also to reduce heat loss. The skin may bear rake marks from other dolphins’ teeth during play or mating, and can become sunburnt if they strand. Touch is a major component of a dolphin’s day. When dolphins play they constantly rub bodies and stroke and pat each other with pectoral fins and flukes.
Not all species of dolphin have a dorsal fin. Those that do, use them for stability. The dorsal fin is cartilage and can be as distinctive as a human face and is used by researchers to identify individuals.
Dolphins propel themselves by up and down movements of their tail flukes. They use their flippers for steering, turning and slowing down. The average dolphin swims at a rate of between 5 and 15 kilometres per hour. If they are being chased they are capable of swimming much faster to above 40 kilometres per hour.
Different dolphin species prefer different water temperatures. The Common Dolphin for example lives in warm waters whereas Belugas and Orcas can survive in arctic water temperatures. A dolphin’s core body temperature is 98ºF whilst the outer body temperature is usually cooler. They have a thick layer of fat, called blubber, just beneath the skin which holds in the heat and keeps out cold.
Dolphins can slow down their hearts thereby using less oxygen than other animals. When diving deep for food, their lungs collapse and the heartbeats become even slower, enabling them to adjust to the greater pressure. They can control where oxygen is sent in their bodies. When they dive they send oxygen to the important brain and heart. Oxygen is stored in their blood and muscles.
Dolphins jump for a number of reasons. Sometimes it is to loosen tiny animals that cling to their skin, sometimes to travel faster or see further, at other times it is to confuse and herd fish they are chasing. They also jump when playing and having fun.
It is believed that, at the end of the Cretaceous Period, the Mesonychidae family diverged, separating cetaceans from other mammals. During this period the family was widespread and diverse. There is strong evidence that the Mesonychidae also gave rise to modern ungulates, such as horses and pigs.
Mesonychidae were terrestrial mammals in the early Eocene, occupying the planes of what is now Africa. They had large bodies and their dentition suggests that the family consisted of carnivores, herbivores and omnivores. Fossils have been found in sediment deposited in estuaries and lagoons, leading to the conclusion that some mesonychids were evolving toward a more aquatic life.
The order Condylarthra was the most likely ancestor between cetaceans and artiodactyla, which gave rise to ungulates. Evidence for this includes fossil record support, similar blood composition, foetal blood sugar, chromosomes, insulin, uterine morphology and tooth enamel microstructure. These similarities, together with skull morphology, lead to the conclusion that the Mesonychidae were the ancestors of modern cetaceans.
Before the rise of the first cetaceans, terrestrial condylarthrans probably colonised the edges of the slow rivers that emptied into the southern and western Tethys Sea, which roughly corresponds to what are now the Mediterranean Sea and Persian Gulf. The assembly of these animals at water reservoirs may have been in a similar manner to mammals, such as hippopotami, of the Savanna tropics today. These ancient creatures, however, were probably more aquatic and less bulky.
At first the ancestors of cetaceans fed on molluscs and slow fish but, as the population grew and competition for resources increased, there was a need for the development of fast reflexes and teeth suitable for catching fast fish. The ability to escape from predators would also have been an issue although not of such importance as feeding.
Changes in the structure of teeth facilitate the understanding of fossil records. The members of the Condylarthra order are thought to have given rise to archaeocetes, the first cetaceans, at the end of the Palaeocene Period. It is possible that the intermediate’ between the Mesonychidae and the first cetaceans, was a seal-like animal that fed in the sea but came to shore to breed. The colonisation of the sea by such creatures began around 50 million years ago.
The Rise of Primitive Cetaceans
During the Palaeocene Period the current Mediterranean Sea and Arabian Gulf formed a semi-enclosed arm of the western part of the ancient Tethys Sea. It was probably in this area, around 50 million years ago, that the condylarthrans started to colonise the coastal fringes and swamps. They were exploiting the ecological niches left at the end of the Cretaceous period by extinct reptiles, including plesiosaurs and icthyosaurs. The warm waters of the Tethys Sea may have expanded during the Eocene Period due to the subsidence of Europe and increased volcanic activity in various parts of the world. The enlarged sea eventually became what is now the Atlantic and Indian Oceans.
Between the first cetaceans and their ancestors there is a lack of fossil information. It is possible that the transitional species was not very successful or widespread so that the few fossils that were formed are very isolated. Another possibility is that the evolution from condylarthrans to cetaceans was very rapid and localised geographically. Some evolutionists now believe that rapid phyletic changes in some animal orders are common.
The world of the first primitive cetaceans, classified under the suborder Archaeoceti, was one of tropical vegetation, swamps and advancing seas. These archaeocetes had elongated bodies and were mainly aquatic. Some were of a moderate size while others may have grown up to 21 metres/68 feet in length. They had reduced hind limbs and long snouts and were well adapted to shallow coastal fringes and open seas.
The oldest verifiable cetacean fossils are Pappocetus Luardi from southern Nigeria and Protoretus Atavus from Egypt. Both were of the Protocetidae family and were likely to have similar behavioural characteristics. A common name for the first cetaceans is Zeuglodonts, from the anachronistic generic name of one of the types, Zeuglodon. The bone structure of these animals was similar to the mammals of the late Cretaceous and early Eocene Periods, with specialisation for grasping fast prey such as fish.
