Background

The Atlantic bottlenose dolphin, Tursiops truncatus, is a small-toothed whale that ranges from Cape Cod to the Gulf of Mexico. These mammals primarily occur in the saltier waters of the lower Chesapeake Bay in summer. They can be observed feeding on schools of fish at Kiptopeake, inside Cape Charles, near the Elizabeth and James rivers, and often are seen swimming in pods of twenty or more individuals. As many as one hundred animals have been seen in the Bay at one time. On a given day a dozen or more can be found in the upper Chesapeake, near the Miles River, and in late summer along the western shore near Annapolis and Baltimore. Occasionally they have ventured up the Potomac River, to Washington, D.C.

The familiar bottlenose belongs to scientific order Cetacea, which includes all whales, and is a member of the suborder Odontoceti, or toothed whales (along with beluga, beaked, and sperm whales and porpoises). Their family is Delphinidae, in which there are thirty species, but only one species of bottlenose, Tursiops truncatus.

Most of the dolphins we find in public aquariums are bottlenose. They are a gregarious species, capable of complex social behaviors. Bottlenose dolphins can reach lengths of 12 feet and can weigh up to 1,400 pounds, but usually weigh 300 to 400 pounds. They are sleeker and more slender than the porpoise. In the Bay the bottlenose feeds on catfish, eels, menhaden, mullet, shrimp, crabs and squid. It can eat between 12 and 15 pounds of fish a day in captivity.

Physical Characteristics

Although it can adapt to many habitats, in the wild the bottlenose seems to prefer to swim within 800 kilometers of shore, and keeps to a "home range." The coastal ecotype that swims in the Bay is adapted to warmer, shallower waters, and is a slimmer variety than the pelagic bottlenose, but their home ranges overlap somewhat. This type has a smaller body and larger flippers for greater maneuverability and heat dissipation. Despite their high profile in aquariums, the bottlenose often is difficult to identify in the wild, because they rarely raise their heads or perform somersaults.

• The Atlantic bottlenose has a high and falcate (curved back) dorsal fin, with a broad base and slightly hooked tip. They have 23-25 pairs of conical interlocking teeth–used for grasping, not chewing–per jaw. Their flippers are long, dark and slender, with pointed tips.
• This dolphin's neck is very flexible: whereas in many ocean dolphins the vertebrae in their necks are fused, in the bottlenose five out of seven of the vertebrae are left unfused, which allows it to bend its neck to a right angle with the rest of its body.
• The bottlenose dolphin has a streamlined body, generally gray to green-gray or gray-brown dorsally, with a white to pinkish-white underside. The coloration may be a form of camouflage–when viewed from above, the dorsal skin blends with the dark waters, and from below the white belly blends with the bright surface of the sea.
• The dolphin's pectoral flippers actually have skeletal remnants of the forelimbs of terrestrial mammals, with the appearance of finger-like bones, which are foreshortened and modified and protected by dense connective tissue and thick cartilage pads.
• The flippers help the dolphin steer, and, in tandem with the flukes (tail fins), slow to a stop. The blood circulation in the flippers also helps maintain their body temperature. Heat from the blood passing through the arteries is partly transferred to a network of veins that are also located in the flippers, which reduces the loss of body heat to the environment. To help the animal reduce excess body heat, circulation is greater in the veins near the flippers' surface and reduced in the veins returning to the main body.
• The tail lobes or flukes are similarly made entirely of fibrous connective tissue and have no bone or muscle.
• Flukes have a similarly constructed circulatory system to help the dolphin conserve body heat in cold water and reduce it in warm. (Modern sharks, in attacking a dolphin, will bite off or tear a fluke. This disables the dolphin and makes it swim in circles until, bleeding and exhausted, it can be dispatched with little risk to the attacker.)
• The dorsal fin, also made of dense connective tissue, is believed to act as a keel, stabilizing the dolphin as it swims. Some dolphin species have no dorsal fin, however, so this may not be the fin's crucial function. The circulatory system in the fin mirrors that found in the flukes and flippers to conserve body heat. Dorsal fins often are curved back, but have variable shapes.
• The bottlenose dolphin has a well-defined rostrum (or snoutlike projection), usually about three inches long.
• Their eyes are located on the sides of the head, near the corners of their mouths. Glands near corners of eye sockets secrete mucus to lubricate and protect the animals' eyes from infective organisms and other impurities while swimming.
• The bottlenose has mall inconspicuous openings for ears, without external flaps.
• Their single blowhole is covered by muscular flap, which provides watertight seal.
• The dolphins' senses of hearing, sight and touch are thought to be acute; their senses of smell and taste are less well known (but they do have taste buds). Their auditory cortex is highly developed, and their auditory nerve contains nearly 68,000 cochlear fibers, twice as many as the human auditory nerve. They can respond to tones within a frequency range of 1 to 150 kHz (compared with the human range of .02 to 17 kHz). Their hearing takes place primarily through their lower jaw, which contains a fat-filled cavity that appears to conduct sound waves to the middle and inner ear and the brain's hearing centers via the auditory nerve.
• The dolphin's acute vision appears to be due to adaptions of the lens and cornea that compensate for the refraction of light that occurs during the transition from aquatic to aerial vision. The dolphin's retinas also contain both rod and cone cells, which may permit the animal to see in both dim and bright light. These cone cells indicate that the dolphin may be able to detect color, although this hypothesis has not been confirmed.
• Bottlenose dolphins are capable of diving to great depths to obtain food. They usually dive between 10 and 150 feet, with the deepest dive on record being nearly 1,800 feet. Their dives usually last between seven and eight minutes, during which the dolphin's heartbeat slows. While underwater the dolphin holds its breath and begins to exhale just before it reaches the water's surface.
• The dolphin's metabolic rate is higher than that of land mammals of similar size, which generates a great deal of body heat.
• Bottlenose dolphins spend about 33 percent of each 24-hour period, sleeping.

