Habitat and Distribution
A. Distribution (Leatherwood and Reeves, 1983).
1. Bottlenose dolphins inhabit temperate and tropical waters throughout the world.
2. In the Pacific Ocean, bottlenose dolphins are found from northern Japan and California to Australia and Chile. They are also found offshore in the eastern tropical Pacific as far west as teh Hawaiian islands. Off the California coast bottlenose dolphins have been observed as far north as Monterey, particularly during years of unusual warmth (Wells, et al., 1990).
3. In the Atlantic Ocean, bottlenose dolphins are found from Nova Scotia and Norway to Patagonia and the tip of South Africa. They are the most abundant dolphin species along the United States from Cape Cod through the Gulf of Mexico.
4. Bottlenose dolphins are also found in the Mediterranean Sea, and in the Indian Ocean from Australia to South Africa.
1. Bottlenose dolphins inhabit the pelagic zone as well as harbors, bays, lagoons, gulfs, and estuaries.
2. In the northwest Atlantic, there seem to be at least two ecotypes (forms) of the bottlenose dolphin. They can be differentiated by skull and body measurements as well as by characteristics of their blood (Hersh and Duffield, 1990).
a. In general, the coastal ecotype seems to be adapted for warm, shallow waters. Its smaller body and larger flippers suggest increased maneuferability and heat dissipation (Hersh and Duffield, 1990). These dolphins frequent harbors, bays, lagoons, and estuaries.
b. In general, the offshore ecotype seems to be adapted for cooler, deeper waters. Certain characteristics of their blood indicate that this form may be better suited for deep diving. Its larger body helps to conserve heat and defend itself against predators (Hersh and Duffield, 1990).
1. Variations in water temperature, migration of food fish, and feeding habits may account for the seasonal movements of some dolphins to and from certain areas (Duffield and Chamerlin-Lea, 1990; Shane, et al., 1986).
2. Some coastal dolphins in higher latitudes show a clear tendency toward seasonal migrations, traveling further south in the winter. Those in warmer waters show less extensive, localized seasonal movements (Shane, et al., 1986).
3. Some coastal animals stay within a limited home range (an area in which individuals or groups regularly move about during day-to-day activities). Home ranges may overlap (Duffield and Chamberlin-Lea, 1990; Wells and Scott, 1991).
Most dolphins undergo seasonal movements, probably as a response to variations in water temperature and food availability.
(Photo by Rrandall S. Wells.)
1. The worldwide population of bottlenose dolphins is unknown. Specific bottlenose dolphin populations in a few areas have been estimated.
a. In the United States Gulf of Mexico, their numbers are estimated to be at least 67,000 (Blaylock, et al., 1995).
b. The populations found in the western North Pacific and along Japanese coasts are estimated at about 35,000 (Klinowska, 1991).
c. In U.S. waters of the western North Atlantic, the bottlenose population is estimated at about 11,700. Of course, at least 9,200 are offshore animals (Blaylock, et al., 1995).
d. The Mediterranean population is estimated at less than 10,000 (Klinowska, 1991).
2. Bottlenose dolphins are not endangered.
3. Chromosome banding techniques have proven useful in bottlenose dolphin population studies. Scientists can identify individuals and determine relationships among dolphins in a group (Duffield and Chamberlin-Lea, 1990).
1. Bottlenose dolphins measured off Sarasota, Florida averaged 2.5 to 2.7 m (8.2-8.9 ft.) and weighed between 190 and 260 kg (419-573 lb.) (Read, et al., 1993).
2. Differences in body size and skull dimensions may be related to habitat differences. The two northwestern Atlantic ecotypes exhibit a pronounced size variance (Herse and Duffield, 1990).
a. In the northwestern Atlantic, small body size is characteristic of the coastal ecotype.
b. Large body size is characteristic of the offshore ecotype.
3. Large bottlenose dolphins in the Pacific may be 3.7 m (12 ft.) and weigh 454 kg (1,000 lb.). In the Mediterranean, bottlenose grow to 3.7 m (12 ft.) or more.
