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Atlantic white-sided dolphins, Lagenorhynchus acutusDolphins (toothed whales)

Marine Life / Marine Vertebrates / Structures & Adaptations to Marine Living »
By Kerry G. Beck, May 2010

Description & Behavior

They’re the acrobats and court jesters of the sea, troops of aerial spinners and wave dancers. Their long shining sleek bodies jet high into the air as they perform a grand ballet with tails propelling them as they skim each wave against the continuous horizon. Mesmerizing to watch, dolphins have been gliding, flipping and dancing into our hearts for centuries.

While no one is exactly sure why these marine mammals perform their fascinating aerial feats, it’s well documented that the playful stunts are an integral part of their behavior. They are well credited throughout history for their playful and social nature as well as superior smarts. Closely related to whales and porpoises, there are over 50 species of dolphin found worldwide.

During the early morning hours, spinner dolphins off Kauai’s Na Pali Coast can be seen leaping from the water at great speeds. The louder the tourists in passing catamarans cheer and clap the more elaborate and lasting the performance, some twirling many times in the air before ever splashing back down. In Florida’s Key West, the more common bottlenose varieties might be seen playing catch with a clump of seaweed or working collaboratively in groups to circle in on and whip fish up into the air to feed on. They spend a great deal of time “fishing,” as an adult male may consume up to 30 pounds or more of fish in a single day. Though we often picture them propelling up to the clouds, they have been known to dive as deep as at least 300 meters into their world below the sea surface.

The pantropical spotted dolphin, Stenella attenuata

Having their bellies as built-in surfboards, they will readily catch a ride on a bow wave and have been doing so for many years. Ancient Greek sailors believed a dolphin riding in their wake was a good omen for a smooth voyage. This was not the case for the pink dolphin of the Amazon River where the indigenous people held if a dolphin was encountered it may tempt unknowing humans into the water and carry them to an underwater world of no return. Sadly and ironically, it’s the pink dolphin facing a fate of no return. That dolphin species, which is in fact pink and has a hump rather than a more common dorsal fin, has landed on the International Union for Conservation of Nature Endangered Species List. Excessive boat traffic, pollution and habitat loss have brought more than a bad omen to this amazing creature. Unfortunately, similar factors pose a threat to all Clymene dolphins, Stenella clymenedolphin species in the oceans of the world. Able to travel at top speeds, swimming over 40 kilometers per hour, dolphins can’t always out swim the threats they face.

While dolphins seem to be quite the chatterboxes, with their wide range of clicks and whistles, hearing may be their most important asset with a technique called echolocation. They have no sense of smell, so eyesight and hearing are essential for hunt and survival. In addition to excellent eyesight, they can hear frequencies many times more than a human. Like bats, dolphins are able to bounce high-pitched sounds toward an object and listen for the echo back, allowing them to get a map of their environment, find food and navigate. But increasing interfering factors like noise pollution, underwater testing conducted in the search for oil and gas sources, naval sonar and live firing exercises are problematic. All these factors can damage a dolphin’s hearing and put great stress on them as they try to escape an unsafe environment. Some even suffer decompression sickness as they try to surface too quickly to escape the noise. Recently researchers at the Bottlenose Dolphin Research Institute conducted a study confirming a link between dolphin vocal emissions, social activities and social signals. And what they can’t hear they can certainly feel.

Irrawaddy dolphins, Orcaella brevirostrisThe dolphin’s skin is very smooth and jam-packed with nerve endings making it many times more sensitive to touch than a human’s. Shedding a layer of skin every two hours, they are very delicate and easily hurt by rough surfaces or garbage and other objects that simply shouldn’t be in their environment. You’ll never find a barnacle hitching a ride on a dolphin. But just imagine how painful it might be to try to wrestle free from a fishing drift or gill net, flee irritating and potentially poisonous contaminants in the water or heal after a run-in with a motor boat blade or jet ski.