Life Cycle

Bottlenose dolphins reach sexual maturity at varying ages, but females often are ready to reproduce between the ages of 5 and 12 years. Males usually become mature between the ages of 10 and 12 years. Females often initiate courtship and mating behaviors, and can be sexually receptive throughout the year.

• The male will often nuzzle the female before mating, and adopt an "S-curve" posture, in which he raises his head and points his flukes down.
• After conception, gestation usually takes 12 months. Calves are born in the water, either tail- or headfirst. The umbilicus snaps either during or just after delivery.
• During birth, an assisting dolphin, which may be either male or female, remains close to the mother and often is the only other dolphin permitted near the newborn calf.
• Calves are usually between 42 and 52 inches long at birth and weigh approximately 44 pounds. Their tail flukes and dorsal fin are pliable at first but gradually become firm, and their dark coloration and the lines on their sides (due to fetal folding) usually disappear within six months.
• Calves nurse within several hours of their birth, suckling from nipples that are concealed in the mother's abdominal mammary slits. Calves nurse for five to ten seconds, between three and eight times an hour, day and night, for up to 18 months. The mother's rich milk (composed of 33 percent fat, 6.8 percent protein, and 58.3 percent water and traces of lactose) helps the calf produce its thick layer of blubber.
• Calves begin to eat a few fish within three or four months. They will begin to vocalize within a few days of birth, but it takes time for them to develop their "signature" clicks and whistles.
• A female could reproduce again within two years, but the average interval is three years.
• Individual dolphins usually have a life span of 20-25 years, but some may live longer than 40 years.

Behavior and Communication

Bottlenose dolphin studies often are conducted on animals in captivity, which means the data may be tainted. However, many interesting behaviors that are common in captivity have been observed in the wild as well. Wild dolphins have been seen "porpoising"–leaping from the water while swimming, sometimes tail first, and doing somersaults. They are powerful swimmers. While they frequently "cruise" at speeds of between 3-7 mph, they are capable of maintaining speeds of 22 mph for short periods, known as "bursts." Occasionally adult females and less frequently adult males have been seen assisting injured dolphins by holding them at the surface so that they could breathe.

Bottlenose dolphins, like other dolphin species, travel in groups called pods, which are coherent, long-term social units. A pod contains about seven animals, of varying ages and genders. For example, mother-calf pairs often travel with other mature females. Juveniles may travel in same-sex or mixed-sex groups. Adult males, however, often travel alone, or in small groups of other adult males, often trios. They rarely travel with juvenile males but may swim with adult females for brief periods. Pods may combine for several minutes or hours to form larger aggregates or herds.

In general, dolphins are known to be sociable animals, although reports of altruistic or social behavior towards humans appear to be exaggerated. They appear capable of recognizing other individuals, even after long periods of separation. Bonds between mothers and calves are particularly long-lasting, as are adult male pair bonds.

Although no firm evidence exists that would suggest dolphins have a "language," they do appear to communicate with one another through production of a variety of sounds, including signature whistles that enable calves to identify their mothers or other members of their pod.

In addition to communicating with their pod members, dolphins use sound to "see" with their ears, through echolocation. They produce clicking sounds, and then receive and interpret the resulting echoes, which enable them to navigate.

Dolphins produce brief directional clicks, which pass through the dolphin's forehead (also known as the "melon"). The melon contains lipids that help focus the sound that the dolphin produces into a sort of sonic "beam." This beam travels 4.5 times faster through water than it would through air, and bounces off objects in the water, producing an echo that the dolphin then interprets. This form of echolocation is most efficient at ranges between 5 and 200 meters. Based on the nature of the echo the bottlenose dolphin can identify the size, distance, contours and speed of objects in the water.

Although the bottlenose dolphin is not endangered (its population in the western North Atlantic is estimated to be 11,700, of which more than 9,000 swim mostly offshore), they apparently are acutely affected by changes in their habitat and by pollution. In 1987 and 1988, nearly 750 bottlenose dolphins were found dead off the east coast of the U.S. Some scientists conjecture that currents brought a toxic algae, Ptychodiscus brevis, also called brevitoxin, to the east coast where menhaden began consuming it. Mackerel consumed the menhaden, and the dolphins ate both species. The fish were immune to the toxin, but the dolphins were not, and it may have weakened their immune systems to other disease.

High concentrations of organochlorines were also found in the carcasses, indicating that pollution also was involved. Between 50 percent and 60 percent of the coastal population died during this period.

To bookmark this page, please use this URL: http://www.chesapeakebay.net/bottlenose_dolphin.htm

Dolphin images courtesy: Photo 1 - Kim Urian / Duke Marine Lab, Photo 2 - Kim Urian / Sarasota Dolphin Research Program

For more information, contact the Chesapeake Bay Program Office:
410 Severn Avenue, Suite 109, Annapolis, MD 21403 / Tel: (800) YOUR-BAY / Fax: (410) 267-5777.

Terms of Use | Privacy Policy | Contact Us
Directions to the Bay Program Office

Last modified: 12/11/03