The Bottlenose Dolphin: Natural History and Ecology
The Bottlenose dolphin is one of the most closely studied cetaceans
in the world. Here at the Kewalo Basin Marine Mammal Laboratory
and The Dolphin Institute, we have made many valuable discoveries
about the sensory systems and cognitive capacities of these animals.
Other marine mammal researchers from all over the world have also
made major contributions to what we know about the bottlenose dolphin.
In this section, you'll find the most up-to-date information about
the bottlenose dolphin's natural history and ecology, including:
- Distribution and Systematics
- Life History and Reproduction
- Feeding
- Predators
- Social Relationships
Distribution and Systematics
The bottlenose dolphin is a cosmopolitan species, in temperate
to tropical zones. In the Atlantic they range from Norway and Nova
Scotia to Patagonia and South Africa, including the Mediterranean
Sea; in the Pacific from Northern Japan and Southern California
to Australia and Chile; and in the Indian Ocean from Australia to
South Africa. A coastal and an offshore form of the species are
generally recognized, with differences found in gross morphology,
hematology, cranial morphology, and parasite faunas. Nuclear and
mitochondrial genetic distinctions between offshore and inshore
bottlenose dolphins have also been described.
The bottlenose dolphin (Tursiops truncatus) is a member
of the order Cetacea, which includes all whales and dolphins. Cetacea
is further divided into to the suborders odontoceti and mysticeti.
Dolphins are odontocetes, or toothed whales; mysticetes, or baleen
whales, utilize baleen (thick keratinized plates) to feed. Bottlenose
dolphins further belong to the family Delphinidae, the oceanic dolphins;
they are the only species in their genus. Several potentially separate
species of bottlenose dolphins have been described, though a general
consensus has yet to be reached. Tursiops aduncus is found in the
Indian Ocean and the tropical Western Pacific ocean; some data from
genetic studies indicate that T. aduncus may actually be more closely
related to the genus Stenella (which includes the striped, spotted
and spinner dolphins). Some researchers refer to the Pacific bottlenose
dolphin as Tursiops gilli, though again it is unclear whether T.
gilli actually represents a separate species.
Life History and Reproduction
Bottlenose dolphins are estimated to live into their forties and
fifties; researchers in Sarasota Bay, Florida, found that females
can live for over fifty years and males for over forty years. Sexual
maturity in females occurs between five and thirteen years of age,
and between eight to thirteen years for males. Mating and calving
times can vary throughout the year but births are predominately
recorded in the spring and fall. Females are seasonally polyestrous
and spontaneous ovulators, generally ovulating between two and seven
times per year with an average cycle length of thirty days. Males
display seasonal changes in their testosterone levels as well.
Bottlenose dolphins have a twelve month long gestation period,
and calves remain with their mothers for several years. Young nurse,
or attempt to nurse, for three to five years, and usually begin
playing with and eating small fish around six months of age.
Feeding
Bottlenose dolphins feed on a wide variety of fish and cephalopods
(such as squid), and occasionally shrimp and small rays and sharks.
They employ a diverse repertoire of feeding behaviors, including
both solitary and social feeding strategies. Some of these strategies
include:
- strand feeding in the tidal creeks of South Carolina and Georgia,
where dolphins pursue schools of mullet onto mud banks and beach
themselves to catch the fish
- crater feeding, where dolphins use echolocation to detect prey
in underneath the sand and dig the prey out with their rostrums
- "whacking" fish with their flukes to stun them
- circling entire schools of fish while one dolphin at a time
darts in to feed
- cooperative feeding with humans; in a town called Laguna, Brazil,
dolphins and human fishermen have evolved a feeding strategy where
dolphins herd schools of fish into the waiting nets of fishermen.
The fishermen wait for a signal from the dolphins to throw their
nets at the appropriate time; the fishermen take home a sizeable
catch, while the dolphins feed on the disoriented fish that escape
the nets. This relationship has been ongoing since 1847, with
humans and dolphins passing the knowledge down through generations.
Finally, some individuals in Shark Bay, Australia, have been observed
carrying sponges on their rostrums; it is hypothesized that they
may use the sponge to protect their rostrum while foraging on
the bottom. If so, this represents the first observed case of
tool use by wild dolphins.
Predators
Sharks are the primary natural predator of bottlenose dolphins.
Bull sharks, tiger sharks, dusky sharks and great white sharks most
commonly prey on the bottlenose dolphin. Some evidence exists that
killer whales and false or pygmy killer whales may also be natural
predators of bottlenose dolphins; scarring consistent with killer
whale and false or pygmy killer whale teeth have been observed on
dolphins, though no attacks have been documented. However, humans
most likely pose the greatest threat to bottlenose dolphin in the
wild; see our conservation (LINK) section for more information.
Social Relationships
The social relationships of bottlenose dolphins in the wild have
been intensely studied in two locations: Sarasota Bay, Florida,
and Shark Bay, Australia. Researchers in these two locations have
discovered much of what we know about the social lives of bottlenose
dolphins through long-term field studies. These studies rely on
photo-identification of the dorsal fins of individual bottlenose
dolphins. Through documenting every sighting of each catalogued
dolphin over many years, researchers are able to put together life
histories of individual dolphins and identify which dolphins are
maternally related. Further, through these field observations, information
on the social, reproductive, foraging and predator avoidance behaviors
of these dolphins has been acquired. In Sarasota Bay, Florida, researchers
led by Dr. Randy Wells of the Mote Marine Lab have studied the resident
community of 100 bottlenose dolphins since 1970. Here, researchers
have found that females have a large network of associates, but
associate most strongly with a group of females called a "band."
These bands form the core of the Sarasota Bay community. Males,
on the other hand, often form very strong associations with another
adult male; these male pairs can endure for many years. In Shark
Bay, Australia, a research team including Dr. Rachel Smolker, Dr.
Richard Connor, Dr. Janet Mann, and Dr. Vincent Janik have studied
the resident population of over four hundred individually identified
bottlenose dolphins since 1984. They have found similar associations
between females in Shark Bay as those found in Sarasota Bay. Shark
Bay males also form strong associations with one or two other males,
and these males engage in herding behaviors towards female dolphins.
Interestingly, these male coalitions can also band together to form
larger coalitions, called alliances, to capture females from other
coalitions or to defend their females from competitors. These alliances
are temporary and allegiances can switch in a matter of hours.
The bottlenose dolphin is likely to continue to be one of the most
frequently studied cetaceans in the wild; its distribution in warm,
coastal waters makes it a popular candidate. Even more, KBMML and
TDI's ongoing research on the cognitive capabilities of the bottlenose
dolphin helps to illuminate field observations and inspire new research
in the field. For further information on the natural history and
ecology of bottlenose dolphins, the following texts may be very
useful:
Reynolds, John E., Wells, Randall S., and Eide, Samantha D. 2000.
The Bottlenose Dolphin: Biology and Conservation. The University
Press of Florida, Miami, Florida.
Mann, Janet, Connor, Richard C., Tyack, Peter L., and Whitehead,
Hal, editors. 2000. Cetacean Societies: Field Studies of Dolphins
and Whales. University of Chicago Press, Chicago, Illinois.
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