Stenella attenuata lives in the Atlantic, Indian, and Pacific oceans. It migrates seasonally to the Japanese coast and is the most common cetacean in the Gulf of Mexico. (Lang 1996, Nowak 1997)
S. attenuata lives in the tropical and subtropical areas of the ocean and seas. Although some live inshore, most members of the species live offshore, where the temperature of the deeper water remains fairly constant. The majority of this species live in between the equator and the Galapagos Islands for the same reason that they tend to live offshore. The home range is hundreds of kilometers in diameter. (Nowak 1997)
S. attenuata is referred to as the pantropical spotted dolphin because its skin becomes spotted as the dolphin grows older. Its dorsal surface is dark gray but covered in paler spots, while its paler ventral surface is covered with dark spots. Another distinguishing feature is the spotted dolphin's bright, white snout. It also has melon, a fatty area located on its forehead. The inshore spotted dolphins tend to be larger in size than offshore dolphins. Males also typically have larger body sizes than females, yet females have longer rostra. The spotted dolphin has between 29 and 37 small, rounded teeth on either side of its upper and lower jaws. It has pectoral fins (on the sides), a dorsal fin (on the central back), and tail flukes. The blowhole, used for breathing and communication, is located on the top of the head. Because S. attenuata has a thin layer of blubber, it has small amounts of stored energy, so it eats high energy foods to make up for the low energy. (Bernard et al. 1989, Lang 1996, Misek et al. 1997, Myers 1997, Nowak 1997, Perrin et al. 1994)
The mean age of sexual maturity for northern offshore female spotted dolphins is estimated to be 11.1 years, which is higher than the estimated age of 9.8 years for southern offshore females. On the other hand, males' average age of sexual maturity is 14.7 years. S. attenuata does not have any particular birthing season, although the number of births does rise in spring and autumn months. The time between births is between 26 and 36 months in the eastern Pacific and approximately 48 months near Japan. The gestation period lasts a little less than a year, and the lactation period can last for 1.5 years or longer. Females usually give birth to a single offspring (Bernard et al. 1989, Chivers et al. 1993, Nowak 1997)
This lactation period is more than three times as long as other large whales. Occasionally females have been known to lactate during a new pregnancy, and some have also been known to give birth to twins, although this is rare. Mother dolphins feed near the surface so that they don't have to leave their calves. Pregnant females up to age 35 have been discovered.
The spotted dolphin is a gregarious animal, swimming in pods of several individuals to several thousand dolphins. The offshore schools tend to be larger in number than those of the inshore dolphins. At the age of two years, young Japanese S. attenuata join pods of other young animals until they achieve sexual maturity. At that time they return to their original pod. Japanese S. attenuata females also have been known to leave their pods during estrous. These two events of age and gender separation do not appear to take place within the tropical Pacific schools. The spotted dolphin often associates with other dolphin species and is commonly sighted with yellowfin tuna. It is an extremely fast animal, capable of swimming between 22 and 28 km/hr. It is an acrobatic species as well, possessing the ability to leap to great heights. It uses echolocation to locate its food. (Myers 1997, Nowak 1997)
The spotted dolphin finds its prey, squid and small fish, near the ocean surface. These dolphins have also been known to feed on isopods and pteropods. Lactating females eat significantly more fish than pregnant or normal spotted dolphins. The lactating female's deviation from the norm is presumably because she requires more energy than normal and pregnant dolphins. More protein and also more energy is obtained from eating fish, rather than from eating the same mass of squid. In addition, fish also contain more calcium and phosphorous, which aid in lactation. Lastly, fish have lower water content, which prevents additional water loss in the lactating female since the consumed fish are hypotonic with the sea water. (Bernard 1989, Lang 1996, Nowak 1997)
The spotted dolphin helps yellowfin tuna fishermen to locate their target. (Nowak 1997)
Regulations designed to lessen the number of dolphins killed by tuna fishermen have increased the cost and complexity of the fishery.
Because S. attenuata tend to swim with yellowfin tuna, Pacific fishermen use sightings of these dolphins to help them locate their yellowfin tuna targets. The majority of S. attenuata deaths are a consequence of yellowfin tuna fishing operations. The enormous nets used to catch these tuna can unintentionally entangle dolphins as well as fish. S. attenuata is the dolphin species that has been affected to the greatest extent by the tuna fish industry. Between 1985 and 1990, almost 130,000 were killed each year because of the tuna fish catching methods. Thanks to United States government regulations, such as requiring improvements in fishing equipment, this number has decreased substantially by 100,000 deaths per year. Some spotted dolphins are killed intentionally by Japanese fishermen. Between 500 and 2000 spotted dolphins are harvested annually in order to be eaten by the Japanese. (Bernard et al. 1989, Chives et al. 1993, Nowak 1997, Perrin et al. 1994)
The current population of spotted dolphins is estimated to be 2.2 million. The government should continue to maintain strict regulations to continue the decrease in mortality among S. attenuata. (Nowak 1997)
Deanna Riseman (author), University of Michigan-Ann Arbor.
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.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
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.
an animal that mainly eats meat
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
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.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
eats mollusks, members of Phylum Mollusca
having the capacity to move from one place to another.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
an animal that mainly eats fish
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
uses touch to communicate
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
breeding takes place throughout the year
Bernard, H.J. and A.A. Hohn. (1989). Differences in Feeding Habits Between Pregnant and Lactating Spotted Dolphins (Stenella Attenuata). Journal of Mammology. 70(1): 211-15
Chivers, S.J. and A.C. Myrick Jr. (1993). Comparison of Age at Sexual Maturity and other Reproductive Parameters for Two Stocks of Spotted Dolphin, Stenella Attenuata. Fishery Bulletin. 91: 611-18
Edwards, E.F. Allometry of Energetics Parameters in Spotted Dolphin (Stenella Attenuata) from the Eastern Tropical Pacific Ocean. Fishery Bulletin. 91: 428-39
Lang, B. Pan-Tropical Spotted Dolphins, Stenella Attenuata. Minerals Management Service Gulf of Mexico OCS Region. http://www.mms.gov/omm/gomr/homepg/regulate/environ/marmam/pan-trop.html
Myers, P. 1997. Animal Diversity Web. http://www.oit.itd.umich.edu/projects/ADW
Nowak, R.M. 1997. Walker's Mammals of the World online version. http://www.press.jhu.edu/books/walker
Perrin, W.F., and G.D. Schnell, D.J. Hough, J.W. Gilpatrick Jr., J.V. Kashiwada. (1994). Reexamination of Geographic Variation in Cranial Morphology of the Pantropical Spotted Dolphin, Stenella Attenuata, in the Eastern Pacific. Fishery Bulletin. 92: 324-46