A large black toothed whale that travels in tight groups and sometimes approaches divers in open water.
Males to about 6 m and 1,360 kg, females to about 4.9 m. Slender and entirely dark grey or black, with a small conical head and no beak at all, and a faint pale patch between the pectoral fins. The dorsal fin is tall, curved and often heavily notched.
The flipper is the feature that settles the identification. Its leading edge carries a distinct hump, giving an S-shaped front margin that no other dolphin has. The pygmy killer whale and the melon-headed whale are the two it is confused with; the short-finned pilot whale is separated by its bulbous forehead and low, very broad-based fin.
It is a deep-water animal, feeding at 300 to 500 m on dives of up to eighteen minutes. It travels in tight groups of ten to twenty within looser clusters of forty or fifty, and it is one of the more curious large dolphins: it will approach a boat or a diver and stay. At Revillagigedo it has been recorded at Socorro and at Roca Partida. It is listed as near threatened, mainly because of longline fishing, and it is harmless.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.