Whales, Dolphins, and Porpoises of the Western North Atlantic Part 26

As they age, the Atlantic spotted dolphins develop grayish-white spots, first low on the sides, spreading upward. During this stage, the cape becomes less distinct, and dark spotting begins to develop on the belly, the spots increasing in number with increasing age. In adult animals, the belly is often extensively covered with dark blotches but never becomes completely black. The lips may be white, and the beak is characteristically tipped with white, a feature which may aid in identification at sea.

The Atlantic spotted dolphins have a spinal blaze and a light line which extends from the flipper to the eye.

Natural History Notes

Little is known of the natural history of the Atlantic spotted dolphins.

The species occurs in herds of up to several hundred individuals, though groups of 50 or fewer (6-10) are more common. They are often seen jumping clear of the water and habitually ride the bow wave of moving vessels. As they do, the distinct cape or band of purplish gray on the back of younger animals and the spotting pattern of older animals may be visible.

Atlantic spotted dolphins feed primarily on squid but may also take carangid fishes, small eels, herring, or anchovies.

May Be confused With

Atlantic spotted dolphins, particularly young animals, may be easily confused with Atlantic bottlenosed dolphins because of the similarities in color pattern and general body shape. However, the Atlantic spotted dolphins have considerable purplish gray in their background colors and the Atlantic bottlenosed dolphins are more dark gray to brownish gray.

This along with considerable differences in the overall sizes of the two species should permit positive identification. In general, the key differences between spotted and bottlenosed dolphins are as follows:

ATLANTIC SPOTTED ATLANTIC BOTTLENOSED DOLPHIN DOLPHIN

MAXIMUM SIZE

To 7.5-8 feet (2.3-2.4 m). To 8-10 feet (2.4-3.1 m) insh.o.r.e, to as much as 12 feet (3.7 m) offsh.o.r.e.

BODY COLOR

Dark purplish gray on back; Dark gray on back; lighter lighter gray on sides and gray on sides; white or pink belly; body becomes on belly (old animals may increasingly spotted with age. have a few spots on belly, but most are not spotted).

HEAD AND BEAK

Head more slender; beak Head robust; beak short; longer; lips and top of snout beak usually uniformly gray often white. (older animals' beak may be white at tip).

NORMAL DISTRIBUTION

Usually found more than 5 Usually more coastal, often miles offsh.o.r.e; most common ascending rivers and entering inside 100-fathom curve. lagoonal and estuarine areas.

Young Atlantic spotted dolphins are so similar in appearance to the Atlantic bottlenosed dolphins that the frequent reports of mixed schools of the two species are probably occasioned by groups of spotted dolphins which include some young, still unspotted animals.

Atlantic spotted dolphins might also be confused with bridled dolphins.

The two can be most readily distinguished by the following characteristics:

ATLANTIC SPOTTED DOLPHIN BRIDLED DOLPHIN

BODY SHAPE

Usually robust, often like More slender, more like that of the Atlantic that of the Atlantic striped bottlenosed dolphin. dolphin.

BODY COLOR

Spotted; purplish gray on Spotted; side of head light back; lighter gray on sides gray; body has stripe from and belly becoming flipper to corner of mouth, increasingly spotted with age. though the stripe tends to As animals becomes more fade as spotting increases.

spotted, cape become less Cape on top of head more distinct. Body has spinal distinct that on Atlantic blaze and light line from spotted dolphin. Body has no flipper to eye. spinal blaze.

At sea the Atlantic spotted dolphins may also be confused with rough-toothed dolphins (p. 135).

Distribution

Atlantic spotted dolphins are a common species in the continental waters of the tropical and warm temperate western North Atlantic. Although they are far more abundant south of Cape Hatteras, they have been reported from the lat.i.tude of Cape May, N.J. (some fishermen claim to have seen them even further north) south through the Gulf of Mexico and the Caribbean to Venezuela. Atlantic spotted dolphins may be replaced around the West Indies by the bridled dolphin.

