The spinner dolphin is a slender species, with an extremely long, thin beak. Also, the head is very slender at the apex of the melon. The dorsal fin ranges from slightly falcate to erect and triangular. In adult males of some populations, the dorsal fin may become so canted forward that it looks as if it were stuck on backwards and the tail stock may become very deepened, with an enlarged post-anal keel of connective tissue. Spinners generally have dark eye-to-flipper stripes and dark lips and beak tips. In spinners, there are 45-62 pairs of very fine, pointed teeth in each jaw. This is more than in any other cetacean species except the franciscana.
Individuals of most spinner dolphin populations have a three-part color pattern (dark gray cape, light gray sides, and white belly) and only minor differences in appearance of males and females. These animals are called Gray’s spinner dolphins (S. l. longirostris).
In the eastern tropical Pacific (ETP), there are three other forms. Eastern spinners (S. l. orientalis) have a monotone steel gray color pattern, with white only as patches around the genitals and axillae. They have the most exaggerated sexual dimorphism. Central American spinners (S. l. centroamericana), previously called Costa Rican spinners, are poorly known, but appear to have a similar color pattern, although it apparently lacks the white ventral patches. Central American spinners are also longer, with longer beaks than eastern spinners. A third type of spinner dolphin in the ETP, often called the whitebelly spinner, appears to represent a hybrid between eastern spinner and Gray’s spinner dolphins. Whitebelly spinners are more robust, with a two-part color pattern and less exaggerated sexual dimorphism than the other stocks in the ETP. Finally, there is a dwarf form of the spinner dolphin (S. l. roseiventris), which is found in waters of Southeast Asia and northern Australia. It has an erect to falcate dorsal fin, a tripartitie color patter, and proportionately large flippers and dorsal fin.
Geographical forms of spinner dolphins have not been well described for most areas, with the exception of the eastern Pacific. Animals of the above described forms, or other undescribed stocks, may exist elsewhere as well.
Newborn spinner dolphins are about 75-80 cm long; adults reach 2 m (females) and 2.4 m (males). They reach weights of at least 77 kg. Eastern spinners are the smaller and Central American spinners the larger of the subspecies in the ETP. Dwarf spinners reach maximum lengths of only about 1.58 m.
Can be Confused With
From a distance, other long-beaked oceanic dolphins can look like spinner dolphins. Spinners are most likely to be confused with clymene dolphins in the Atlantic, but careful attention to color pattern differences and head and body shape differences will allow them to be distinguished. In any ocean, they may be confused with pantropical spotted, striped, and common dolphins. Careful attention to beak length, dorsal fin shape, and color patterns should allow for accurate identifications.
The range of the spinner dolphin is pantropical and nearly identical to that of the pantropical spotted dolphin, encompassing oceanic tropical and subtropical zones in both hemispheres. Limits are near 40°N and 40°S. Unlike pantropical spotted dolphins, spinners often rest in shallow, coastal waters, and may spend days in sandy-bottomed bays of oceanic islands.
Ecology and Behavior
The spinner dolphin’s habitat is in coastal waters, with the exception of the ETP.
The spinner dolphin is named for its habit of leaping from the water and spinning up to seven times on its long axis before falling back to the water. This is one of the most aerial of all dolphins, and in most areas they are active bowriders (the main exception is the ETP, where these dolphins have been harassed by fishermen, who encircle them to catch tuna swimming below). Herd sizes range from less than 50 up to several thousand. Associations with spotted dolphins are common in the ETP, and they occasionally associate with several other marine mammal species. Their association with spotted dolphins and yellowfin tuna results in their entanglement in tuna purse seines in the ETP.
The behavior of Hawaiian spinner dolphins has been well studied. Moderately-sized groups of dolphins move into shallow sandy bays to rest in the daytime, and then move offshore in the late afternoon/evening for nighttime feeding in nearby continental slope and oceanic waters. Dolphins are highly aerial during the ascent from rest to foraging.
Calving peaks in different populations range from late spring to fall after a gestation of ~10.5 months. Females lactate 1-2 years, and the calving interval is about 3 years. Maximum age is thought to be 20 years.
Feeding and Prey
Spinner dolphins feed by seizing prey. They have a broad diet dominated by fish, squid, and other invertebrates. Most spinners feed predominantly at night, on midwater fish and squid, and rest during much of the day. Dwarf spinners are exceptional, however, and presumably feed during daylight hours on reef-associated organisms.
The IUCN lists the spinner dolphin as “lower risk/conservation dependent”, in that conservation measures are in place that address the concerns for spinners. This is the second-most important species of dolphin involved in the Eastern Tropical Pacific (ETP) tuna purse seine fishery (after the pantropical spotted dolphin). The eastern spinner dolphin population is estimated to have been reduced to less than 1/3 its original size by the kill. Other incidental kills occur throughout the range in a number of different fisheries, including driftnets, purse seines, and trawls.
In some cases, human use of bycaught dolphins has led to direct fisheries. Direct kills occur in several areas, including the Caribbean, Sri Lanka, the Philippines, and Indonesia, and occasionally Japan. Their habit of resting in coastal waters leads to problems of harassment by dolphin-watching boats in a number of areas. In 2000, NMFS estimated the Hawaiian stock of spinners to be 3,184 (CV = 0.37) and in 1995, the Northern Gulf of Mexico population was likewise estimated at 6,316 (CV = 0.43). The population of spinner dolphins in the ETP is estimated to be 1,157,746 (CV = 0.43).
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