4. On average, full-grown males are slightly longer than females, and considerably heavier. As juveniles, however, females grow at a faster rate until about 10 years of age (Read, et al., 1993).
B. Body shape.
A bottlenose dolphin has a sleek, streamlined, fusiform body.
1. Coloration is a nondescript gray to gray-green or gray-brown on the back, fading to white on the belly, lower jaw, and anal regions. The belly may be pinkish.
2. This coloration, a type of camouflage known as countershading, may help conceal a dolphin from predators and prey. When viewed from above, a dolphin's dark back surface blends with the dark depths. When seen from below, a dolphin's lighter belly blends with the bright surface of the sea.
3. Older animals in some regions sometimes show an inconspicuous spotting along their sides and on their bellies.
D. Pectoral flippers.
1. A dolphin's forelimbs are pectoral flippers. Pectoral flippers have all the skeletal elements of the forelimbs of terrestrial mammals, but they're foreshortened and modified.
2. The skeletal elements are rigidly supported by connective tissue. Thick cartilage pads lie lengthwise between the bones.
3. Pectoral flippers are curved slightly and pointed at the tips.
4. Dolphins use their pectoral flippers mainly to steer and, with the help of the flukes, to stop.
5. Blood circulation in the flippers adjusts to help maintain body temperature.
a. Arteries in the flippers are surrounded by veins. Thus, some heat from the blood traveling through the arteries is transferred to the venous blood rather than the environment. This countercurrent heat exchange aids dolphins in conserving body heat.
b. To shed excess body heat, circulation increases in veins near the surface of the flippers and decreases in veins returning to the body core (Ridgway, 1972).
1. Each lobe of the tail is called a fluke.
2. Flukes are flattened pads of tough, dense, fibrous connective tissue, completely without bone or muscle.
3. Longitudinal muscles of the back and caudal peduncle (tail stalk) move flukes up and down to propel a dolphin through water.
Dolphins propel themselves forward by moving their flukes up and down.
4. The total spread of the flukes is about 20% of the total body length.
5. Like the arteries of the flippers, the arteries of the flukes are surrounded by veins to help conserve body heat in cold water.
F. Dorsal fin.
1. Like the flukes, the dorsal fin is made of dense, fibrous connective tissue, with no bones.
2. The dorsal fin may act as a keel. It probably helps stabilize a dolphin as it swims, but is not necessarily essential to a dolphin's balance. (Some dolphin species lack dorsal fins.)
3. As in the flukes and the flippers, arteries in the dorsal fin are surrounded by veins to help conserve body heat in cold water.
4. The dorsal fin is often falcate (curved back), although the shape is quite variable. It is located at the center of the back.
1. A bottlenose dolphin has a well-defined rostrum (snoutlike projection), usually about 7-8 cm (3 in.) long, marked by a lateral crease.
2. Teeth are conical and interlocking.
a. They are designed for grasping (not chewing) food.
b. The number of teeth varies considerably among individuals. Most individuals have 20 to 25 teeth on each side of the upper jaw and 18 to 24 teeth on each side of the lower jaw, a total of 76 to 98 teeth (Rommel, 1990).
A bottlenose dolphin may have as many as 98 conical teeth.
3. Eyes are on the sides of the head, near the corners of the mouth. See also eyesight.
Glands at the inner corners of the eye sockets secrete an oily, jellylike mucus that lubricates the eyes, washes away debris, and probably helps streamline a dolphin's eye as it swims. This tearlike film may also protect the eyes from infective organisms (Young and Dawson, 1992).
4. Ears, located just behind the eyes, are small inconspicuous openings, with no external pinnae (flaps). See hearing.
5. A single blowhole, located on the dorsal surface of the head, is covered by a muscular flap. The flap provides a water-tight seal (Ridgway, 1972).
a. A bottlenose dolphin breathes through its blowhole.
b. The bottlenose is relaxed in a closed position. To open the blowhole, a bottlenose dolphin contracts the muscular flap.
