The body color of Tursiops
truncatus on the dorsal surface ranges from gray to gray-green or dark gray,
fading to white on the ventral surface. Older individuals sometimes show an
inconspicuous spotting pattern along the ventral surface and sides. Body form is
fusiform and streamlined.
One species of bottlenose dolphin
is recognized; however, biochemical evidence suggests that at least 2 distinct
ecotypes exist. These ecotypes can be differentiated from differences in blood
chemistry, skull measurement and body size. In general, the coastal ecotype has
a somewhat smaller body and larger flippers than the offshore ecotype. It is
suggested that the larger flippers increase maneuverability and heat exchange in
the harbors, bays and lagoons frequented by these dolphins. The offshore ecotype
appears to be better adapted for deep diving, and for colder, deeper waters. Its
larger body aids in heat conservation, and is a defense against attack from
potential predators (Hersh and Duffield 1990).
II. HABITAT AND
Tursiops truncatus is a cosmopolitan species found
in both tropical and temperate waters of the world's oceans (Wells and Scott
1999). In the Atlantic Ocean, bottlenose dolphins range from Nova Scotia to
Patagonia, and from Norway to South Africa. In the Pacific, they are found from
Japan to Australia, the Hawaiian Islands, and from southern California to Chile.
Tursiops truncatus is found
throughout the Indian River Lagoon, with a resident population estimated to be
between 200 and 800 individuals. This population may at times be augmented by normally coastal individuals which utilize the Lagoon at certain times during the year, or even more sporadically.
A five year study is currently being undertaken by the Harbor Branch Dolphin
Research and Conservation Program in which a photographic catalog of Indian River
Lagoon dolphins will be compiled. Future research will attempt to understand
issues such as population size and structure, movement patterns, home range
boundaries, social interactions, and individual life histories of Indian River
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
Differences in body size and
skull measurements in bottlenose dolphins are possibly related to habitat
differences in the 2 ecotypes observed in the western North Atlantic Ocean.
There are also apparent size differences between Atlantic and Pacific
populations. Large Pacific bottlenose dolphins have been measured at 3.7 m (12
ft.) and 454 kg (1000 lbs.), while bottlenose dolphins measured off the Florida
coast averaged 2.6 m (8.5 ft.) and weighed 190-260 kg (419-572 lbs.) (Hubbs Sea
The average lifespan for
bottlenose dolphins is approximately 20 years; however, some bottlenose dolphins
have lived as long as 48 years. Age may be established for dolphins by dental
analysis. As dolphins age, they produce growth layer groups (GLGs) on their
teeth. By examining a sliced cross section of tooth and counting the growth
layer groups, age may be established.
The worldwide population of
Tursiops truncatus is not known. However, populations sizes have been measured
in some areas. The U.S. population has been estimated at approximately 79,000,
of which 12,000 inhabit the waters of the western North Atlantic ocean, and
67,000 inhabit the Gulf of Mexico and Florida areas.
Swimming speed and duration in
the bottlenose dolphin are inversely proportional; high speed bursts of swimming
may last for only seconds, while low speed swimming may last for extended
periods of time. Swimming speeds in the bottlenose dolphin average approximately
5-11 kph (3-7 mph). Burst speeds have been measured at 29-35 kph (18-22 mph).
Dolphins do not generally need to
make deep dives in order to capture prey. However, they are capable of diving
deeply for up to 10 minutes at a time. The maximum depth of a trained dive
performed by a dolphin was 547 m (1795 ft.) (Ridgeway 1990).
Sexual maturity in dolphins is
highly variable. On average, females become sexually mature when they reach 2.3
m (7.5 ft.) and are between 5-12 years of age. Males mature at 2.4 m (8 ft.) or
10-12 years of age.
Bottlenose dolphins may breed
throughout the year, however breeding peaks have been observed. These peaks vary
with geographic location, but generally coincide with the calving season. In
Florida, the calving season peaks in May; however,
among Indian River Lagoon dolphins there is a bimodal peak, with most births
occurring in April and August.
IV. PHYSICAL TOLERANCES
The fusiform body shape of this
species and its reduced limb size act to decrease the amount of exposed surface
area, thus conserving heat. A thick layer of blubber underlies the skin and acts
as both an insulation layer and an energy reserve. The blubber layer generally
accounts for 18-20% of a dolphin's body weight.