During the Palaeocene and Eocene periods there was a large amount of conflict for resources. Large populations developed and then collapsed due to changes in the ecological balance brought on by their own existence. Since evolution usually occurs in unfavourable conditions where the size of population is reduced, it was particularly accelerated during this period.
Natural selection at this time favoured adaptations for the capture of fast moving and agile fish rather than fresh water estuarine molluscs and slow fish. The dentition of Archaeocetes was heterodont, meaning that the incisors, cuspids and molars were differentiated as in terrestrial mammals. As modern cetcea, these primitive cetaceans had dense ear bones, long palates, nostrils on top of the snout, space around the ear bones for fatty deposits and air sacs to isolate the ear from the skull. The body was elongated with a long tail, short neck and reduced hind limbs. The front limbs were paddle-shaped and there was a point of flexion in the tail vertebrae allowing up-down movement as well as side-to-side movement.
The first cetaceans were not likely to have been as well adapted physiologically to a marine existence as modern cetaceans. Even later specimens were limited to warm water and the archaeocetes could only sustain short, shallow dives. The many limitations in the body made competition with more advanced cetaceans, during the Oligocene Period, impossible. Although dominant in the Eocene Period, diversity fell during the Oligocene. The Oligocene Period is known for low diversity in general especially in the western South Pacific. The date of the last remains of this suborder, found in France, cannot be confirmed as to whether they were from the early or middle Miocene period.
During the Oligocene period, between 38 and 25 million years ago, archaeocetes were replaced by members of at least four families; the Agorophiidae and Squalodontidae which were primitive odontocetes, and the Aetiocetidae and Cetotheriidae which were early mysticetes. Life became even more aquatic for the cetaceans during this time as the external nostrils shifted backward, structures formed to seal the animals from the water, and the long mobile neck, functional hind limbs and most of the pelvic girdle were lost. Any remaining pelage that associated these creatures with their terrestrial ancestors was also lost. The body became more torpedo-shaped and a dorsal fin developed. The latter was probably common to all species at first but was later lost by some such as the Right Whale Dolphin. Horizontal tail flukes developed during this period.
Any functions that were not useful for a marine existence were quickly selected out of the cetaceans. The evolution was aggressive with many adaptations for survival, including resistance to the accumulation of haemoglobin, tolerance of low levels of oxygen, a hypodermal blubber layer for the storage of nutrients, sophisticated control of body temperature and telescoping of the front skull.
The most primitive odontocetes still had heterodont dentition but, by the late Oligocene, the teeth had been modified in some specimens to form long rows of many sharp uniform teeth with single roots and conical crowns; a condition known as homodonty. Almost all present day dolphins have homodont dentition, with Risso’s Dolphin being an exception. Some highly derived odontocetes, notably the Norwhale, lost or reduced parts of their dentition or developed specialised teeth. The odontocetes underwent extreme modifications to the design of the skull, adapting for acoustic scanning and diving. The melon and modern nasal passages developed, as did the ability to perform echolocation when the skull was telescoped. There were already some adaptations that aided echolocation, as the isolation of the ear bones with body fat and air sacs allowed for directional hearing.
The earliest true odontocetes were of the Agorophiidae family. They were short-beaked whales with triangular shark-like teeth. These gave rise to Squalodonts, the behaviour of whom may have resembled the Killer Whale although morphologically they were quite different. Most were large with bodies of at least 3m/10 feet in length with almost completely telescoped skulls.
Squalodonts gave rise to a family of primitive dolphins, traditionally called Eurhinodelphinidae, but now termed Rhabdosteidae. These creatures had extremely long snouts and most species were approximately 3m/10 feet in length. Fossils have been found in Europe, North America, South America and freshwater deposits of Australia, from the Early and Middle Miocene. The skulls were fully telescoped with many homodont teeth. However, unlike modern dolphins, the skulls were still symmetrical at this time.
From Primitive Odontocetes to Delphinidae
During the early Miocene, the melon developed in its modern form and the acoustic systems were refined. Various dolphin-like families, now extinct, flourished. The Squalodelphinadae were the earliest known odontocetes with asymmetrical skulls. Two taxa have been identified: Squalodelphis from northern Italy and Diochotichus from Argentina. These creatures were small, probably less than 3 metres, and had homodont teeth. The skulls were telescoped in a manner similar to the Rhabdosteidae and many derived species of Squalodonotodae. Some believe that beaked whales evolved from this species.
In modern times, the Delphinoidea family contains most of the living cetacean species, including the Delphinidae, Phocoenidae and Monodontidae families, as well as the now extinct Albireonidae and Kentriodontidae families which are thought to be ancestors. Most members of the Kentriodontidae were small with lengths up to 2 m/6.5 feet. They had short beaks with many homodont teeth. Although diverse during the Middle and Late Miocene in both the Atlantic and Pacific Oceans, there are no specimens less than ten million years old.
The Delphinidae, Phocoenidae and Monodontidae families are genetically very similar, with only a 10-15% variation in C-heterochromatin. Almost all have the same chromosome number, 2n=44. The one exception is the Orca, with only one pair of t chromosomes, which has short arms and satellite structures. The m chromosomes are also very small in the Orca.