A. Social structure.
1. Bottlenose dolphins live in groups called pods (Scott, Wells, and Irvine, 1990).
a. A pod is a coherent long-term social unit.
b. The size of a pod varies significantly with its composition. On the west coast of Florida, mean pod size is about seven animals (Scott, Wells, and Irvine, 1990).
c. In the wild, pod composition and structure are based largely on age, sex, and reproductive condition (Wells, 1991).
(1) Researchers on the eastern U.S. coast commonly sight mother-calf pairs and pods of mature females with their most recent offspring (Wells, 1991).
(2) Subadults typically occur in mixed-sex and single-sex groups (Wells, 1991).
(3) Adult males are often observed alone, or in pairs or occasional trios (Wells, 1991). Adult males commonly move between female groups in their range, and may pair up with females for brief periods. Adult males rarely associate with subadult males (Wells, 1991; Herman, 1980).
2. In general, size of pods tend to increase with water depth and openness of habitat. This may be correlated with foraging strategies and protection (Shane, et al., 1986).
3. Several pods may join temporarily (for several minutes or hours) to form larger groups called herds or aggregations. Up to several hundred animals have been observed traveling in one herd (Shane, et al., 1986).
4. Researchers have identified certain factors that tend to cause a pod to either draw together or to disperse somewhat (Herman, 1980).
a. Factors that tend toward cohesion include protection, fright, and familial associations.
b. Factors that tend toward dispersion include alertness, aggression, and feeding.
5. There may be a social hierarchy within a group of bottlenose dolphins.
B. Social behavior.
1. Dolphins in a pod appear to establish strong social bonds. Behavioral studies suggest that certain animals prefer association with each other and recognize each other after periods of separation. Field observations suggest that mother-calf bonds are long-lasting.
a. Mother-calf bonds are long-lasting; a calf typically stays with its mother three to six years or more (Wells, 1991).
b. Adult male pair bonds are strong and long-lasting. Male pairs often engage in a number of cooperative behaviors (Wells, 1991).
2. Bottlenose dolphins establish and maintain dominance by biting, chasing, jaw- clapping, and smacking their tails on the water (Shane, et al., 1986; Herman, 1980).
3. Dolphins often show aggression by scratching one another with their teeth, leaving superficial lacerations that soon heal (Shane, et al., 1986). Traces of light parallel stripes remain on the skin of the dolphin. These marks have been seen in virtually all species of dolphins. Dolphins also show aggression by emitting bubble clouds from their blowholes.
4. During courtship, dolphins engage in head-butting and tooth-scratching (Shane, et al., 1986).
5. Bottlenose dolphins often hunt together. See Methods of collecting food.
Dolphin courtship behavior includes twisting, nuzzling, and tooth-scratching.
C. Daily activity cycles.
1 . Observations indicate that dolphins undergo daily cycles of activity.
2. Bottlenose dolphins are active to some degree both day and night (Shane, et al., 1986).
3. Social behavior comprises a major portion of bottlenose dolphins'daily activities (Shane, et al., 1986).
4. Feeding usually peaks in the early morning and late afternoon (Shane, et al., 1986).
D. Individual behavior.
1 . Dolphins frequently ride on the bow waves or the stern wakes of boats. This is probably adapted from the natural behavior of riding ocean swells, the wakes of large whales, or a mother dolphin's "slip stream" (hydrodynamic wake) (Shane, et al., 1986).
2. Dolphins have been seen jumping as high as 4.9 m (1 6 ft.) from the surface of the water and landing on their backs or sides, in a behavior called a breach.
3. Both young and old dolphins chase one another, carry objects around, toss seaweed to one another, and use objects to solicit interaction. Such activity may be practice for catching food.
In a common behavior called a breach, a bottlenose dolphin jumps out of the water and lands on its side.
E. Protection and care.
1 . Large adult males often roam the periphery of a pod, and may afford some protection against predators (Herman, 1980).
2. Researchers have observed scouting behavior in bottlenose dolphins. An individual may investigate novel objects or unfamiliar territories and "report" back to the pod (Herman, 1980).
3. Bottlenose dolphins may aid ill or injured pod mates. They may stand by and vocalize, or they may physically support the animal at the surface so it can breathe.
