Dolphins: Delphinidae

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DOLPHINS: Delphinidae

KILLER WHALE (Orcinus orca): SPECIES ACCOUNTS
COMMON BOTTLENOSED DOLPHIN (Tursiops truncatus): SPECIES ACCOUNTS
SPINNER DOLPHIN (Stenella longirostris): SPECIES ACCOUNTS

PHYSICAL CHARACTERISTICS

Dolphins are found in all oceans and many rivers of the world. They are often confused with other aquatic animals. Dolphins arose from the same ancestor as porpoises, but have been a separate family for at least eleven million years. In addition, the common names of some dolphins lead to confusion. For example, the killer whale is actually a dolphin. With genetic testing now available, some re-classification of individual dolphin species is occurring.

Dolphins have long, streamlined, torpedo-shaped bodies adapted to life in the ocean. Generally they are fast, acrobatic, agile swimmers. The bones in what would be the hand and arm of a land animal are compressed into a web of bones to make flippers. Their back legs are so reduced that all that remains are a few internal pelvic bones. They have strong, muscular tails. Dolphins breathe through a single blowhole on top of their head. All dolphins have a melon, a fatty organ on their forehead that they use for echolocation. Echolocation (eck-oh-loh-KAY-shun) involves making sounds that bounce off objects. Sense organs pick up the echo or reflected sound and use information about the echo's timing, direction, and strength to determine the location of objects. They have a single type of cone-shaped tooth, but the number of teeth ranges from four to about 260, and the size varies with the size of the species. Dolphins are able to taste, but not smell.

Within this family there are many physical differences in size and color. The smallest dolphin is the endangered Hector's dolphin. They are about 4.5 feet (1.4 meters) long and weigh about 117 pounds (53 kilograms). The largest is the killer whale, which can measure 30 feet (9 meters) and weigh 12,000 pounds (5,600 kilograms). Dolphins come in many colors, including black, white, gray, tan, brown, orange, and pink. Some have distinctive color patterns, while others are a single color.

GEOGRAPHIC RANGE

Dolphins are found in every ocean and sea and in many major river systems. They are the largest family of cetaceans.

HABITAT

Dolphins live in salt water, fresh water, and brackish water, a mixture of salt and fresh water. They live in both the open ocean and in coastal waters, although more live shallow water. Their distribution is determined mainly by the availability of prey.

DIET

Dolphins are carnivores, meat eaters. They eat fish and squid and capture their food one fish at a time. The type of fish they prefer depends on the zone of the ocean that they inhabit. Killer whales eat fish, but they also hunt seals, sea lions, other dolphins, whales, porpoises, and sea birds.

Dolphins use echolocation to navigate and find prey. Echolocation allows dolphins to use high-pitch sounds that bounce off objects in order to determine their location. In some species, echolocation is so sensitive that it can locate an object less than 0.5 inch across (1.25 centimeters) at a distance of 50 feet (15 meters).

BEHAVIOR AND REPRODUCTION

Dolphins are highly intelligent social animals. Many species appear to live in cooperative groups, groups that work together. They may live in groups called pods of fewer than five or as many as several thousand. To some extent, group size depends on the availability of food. Within a large group, animals often separate by age and sex.

Dolphins have excellent hearing and communicate with each other by producing a variety of different sounds, often identified as "clicks," "pulses," and "whistles." Some of these sounds may be identifiers for individual animals, but this communication is not well understood. Dolphins living in clear water may also communicate by flipping and flashing patches of color on their bodies.

There are many examples of dolphins working cooperatively. They may work together to locate and round up a school of fish or chase them into shallow water or to attack a predator, an animal that hunts them for food. They have been seen helping newborn or injured animals to the surface to breathe. They are best known for their acrobatics. They often leap and spin out of the water, sometimes in large, coordinated groups. They are curious and playful. Some dolphins will catch a ride on the waves a boat makes as it passes through the water. Dolphins can be taught behaviors or tricks when in captivity.

Dolphins mate and give birth in the water. From an early age, both sexes do a lot of touching and stroking, rubbing and sex play behavior with their own and the opposite sex. Sexual maturity, the ability to reproduce, occurs when individuals are between five and sixteen years old. Larger species tend to mature later than smaller ones. A single calf is born after a pregnancy lasting ten to fifteen months.

The bond between mother and calf is extremely important and may last many years. Calves begin to catch fish when they are a few months old, but may continue to nurse for three-and-a-half years or more. Even after they are weaned, no longer nursing, they remain with their mother for a year or longer.

DOLPHIN COMMUNICATION

Scientists who have recorded dolphin whistles have found that individual animals react much more strongly to the whistle of an individual that is related to them than to a whistle of a stranger. It appears that each dolphin has a signature whistle all its own that is recognized by its family.

DOLPHINS AND PEOPLE

Dolphins are familiar to most people from exhibitions at marine parks and movies and television programs such as "Flipper." Dolphin-watching tours attract thousands of ecotourists, who travel to observe these animals without interfering with them. More controversial are resorts where tourists can swim with captive dolphins. Dolphins are hunted for food in some places in the world. They are also trained by the United States military to retrieve small underwater objects.

CONSERVATION STATUS

The conservation status of dolphins depends upon the species. Hector's dolphin is considered Endangered, facing a very high risk of extinction in the wild, because it is often killed accidentally by fishing gear. Population estimates are not available for most species.

Dolphins are threatened by hunting, accidental capture in fishing nets, pollution, and capture for display in captivity. In the 1990s public pressure resulted in the development of dolphin-free fishing nets and the sale of dolphin-free tuna. These changes have resulted in a substantial decrease in the number of dolphins accidentally harvested during fishing. Dolphins are protected in the United States under the 1972 Marine Mammal Protection Act and are the focus of many conservation and research organizations.