Only a small portion of the population may be purposely hunting dolphin but anyone can take action to protect them from harm. Preserving the dolphin isn’t only beneficial to their overall survival but could also directly benefit modern medicine. Recently at a meeting of the American Association for the Advancement of Science the National Marine Mammal Foundation reported on a study concluding that blood chemistry of a fasting dolphin resembled that of humans with diabetes. The researchers concluded that dolphins could be the most realistic model for studying diabetes in humans.

Common bottlenose dolphins, Tursiops truncatusIn addition to all of their captivating characteristics, dolphins are really smart. Anyone who has ever worked with dolphins has noted their natural ability to learn, problem solving ability and sometime mischievous streak. They’ll also stick together in times of trouble. In the wake of Hurricane Katrina, eight bottlenose dolphins were forced out to sea when their marine facility was destroyed by the storm. Before being rescued two weeks later, all eight dolphins survived injuries and stuck together becoming a symbol of hope and strength for the Gulf Coast, a region with an ecosystem that is now under brutal attack again, this time from the devastating oil spill.

What They Look Like

Although their color varies among species, the basic coloring of dolphins is a shade of gray with a white underbelly. Their large mouth, curved into a wide smile, houses up to 250 small teeth. They can vary in size from 1.2 m and 40 kg to 9.5 m and more than 9,000 kgs. Dolphins breath through a blowhole on the top of their head. They only have a few hairs at the tip of their snout, or rostrum, at birth. However, the boto or Amazon river dolphin, which has poor eyesight, does have some small hairs on the rostrum believed to aide as an additional tactile sense. Looking at a dolphin up close there are many unique features beyond the curved mouth that appears set in a constant and mischievous smile. They have a streamlined fusiform body built for speed and equipped with the tail fin, or fluke, for propulsion. Using their entire tail section they control direction with the dorsal fin, found on some species, providing extra stability. No dorsal fin is the same on any dolphin, it's a key distinguishing feature like a unique face or fingerprint.

How They Swim

Fast! The speedy marine mammals, can swim up to 40 kilometers per hour or faster for an hour or longer. They use their tail fin, or fluke, for propulsion and pectoral fins and tail for direction. Those who have a dorsal fin use that for added stability.

How They Communicate

Capable of producing a wide repertoire of sounds, the dolphin uses nasal air sacs located just below the blowhole. Three distinct categories of sound include frequency modulated whistles, burst-pulsed sounds and clicks. Dolphins communicate with their whistles and burst-pulsed sounds and some can identify themselves using a signature whistle. Clicks are mostly directional when they use echolation. For example, a click rate increases when a dolphin approaches an object of interest. These are some of the loudest sounds made by marine mammals.

Do They Sleep?

Dolphins sleep with only one brain hemisphere in slow-wave sleep at a time, which allows them to stay conscious enough to breathe and keep a watch out for any predators or other threats. In captivity dolphins have been found to enter a full sleep state where both eyes are closed and there is no response. This may be because they are not preyed upon in captivity.

World Range & Habitats

Where They Live

There are about forty-eight species of ocean dolphins and five species of river dolphins. They are found in all oceans of the world where they can be found predominantly in the shallower seas of the continental shelves. Very social animals, they tend to live in pods of up to a dozen members. However, in some areas where there is an abundance of foods, superpods can form with more than 1,000 dolphins. Dolphins also will stay and try to take care of any injured or ill members of their group. They also have been found assisting other species and protecting swimmers from sharks by swimming protective circles around the swimmers or charging the sharks to stave them off.

Feeding Behavior (Ecology)

What They Eat

Fish, fish and more fish. An adult male orca can eat up to 290 kgs of fish or more in one day. Squid and an occasional crustacean are added delicacies on a dolphin’s menu. They are all carnivores. To feed many times members of a pod will surround and circle a school of fish. Dolphins have also been observed corralling fish into shallow water where they can be more easily captured. They can also cooperatively herd fish into a “bait ball” where the hungry dolphins take turns plowing into the ball to feed on the stunned fish.

While there are few threats to dolphins from other marine life who want to eat them, they occasionally will fall prey to some larger shark species including bulls, great whites and tiger sharks. The largest dolphin is the orca, commonly known as the killer whale due to its huge size, is the ocean's very top predator.