F. Interaction with other species.
1 . Bottlenose dolphins have been seen in groups of toothed whales such as pilot whales, spinner dolphins, spotted dolphins, and rough-toothed dolphins.
2. Bottlenose dolphins have been seen riding the pressure waves of gray whales, humpback whales, and right whales (Shane, et al., 1986). They often force Pacific white-sided dolphins away from prime spots in the waves (Herman, 1980).
3. Dolphins respond to sharks with tolerance, avoidance, and aggression. Tiger sharks elicit the strongest responses from dolphins (Shane, et al., 1986). Researchers have observed dolphins attacking, and sometimes killing, sharks in the wild (Herman, 1980).
4. Some individuals in the wild regularly solicit attention, such as touching and feeding, from humans (Shane, et al., 1986).
Birth and Care of Young
Gestation period is about 12 months (Schroeder, 1990; Hansen, 1990; Odell, 1975).
B. Birth seasons.
1. Worldwide, calves are born throughout the year (Schroeder, 1990; Hansen, 1990; Odell, 1975).
2. Seasonal calving peaks vary by area (Schroeder, 1990; Hansen, 1990; Odell, 1975).
a. Bottlenose dolphins along the west coast of Florida show a calving peak in May (Urian, et al., 1996).
b. Most dolphin births along coastal Texas waters occur in March (Urian, et al., 1996).
c. Peak calving appears to be bimodal for dolphins in Florida's Indian River Lagoon; most births occur in April and August (Urian, et al., 1996).
d. Bottlenose dolphins in the Pacific Ocean along the coast of southern California have shown a calving peak in the fall.
C. Frequency of birth.
A female dolphin caon potentially bear a calf every two years, but calving intervals generally average three years.
1. Calves are born in the water. Deliveries can be either tail-first or head-first. The umbilical cord snaps during or soon after delivery.
2. Sometimes an assisting dolphin may stay close to the new mother and calf. Although this assisting dolphin often is referred to as an "auntie" dolphin, it may be male or female. This "auntie" dolphin is often the only other dolphin a mother allows near her calf (Herman, 1980).
E. Calf at birth.
video clip of a Commerson's dolphin birth. (2 Mb)
1. The calf is approximately 106-132 cm (42 to 52in.) long and weighs about 20kg (44 lb.).
2. In the first few days after birth, the dorsal fin and tail flukes are flaccid and pliable, but gradually become more stiff.
3. Calves are darker than adults and show several vertical, light-colored lines on their sides, a result of fetal folding. These lines disappear within six months.
The light-colored lines on the sides of a dolphin calf are a result of fetal folding.
F. Care of the young.
a. Calves nurse under water, close to the surface.
b. The calf suckles from nipples concealed in abdominal mammary slits.
c. Observations in zoological parks show that nursing usually begins within six hours of birth. A calf nurses as often as four times per hour for the first four to eight days (Schroeder, 1990).
d. Each nursing instance usually lasts only about five to ten seconds. A calf nurses three to eight times per hour throughout the day and night (Cockcroft and Ross, 1990).
e. Milk is composed of 33.0% fat, 6.8% protein, and 58.3% water (Oftedal, 1984), with traces of lactose. The rich milk helps the baby rapidly develop a thick insulative layer of blubber.
f. A calf may nurse for up to 18 months (Barros and Odell, 1990).
2. A mother dolphin stays close to her calf and attentively directs its movements. The baby swims close to its mother and is carried in the mother's "slip stream," the hydrodynamic wake that develops as the mother swims. This helps the baby to swim and enables the mother and calf to stay up with the group.
3. There is probably a considerable amount of learning involved in mothering.
G. Calf development.
1 . Bottlenose dolphin breeding colonies in marine zoological parks continue to provide a unique opportunity to observe and quantify aspects of dolphin biology.
2. In zoological environments, calves begin to take a few fish at about three to four months, when their teeth begin to erupt. Calves begin to eat fish when they reach about 130 to 150 cm (51-59 in.) (Barros and Odell, 1990).
3. Within a few days of birth a calf can vocalize, but signature characteristics develop with age (Caldwell, Caldwell, and Tyack, 1990).