KILLER WHALE (Orcinus orca): SPECIES ACCOUNTS

Physical characteristics: Killer whales, or orcas, are the largest dolphins, measuring 30 feet (9 meters) and weighing up to 12,000 pounds (5,600 kilograms). They have a striking black and white pattern of mainly black above and white below, and they have the tallest dorsal fin of any cetacean. The dorsal fin can reach 6 feet (2 meters) in height. They are the top predators in the ocean.

Geographic range: Killer whales live in all the oceans of the world, but are most abundant in cold water areas such as the Arctic and Antarctic.

Habitat: Killer whales prefer cold water, but can live in warmer temperatures. They tend to live in water that is less than 650 feet (200 meters) deep. Rarely they have been known to swim up rivers such as the Columbia in the United States and the Thames in England.

Diet: Killer whales have the most varied diet of any dolphin. They hunt fish, seals, sea lions, other dolphins, porpoises, and whales. Their diet depends primarily on what is available in their region of the ocean. They are swift swimmers and hunt in packs. They can successfully attack a blue whale, the largest animal on the planet, or a great white shark. In some places they chase sea lions up onto the beach and attack them. They are able to swallow a small seal whole. Adults eat 3 to 4 percent of their body weight daily.

Behavior and reproduction: Killer whales live in stable pods. Males often stay with their mothers for years after they are weaned. Like other dolphins, they use echolocation and make sounds to communicate with members of their pod.

Killer whales have pregnancies that last from fifteen to eighteen months and produce a single calf. Calves stay dependent on their mothers for several years. New calves are born only every three to eight years.

Killer whales and people: Most people know of killer whales from exhibits at marine parks and movies. Since they can regularly be seen near shore, they are often the object of dolphin-watching tours. In 1985, the first killer whale was successfully born in captivity. That whale lived to adulthood and produce calves of her own. It is not clear how long killer whales live in the wild. Estimates range from thirty to fifty years.

Conservation status: Killer whales are not threatened. Their main threat appears to be pollution of their habitat. ∎

COMMON BOTTLENOSED DOLPHIN (Tursiops truncatus): SPECIES ACCOUNTS

Physical characteristics: Common bottlenosed dolphins, also called Atlantic bottlenosed dolphins, range in size from 8 to 12.5 feet (2.5 to 3.8 meters) and in weight from 500 to 1,100 pounds (227 to 500 kilograms). These dolphins can be colored brown to gray on their backs and light gray to white on their bellies. There are several distinct subpopulations in different regions of the world.

Geographic range: These dolphins are found worldwide in warm and temperate, moderate temperature, waters. In the United States they are the most abundant dolphin along the Atlantic coast from Massachusetts to Florida.

Habitat: Common bottlenosed dolphins prefer warm shallow water and are often found along the coast in harbors and bays, although they also inhabit open ocean.

Diet: These animals eat fish, squid, and shrimp. They often feed cooperatively, herding fish together to make them easier to catch. In

shallow water they may chase fish into a sandbar where they are trapped.

Behavior and reproduction: Common bottlenosed dolphins form pods of varying size. Pods in the open ocean seem to be larger than those close to shore. The pods are moderately stable and tend to migrate in order to follow the fish. Those pods living in cooler waters usually migrate to warmer water in the winter.

Common bottlenosed dolphins are curious and playful. They often ride the waves produced by the passage of a boat through the water. They can jump as high as 16 feet (4.9 meters) out of the water.

Female bottlenosed dolphins have their first calf between the ages of five and twelve years. Pregnancy lasts about twelve months and produces a single calf. Calves stay with their mothers for about three years, after which another calf is born. Bottlenosed dolphins have been successfully born and raised in captivity.

Common bottlenosed dolphins and people: Common bottlenosed dolphins are the dolphins most frequently exhibited in marine park shows. They are very acrobatic and can be taught many behaviors in captivity.

Conservation status: Not enough information is available to give the common bottlenosed dolphin a conservation rating, however, they do not appear to be threatened. The main threat to their habitat is pollution. ∎

SPINNER DOLPHIN (Stenella longirostris): SPECIES ACCOUNTS

Physical characteristics: Spinner dolphins, also called long-snouted dolphins, are known for their acrobatic displays. Spinner dolphins are about 7.7 feet (2.3 meters) long and weigh about 170 pounds (78 kilograms). Males are usually larger than females. They vary in color from individuals that are all gray to ones having black backs, gray sides, and white bellies.

Geographic range: Spinner dolphins are found worldwide in tropical and subtropical waters.

Habitat: Spinner dolphins mainly live in the open ocean, although they may come into shallow waters to feed.

Diet: Spinners are carnivores. They tend to feed at night and eat mainly fish, squid, octopus, and shrimp.

Behavior and reproduction: Spinner dolphins form schools or pods that may contain more than 1,000 individuals. They are very social and communicate with each other by sound and touch. They are best known for their ability to leap out of the water and turn on their longitudinal, long, vertical, axis. Some can spin as many as seven times on one jump. This behavior gave them their common name.

Less is known about the reproductive behavior of spinner dolphins than some other species because they live farther out in the ocean and they do not survive well in captivity. Females produce one calf after about a ten-and-a-half-month pregnancy. New calves are born about every three years.

Spinner dolphins and people: Spinners were the first dolphins captured for display in marine parks because of their ability to leap and spin, but they do not survive well in captivity. Their amazing leaps and spins attract ecotourists who want watch these animals in their natural habitat. Because they often associate with tuna, they are sometimes accidentally killed by fishing gear.

Conservation status: Spinner dolphins are not threatened. ∎