Orcas (formerly known as the killer whale), Orcinus orca

Some infrequent incidents of orca’s attacking smaller dolphins or calves have been documented. Perhaps one of the most inexplicable threats comes from Japan where thousands of bottlenose dolphins are slaughtered each year for their meat.

Rough-toothed dolphins, Steno bredanensisLife Histories

How They Reproduce

Dolphins are viviparous, meaning that their eggs develop inside the female and the embryo derives nutrition from the mother. Dolphins mate belly to belly. Because this is also a social behavior, some species engage in lengthy activity, however, copulation is brief, about 10 seconds, and may be repeated several times over a short period. In some cases males will fight viciously over females during the breeding season and a hierarchy based on size is generally established among males. Females become sexually mature at five to 12 years of age, while males are mature at nine to 13 years. Their reproductive organs are located on the underside of the body. Males have two slits, one concealing the penis and one further behind for the anus. The female has one genital slit, housing the vagina and the anus. A mammary slit is positioned on either side of the female's genital slit. Dolphins usually produce offspring once every two to three years. For most dolphin species, gestation lasts for 12 months and lactation lasts from 12 to 18 months. Despite the long nursing period, young dolphins begin eating solid food when they are less than six months old. Mother and calf often remain closely associated until the young dolphin is four or five years old.

Conservation Status/Additional Comments

Yangtze river dolphins, Lipotes vexilliferHow You Can Help

Other than some humans, dolphins have few natural enemies. Yet their future is uncertain. Many dolphins are killed each year when they get entangled in fishing nets. Others are suffering due to contamination of their environment and death and injury after run-ins with boats and jet skis. In Japan dolphins are herded toward shore and killed with harpoons in bloody hunts, a practice that conservation organizations have condemned. Pesticides, heavy metals, plastics and other agricultural pollutants also harm their environment. There are a number of organizations engaged in ways to protect the future for the dolphin. Visit the sites listed below under Further Research and read more on how you can help ensure their future.

Taxonomy

There are an estimated 53 species of dolphin in 24 genera. Dolphins are related and classified in the following way:

Kingdom Animalia Phylum Chordata Subphylum Vertebrata Class Mammalia Subclass Theria Infraclass Eutheria Order Cetacea Suborder Odontoceti (toothed whales)

Hybrid dolphins

Three dolphins were found beached on the coast of Ireland in 1933 and were reportedly hybrids between Risso's and bottlenose dolphins. This mating was apparently later repeated in captivity producing a hybrid calf. A bottlenose dolphin and a rough-toothed dolphin in captivity were also reported to have produced hybrid offspring. A common dolphin-bottlenose dolphin hybrid is kept at SeaWorld in California. Other dolphin hybrids are also kept in captivity around the world or have been reported in the wild, such as a bottlenose-Atlantic spotted dolphin hybrid. The best known dolphin hybrid is the wolphin, a false killer whale-bottlenose dolphin hybrid which is fertile. Two wolphins are currently captive at the Sea Life Park in Hawaii; the first was born in 1985 from a male false killer whale and a female bottlenose dolphin. Wolphins have also apparently been observed in the wild.

The vaquita, Phocoena sinus

Evolution

Dolphins, whales and porpoises have all evolved from land dwelling ancestors, like all marine mammals, most likely of the Artiodactyl order. The ancestors of dolphins are thought to have entered the water about fifty million years ago, in the Eocene epoch.

Long-beaked common dolphins, Delphinus capensisInterestingly, the bones in dolphin pectoral fins closely resemble those of the human hand and their skeletons also have two small pelvic bones which appear to be vestigial hind legs. In 2006 an unusual bottlenose dolphin was captured in Japan with small pelvic fins on each side of its genital slit, which scientists believe to be a more pronounced development of these vestigial hind limbs.

Evolving in water over time, dolphins became more streamlined and lost their hind limbs in favor of tails that enable them to swim faster and dive deeper after prey. For example, the common dolphin can swim at speeds up to 64 kph.

Further Research

Conservation, research and news:

Photos:

References

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