Within this range, the Atlantic spotted dolphins appear to be generally restricted to the waters outside the 100-fathom curve, most commonly more than 5 miles offsh.o.r.e. However, populations in the Gulf of Mexico move insh.o.r.e in the late spring, and may approach close to sh.o.r.e during spring and summer.

Stranded Specimens

Stranded Atlantic spotted dolphins may be difficult to distinguish from bridled dolphins. If the color pattern is still clearly visible, the differences in coloration described above, particularly those of the head, and the presence or absence of a spinal blaze may be used. But since external appearance other than coloration are often very similar, specimens should be photographed from as many aspects as possible and the entire specimen or the roughed-out complete skeleton transported to a museum for preparation and examination. Tooth counts recorded for the two species to date are also very similar.

[Ill.u.s.tration: Figure 115.--Atlantic spotted dolphins beside a research vessel off Beaufort, N.C., September 1965. Adults of this species can be identified by the spotting pattern and the white coloration of the lips.

(See also bridled dolphin, p. 108.) Young animals which lack spots may be confused with the Atlantic bottlenosed dolphins though they are smaller and are purplish gray on the back. (_Photo by G. T. Green._)]

[Ill.u.s.tration: Figure 116.--A side view of two female Atlantic spotted dolphins from off St. Augustine, Fla. in the tank at Marineland of Florida. Note the tall falcate dorsal fin, pointed on the tip and varying slightly in shape between the two individuals, and the spots on the body. (_Photo by S. Leatherwood._)]

[Ill.u.s.tration: Figure 117.--A series showing the development of the color pattern of the Atlantic spotted dolphins from Florida. Newborn or young animals are dark purplish gray on the back, grading to immaculate white on the belly. As they mature, animals develop light spots, first on the lower sides, then higher on the back, and dark spots on the belly. As spotting increases, the cape becomes less distinct. (_Photos by A. Solis (a), D. K. Caldwell (b, c), and courtesy of Marineland of Florida (d, e, f)._)]

[Ill.u.s.tration: Figure 118--Juvenile Atlantic spotted dolphins at sea in the southeastern Caribbean. Although spotters can be either relatively short-snouted and chunky or long-snouted and slightly built, the spinal blaze, flipper-to-eye stripe, white lips, and falcate dorsal fin can be used to identify them. (_Photo by D. Poppe._)]

[Ill.u.s.tration: Figure 119.--A captive Atlantic spotted dolphin from off St. Augustine, Fla. This species has from 30 to 36 teeth in each upper jaw and from 28 to 35 in each lower jaw, fewer than all other Stenella except perhaps the bridled dolphin, fewer than the saddleback dolphins, but more than the Atlantic bottlenosed dolphins. (_Photo by S.

Leatherwood._)]

BRIDLED DOLPHIN (T)

_Stenella frontalis_ (G. Cuvier 1829)

Other Common Names

Bridled spotted dolphin, Cuvier's dolphin, gamin (St. Vincent), bridled porpoise.

Description

Bridled dolphins, the second species of spotted dolphins in the western North Atlantic, reach an adult length of at least 7 feet (2.1 m). Like the other, the Atlantic spotted dolphin (p. 104), bridled dolphins are characterized by light grayish-white spots on the dark dorsal portions of the body and dark spots on the light ventral surface. Other details of the coloration differ somewhat. Bridled dolphins are dark gray on the back, fading to lighter gray on the sides and belly. They lack the spinal blaze characteristic of Atlantic spotted dolphins. Except on the head, the border between the back and side colors is indistinct. On the head, the cape (the dark color of the top of the head) is distinct. In the light gray of the side of the head are the markings from which the species derives its common name, "bridled dolphin." These are a black circle around the eye with an extension to the junction (apex) of the rostrum and the melon (present in nearly all dolphins) and a broad black stripe from the origin of the flipper to the corner of the mouth. This mouth-to-flipper stripe tends to fade as spotting increases. Both the upper and lower lips are white or pinkish.

Natural History Notes

Virtually nothing is known of the natural history of bridled dolphins except that they have been observed in small herds of from 5 to 30 individuals and sometimes ride the bow wave of a vessel.