Stenella frontalisAtlantic spotted dolphin

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Geographic Range

Stenella frontalis, the Atlantic spotted dolphin, is found in the tropical and temperate waters of the Atlantic Ocean (Wilson and Reeder, 1993).

Habitat

Along the southeastern and Gulf coasts of the U.S., Stenella frontalis inhabits the continental shelf, usually within 250-350 km of the coast. In the Bahamas, the Atlantic spotted dolphin spends most of its time in the shallow water over sand flats. (Ridgway, 1994; http://whales.ot.com, 1999).

Physical Description

As the common name spotted dolphin suggests, S. frontalis has a spotted color pattern on its body. These spots are not present at birth, and generally do not appear until the onset of weaning. The first spots to appear on the calves are dark spots on the animal's ventral surface. As the dolphin approaches puberty, the ventral spots increase in number and size and pale dorsal spots appear as well. The number of spots continues to increase with age, similar to the development of spotting in Stenella attenuata. There is a large amount of variation in the adult color pattern, between populations and between individuals. At times some individuals become so heavily spotted that they appear white from a distance. Spotting seems to decrease with the distance from the continental shores of North America. In the Azores some specimens have had few or no ventral spots, but well developed dorsal spotting.

The beak of S. frontalis is long and narrow, a typical feature of all Stenella dolphins. S. frontalis has a robust head and body, that make it larger in size, but not length, than S. attenuata. Proportionately larger flippers, flukes and dorsal fins are also characteristic of S. frontalis. The average adult body length of the Atlantic spotted dolphin is 166-229cm. The adult S. frontalis females tend to be slightly larger than the males, and an average adult weight is approximately 200 pounds (90k).

The skull of the Atlantic spotted dolphin varies in size with individuals and with geographical region. Skull size is generally correlated with body size. S. frontalis has small conical teeth, 3-5mm in diameter. In each rostral row there are 32-42 teeth, and 30-40 teeth in each mandibular row. S. frontalis have on average a distally broader rostrum and fewer but larger teeth than S. attenuata. At times differentiating between these two spotted dolphins is difficult, especially in areas where they converge geographically. (Ridgway, 1994; http://wwwa.com/dolphin/index.html, 1999; http://whales.ot.com, 1999).

  • Average mass
    90 kg
    198.24 lb

Reproduction

Females are generally sexually mature at 9 years. Males do not reach sexual maturity until their 12th year. There is evidence of year round mating, and gestation is between 11 and 12 months long. Calves are normally born in May and September. There have been some observations of pods segregated by reproductive status as well as sex and age. ( http://whales.ot.com, 1999).

  • Breeding season
    There is evidence of year round mating
  • Range gestation period
    11 to 12 months
  • Average age at sexual or reproductive maturity (female)
    9 years
  • Average age at sexual or reproductive maturity (male)
    12 years

Behavior

Dolphins, including S. frontalis, are highly intelligent and social animals. They live in close knit groups called pods, that involve complex social organization with individual recognition and bonding. Pods range in size from a few dolphins to several thousand in offshore regions. Generally pods consist of less than 50 individuals. Atlantic spotted dolphins often school with other species, such as spinner dolphins. The pods of S. frontalis vary in make-up. Segregation by age, sex, and reproductive status has been observed.

The Atlantic spotted dolphin is a vigorous swimmer, quite active at the surface doing forward flips and hurling itself into the air. S. frontalis also has a complex communication system that is made up of narrow-band whistles. These whistles differ enough between individuals that the human ear can distinguish between individual dolphins. The dolphins are able to communicate with each other by whistling, clicking their tongues, cackling and uttering sharp cries. (Ridgway, 1994; http://whales.ot.com, 1999).

Communication and Perception

Food Habits

The diet of Stenella frontalis varies with location. They eat a variety of invertebrates, as well as small eels and herring. They have even been known to follow trawlers to eat discarded fish. Other feeding habits include feeding at or near the surface and "tracking" schools of small fish. (Ridgway, 1994; http://whales.ot.com, 1999).

  • Animal Foods
  • fish

Economic Importance for Humans: Positive

In the past dolphin flesh was considered a delicacy. Besides being used for food, certain parts of its body were used for medicinal purposes. For example, the oil from the liver was used to treat ulcers. Today zoologists are interested in dolphins because they have a high intelligence level. Due to their high intelligence level, dolphins have been trained to help in underwater salvage operations and have even taken part in military exercises. (Stephen, 1973).

Economic Importance for Humans: Negative

The cost and complexity of the tuna fishery has been increased because of regulations that have been designed to lessen the number of dolphins killed by tuna fisherman.

Conservation Status

Stenella frontalis is listed in Appendix II of the Convention of International Trade in Endangered Species of Wild Fauna and Flora (CITES). Those listed in Appendix II, as stated on the CITES web site,are "species which although not necessarily threatened with extinction may become so unless trade is subject to strict regulation." As well as non-threatened species that must be subject to regulation in order to control threatened species. ( http://www.wcmc.org.uk/CITES/english/index.html).

Other Comments

If a dolphin is in distress it can call out for help. The dolphin puts out an intermittent distress signal that alerts the other dolphins and they hurry to help it. S. frontalis is known to aid other dolphins in its pod that are in distress. If a member of the group is wounded or sick, the others will take turns supporting it in the water until it recovers or dies. (Ridgway, 1994; Stephen, 1973).

Contributors

Crystal Allen (author), University of Michigan-Ann Arbor, Phil Myers (editor), Museum of Zoology, University of Michigan-Ann Arbor.

Glossary

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

piscivore

an animal that mainly eats fish

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

year-round breeding

breeding takes place throughout the year

References

"CITES" (On-line). Accessed October, 1999 at http://www.wcmc.org.uk/CITES/english/index.html.

"The Wild Dolphin Project" (On-line). Accessed October, 1999 at http://wwwa.com/dolphin/index.html.

"Whale Songs" (On-line). Accessed October, 1999 at http://whales.ot.com.

Ridgway, S., R. Harrison. 1994. Handbood of Marine Mammals. San Diego: Academic Press Inc..

Stephen, D. 1973. Dolphins, Seals and Other Sea Mammals. New York: G.P. Putnam's Sons.

Wilson, D., D. Reeder. 1993. "Mammal Species of the World" (On-line). Accessed 12 March 2001 at http://www.nmnh.si.edu/msw/.