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