(Gray, 1828) - Spinner dolphin
The spinner dolphin is a slender dolphin, 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 stocks, 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.
Spinner dolphins generally have dark eye-to-flipper stripes and dark lips and beak tips. There are 3 subspecies known. The individuals of most spinner dolphin stocks in the world have a three- part colour pattern (dark grey cape, light grey sides, and white belly) and only minor differences in appearance of males and females. These animals (illustrated above) are called Gray's spinner dolphins (S. l. longirostris).
In the eastern tropical Pacific, there are 3 other forms. Eastern spinner dolphins (S. l. orientalis) have a monotone steel grey colour pattern, with white only as patches around the genitals and axillae. They have the most exaggerated sexual dimorphism. Central American spinner dolphins (S. l. centroamericana), previously called Costa Rican spinners, are poorly known, but appear to have a similar colour pattern, although it may lack the white ventral patches. A third type of spinner dolphin in the eastern tropical Pacific, often called the whitebelly spinner dolphin, appears to represent a hybrid between eastern spinner and Gray's spinner dolphins. Whitebelly spinners are more robust, with a two-part colour pattern and less exaggerated sexual dimorphism than the other stocks in the eastern tropical Pacific.
Geographical forms of spinner dolphins have not been well described for most areas, with the exception of the eastern Pacific; although a dwarf form has been described from the Gulf of Thailand. Animals of the above described forms, or other undescribed stocks, may exist elsewhere as well.
In spinner dolphins, there are 45 to 62 pairs of very fine, pointed teeth in each jaw. This is more than in almost any other cetacean species.
Sub species description
Gray's spinner dolphin (S. l. longirostris ): Colour pattern three-part (white belly, light grey sides, dark grey cape); dorsal fin falcate to erect; body more robust than in other forms; post-anal hump of adult males nearly absent; distribution worldwide, except eastern tropical Pacific.
Eastern spinner dolphin (S. l. orientalis): Pigmentation monotone grey, with light patches around genital area and axillae; dorsal fin triangular to canted forward (extremely canted in adult males); adult males with deepened tail stock and enlarged post-anal hump; maximum size 2 m; known distribution limited to the eastern tropical Pacific east of 145°W.
Central American spinner dolphin (S. l. centroamericana): Pigmentation monotone grey; apparently, no light patches around genital area and axillae; dorsal fin triangular to canted forward (extremely canted in adult males); adult males with deepened tail stock and enlarged post-anal hump; known distribution limited to 80 km offshore from southern Mexico to Panama in the eastern tropical Pacific.
Whitebelly spinner dolphin (hybrid between S. l. longirostris and S. l. orientalis): Body slightly more robust than above 2 forms; colour pattern largely bipartite, with dark dorsal cape, and white belly and lower sides; dorsal fin slightly falcate to slightly canted (tending towards canted in adult males); post-anal hump of adult males small to moderate; distribution limited to offshore eastern tropical Pacific.
Can be confused with
From a distance, other long-snouted oceanic dolphins can look like spinner dolphins. Spinner dolphins are most likely to be confused with clymene dolphins in the Atlantic, but careful attention to colour pattern differences and head and body shape differences will allow them to be distinguished.
Newborn spinner dolphins are about 75 to 80 cm long; adults reach 2 m (females) and 2.4 m (males). They reach weights of at least 77 kg. Eastern spinner dolphins are the smaller and Central American spinner dolphins the larger of the subspecies in the eastern tropical Pacific.
The range of the spinner dolphin is 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.
Biology and Behaviour
The spinner dolphin is named for its habit of leaping from the water and spinning up to 7 times on its long axis, before falling back to the water. This is one of the most aerial of all dolphins. Herd sizes range from less than 50 up to several thousand. Associations with pantropical spotted dolphins are common in the eastern tropical Pacific.
Calving peaks in different populations range from late spring to autumn.
Spinner dolphins of most populations feed predominantly at night, on mid-water fish and squid, and rest during much of the day. Their association with pantropical spotted dolphins and yellowfin tuna results in their entanglement in tuna purse seines in the eastern tropical Pacific.
Both the whitebelly and eastern spinner forms have been heavily involved in the tropical Pacific tuna purse seine fishery. The numbers of eastern spinners have been reduced significantly in the last few decades by this fishing practice. Catches of spinner dolphins also occur in the Caribbean, Australia, Japan, Thailand, the Philippines, and Sri Lanka; in this last area up to 15,000 are killed each year in gillnets and by hand-harpooning. There are likely to be fisheries interactions off West Africa.