As with all whales, bottlenose
dolphins utilize a countercurrent heat exchange system to regulate internal body
temperature. The circulatory system is designed to conserve heat in cold
temperatures and to dissipate heat in warmer temperatures. Arteries in the
flippers, flukes and dorsal fin are surrounded by veins. In cold temperatures,
heat from blood warmed in the core of the animal is transferred to venous blood
rather than to the external environment, thus conserving body heat. Another
mechanism used to conserve body heat occurs during deep diving, when blood from
the extremities and body surface is shunted to tissues underlying the blubber
In warm temperatures, dolphins
sometimes need to dissipate excess body heat generated by burst swimming or
other strenuous activities. In these cases, blood flow to the veins of the
extremities is increased, while circulation to the veins of the body core is
decreased. In this manner, excess heat is released from the body to the external
V. COMMUNITY ECOLOGY
The diet of Tursiops truncatus is
primarily composed of fish, however squid and shrimp are also taken. Over 45
fish species are utilized as prey by bottlenose dolphins, with sciaenid fish
(croakers) being most abundant in the diets of Indian River Lagoon dolphins. As
most prey species do not school and are associated closely with seagrasses, it
has been proposed that bottlenose dolphins may locate prey by passive listening.
A single dolphin may eat 4 - 5% of
its body weight daily, with nursing mothers consuming approximately 8% of their
body weights daily. Feeding behaviors are highly flexible and adapted to
particular habitats and food resources. Dolphins feed by swallowing prey species
whole, or by breaking larger prey items into more manageable sizes by shaking or
Dolphins are often cooperative
hunters in open waters. Pods may surround large schools of fish, herding them
into tight masses and often stunning them by use of the tail flukes. When the
school is dense enough, the dolphins take turns swimming through the school to
feed. Dolphins also have been observed to herd schools of fishes against
shorelines in order to trap them in shallow water where they may be easily fed
Bottlenose dolphins have also
been known to feed in association with commercial fishing operations, feeding on
the incidental catch of shrimp trawls.
It is as yet unclear whether
bottlenose dolphins face competition from other apex predators in the Indian
River Lagoon. Possible candidates for competitors include the bull shark (Carcarhinus
leucas) and the American Alligator (Alligator mississippiensis) which, along with
the dolphin, have been implicated in damage caused to fishing gear in the Indian
River Lagoon. It must be stressed, however, that there is no clear evidence that
the diets of these species overlap. Large sharks such as tiger sharks, dusky
sharks and bull sharks are predators of bottlenose dolphins. Dolphins respond to
the presence of these animals with tolerance, avoidance and aggression. Tiger
sharks have been observed to elicit the strongest responses in dolphins, and
they have been observed attacking and killing tiger sharks in the wild. The
American alligator has also been observed to prey upon stranded dolphins,
however, attacks by alligators are a rare occurrence.
Field observations of wild
dolphins indicate that daily activity cycles occur, with dolphins being active
to some degree in both day and night. A major portion of a dolphin's daily
activities center around social behaviors and interactions. Dolphins in a pod,
or social group, establish strong bonds. Studies have suggested that individual
dolphins often show preferences for some individuals within the group over
others, and are able to recognize each other even after long separations. Bonds
between adult male pairs are strong and may last for long periods of time, with
males showing cooperation in many activities. Mother-calf bonds are also long
lasting, and calves may remain with their mothers for 3- 6 years or longer.
Feeding activities peak in the
early morning and late afternoon with dolphins often hunting cooperatively.
Tursiops truncatus and the West
Indian Manatee exist as the only marine mammals to inhabit the Indian River
Lagoon. However, given that manatees are herbivorous and dolphins are
carnivorous, it is unsurprising that they are not necessarily found in
association with one another.
Bottlenose dolphins outside the
Indian River Lagoon have been observed in groups of pilot whales, spinner
dolphins, and spotted dolphins. They have also been observed riding the pressure
waves of large whales such as humpback, gray, and right whales.
VI. SPECIAL STATUS
Notes on Special Status:
All whales and dolphins in U.S.
waters fall under the jurisdiction of the National Marine Fisheries Service
(NMFS) and are protected under the Marine Mammal Protection Act of 1972. This
act made it illegal to harass or hunt any marine mammal in U.S. waters, but
allows certain exceptions including native subsistence hunting, and taking
dolphins and whales as incidental catch in commercial fishing.
Cost in the IRL:
Six of the ten most important
fish species to the bottlenose dolphin are also commercially important in the
Indian River Lagoon. These include the spotted sea trout, striped mullet,
Atlantic croaker, spot, weakfish and kingfish. Barros and Odell (1993), using a
hypothetical population of 400 resident dolphins in the Indian River Lagoon,
estimated the total annual consumption by dolphins of these prey species to be
598 metric tons per year. This total was compared to the 1990 total annual catch
of 1,113 metric tons by the commercial fishery within the lagoon. Given the
different market values of the fish species taken, the estimated impact on the
fishery by dolphins was similar to the landings by commercial fisheries, and was
estimated to be over 1.1 million dollars per year.
Report by: K. Hill,
Smithsonian Marine Station
with thanks to N. Barros, Hubbs-Sea World Research Institute
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Page last updated: July 25, 2001