Atlantic white-sided dolphins reach about 9 feet (2.7 m) in maximum length and are robust in form with a small but distinct beak (less than 2 inches [5.1 cm] long).
The dorsal fin is tall, distinctly back curved, and pointed on the tip. The tail stock is extremely thick and does not narrow laterally until very near the tail flukes.
The back is distinctly black, the belly white. The sides have zones of gray, tan, and white.
The single most distinctive feature of Atlantic white-sided dolphins is an elongated oval zone of white and yellowish white along the sides from just below the dorsal fin to the area above the anus. These patches of lighter coloration, clearly demarcated from each other and from the surrounding coloration, are frequently visible simultaneously with the dorsal fin as the animals roll at the surface to breathe. Even alone this feature permits positive identification of the species. The dorsal fin is often part gray, part black. The beak is all black.
Natural History Notes
Atlantic white-sided dolphins are known to congregate in herds of perhaps a thousand animals, though smaller herds are far more common. The species is often wary of ships and does not ordinarily ride the bow wave. Like a number of other species, white-sided dolphins have been reported in association with Atlantic pilot whales.
May Be Confused With
At sea, Atlantic white-sided dolphins are most likely to be confused with the white-beaked dolphins, with which they overlap in distribution. Though they are very similar in general appearance, the two can be distinguished in the following ways:
| Atlantic White-Sided Dolphin | White-Beaked Dolphin |
|---|---|
| COLORATION | |
| Elongated band of yellowishwhite and white along side,visible behind and belowdorsal fin as animal rolls. | Two grayish areas—one infront, the other below andbehind dorsal fin, visible asanimal rolls. |
| BEAK | |
| All black. | Sometimes white in parts ofrange though western Atlanticanimals are usually dark. |
| MAXIMUM SIZE | |
| To 9 feet (2.7 m). | To 10 feet (3.1 m). |
| DORSAL FIN | |
| Often part black, part lightergray. | Uniformly dark. |