Whales, Dolphins, and Porpoises of the Western North Atlantic Part 27

May Be Confused With

At sea, bridled dolphins may be confused with Atlantic spotted dolphins or spinner dolphins. Differences by which they may be distinguished from the first are tabularized on p. 105. Differences between spinner and bridled dolphins permitting identification at sea are as follows:

BRIDLED DOLPHIN SPINNER DOLPHIN

COLORATION

Distinct cape on top of head; Dark gray on back; tan or side of head light gray; yellowish tan on sides; white distinct stripes from flippers on belly, to corner of mouth and from dark circles around eye to apex of melon.

BEAK

Shorter and more slender; all Extremely long and slender; black; lips white. dark gray to black on top, white below; lips black.

DORSAL FIN

Uniformly dark gray. Often lighter gray in middle, dark around border.

Distribution

Bridled dolphins occur in tropical and subtropical waters primarily near coastal areas and islands, but are best known from the West Indies. They have been reported from the Antilles, from Texas, and from Florida north to North Carolina. It has been speculated that this species replaces the Atlantic spotted dolphin around the West Indies.

Bridled dolphins have not yet been described from the South American coast.

Stranded Specimens

Bridled dolphins have from 29 to 34 teeth in each upper jaw and from 33 to 36 in each lower jaw. They can be distinguished from spinner dolphins, which have 46-65 teeth in each jaw, by this character alone.

They may be distinguished from spotted dolphins only if the color pattern of the head is clearly visible. If it has faded, the specimen will probably require museum preparation and examination before it can be positively identified.

[Ill.u.s.tration: Figure 120.--A bridled dolphin harpooned in the commercial whale fishery off St. Vincent, Lesser Antilles. If the color pattern has faded, bridled dolphins cannot be readily distinguished from the Atlantic spotted dolphins and must be sent to a museum for preparation and examination of the skull and skeleton. (_Photo by W. A.

Huck, courtesy of Marineland of Florida._)]

[Ill.u.s.tration: Figure 121.--A bridled dolphin from St. Vincent, Lesser Antilles. Note the dark cape of the back, the lighter side, and the dark stripes from the eye to the snout (found in most dolphin species) and the flipper to the gape, a feature which fades as the animal's spotting increases. Together these two features comprise the "bridle" from which the common name derives. Note also the white lips and the white lower jaw. (_Photo by J. R. Sullivan._)]

SPINNER DOLPHIN (T)

_Stenella longirostris_ Gray 1828

Other Common Names

Long-snouted dolphin, long-beaked porpoise, spinner porpoise, rollover (St. Vincent).

Description

Spinner dolphins reach a maximum length of about 7 feet (2.1 m). The body is slender. The beak varies from extremely long and slender (Fig.

123) to relatively short (Fig. 125); the beak is usually dark on top and clean white below, though there may be some white above. The tip of the snout and the lips are distinctly black, while those of both species of spotted dolphins are light. The back is dark gray to black, the sides are tan to yellowish brown, and the belly is white. Some of the larger animals appear almost all black with faint, light speckling. The dorsal fin is generally moderately falcate, but may be almost triangular in adult males. It is often a lighter gray near the middle, bordered by black or dark gray.

Natural History Notes

Spinner dolphins derive their common name from their habit of leaping clear of the water and spinning on their longitudinal axis. The reasons for this behavior are unknown. Individuals may rotate 2 times, or more, in one leap but spinning behavior is not observed as frequently in the western North Atlantic as it is in the eastern tropical Pacific.

Spinner dolphins occur in herds of up to several hundred individuals and are often seen jumping clear of the water, working the sea surface into a froth. They frequently come to the bow of a boat from considerable distances to ride in the bow wave and may ride for protracted periods.

May Be Confused With

Spinner dolphins may be confused with saddleback dolphins. Both species occur in large herds and often come to moving vessels to ride the bow wave. The two can be distinguished, however, by these differences:

SPINNER DOLPHIN SADDLEBACK DOLPHIN

COLORATION

Dark gray on back; tan or Dark gray to brownish gray yellowish tan on sides; white on back; white on belly with on belly; lacks crisscross crisscross or hourgla.s.s pattern pattern on sides; distinct of tan to yellow on sides; black stripe from flipper to distinct black stripe from eye. flipper to middle of lower jaw.

Spinner dolphins might also be confused with bridled dolphins, but may be distinguished by the differences summarized on p. 108.

Distribution

Spinner dolphins are distributed in oceanic and coastal tropical waters.

Though one specimen was collected from South Carolina, they have been more frequently reported from both coasts of Florida, the Gulf of Mexico, the Caribbean, and the West Indies. They have also been reported from Venezuela. They are said to be the most abundant dolphin species from the southeastern Caribbean. Some Pacific spinner dolphins are distributed in oceanic zones. Atlantic spinners may be abundant in offsh.o.r.e tropical waters as well.

Stranded Specimens

Stranded spinner dolphins are most readily identified by the extremely long rostrum and the 46-65 teeth, far more than any other species of dolphin. Note that the striped dolphins may have from 43 to 50 teeth per side in each jaw. However, striped dolphins are decidedly larger (to about 9 feet [2.7 m]), have a shorter beak, and are distinctly marked with dark stripes from the eye to the flipper, from the eye to the a.n.u.s, and from the area behind the dorsal fin forward, towards but not reaching the head.

Saddleback dolphins also have from 40 to 50 teeth on each side but are also easily distinguishable by the differences in coloration discussed above for living animals at sea.

[Ill.u.s.tration: Figure 122.--Spinner dolphins occur in large herds in tropical waters. As ill.u.s.trated by these photos of animals off Venezuela in 1969, spinner dolphins often leap clear of the water and may come to a moving vessel from considerable distances away to frolic in its bow wave. (_Photo by M. Bartlett._)]

[Ill.u.s.tration: Figure 123.--A spinner dolphin jumping close beside a research vessel off the Virgin Islands. The distinctive color pattern (gray on the back, tan on the sides, and white on the belly) is clearly visible. The black-tipped rostrum and the black lips are key characters to this species. (_Photo by C. McCann._)]

[Ill.u.s.tration: Figure 124.--Spinner dolphins are active bow riders and may stay with a vessel for long periods of time. (_Photo from off the Virgin Islands by H. E. Winn._)]

[Ill.u.s.tration: Figure 125.--Although all spinner dolphins so far examined have the same basic characteristics, the degree of expression of those characteristics varies from individual to individual or area to area. These small short-snouted dolphins (those on bottom stranded near St. Petersburg, Fla. and maintained alive by the Aquatarium in that city, and those on top photographed at sea, off the northwestern Africa coast in 1972) are spinners, although their cla.s.sification is uncertain.

There may be several species or geographical races of spinners in the Atlantic. (_Photos courtesy of W. F. Perrin (top) and Aquatarium (bottom)._)]