The spinner dolphin is a small cetacean with a slim build. Adults are typically 129-235 cm long and reach a body mass of 23-79 kg. This species has an elongated rostrum and a triangular or sub-triangular dorsal fin. Spinner dolphins generally have tripartite color patterns.
The dorsal area is dark gray, the sides light gray, and the underside pale gray or white. Also, a dark band runs from the eye to the flipper, bordered above by a thin, light line. However, the spinner dolphin has more geographic variation in form and coloration than other cetaceans. In the open waters of eastern Pacific, dolphins have relatively small skulls with short rostra. A dwarf form of spinner dolphin occurs around southeast Asia.
In these same subspecies, a dark dorsal cape dims their tripartite color patterns. Further offshore, subspecies tend to have a paler and less far-reaching cape. In certain subspecies, some males may have upright fins that slant forward. Some populations of spinner dolphin found in the eastern Pacific have bizarre backwards-facing dorsal fins, and males can have strange humps and upturned caudal flukes.
In certain regions, such as Hawaii and northern Brazil, dolphins spend the daytime resting in shallow bays near deep water. At dusk, they travel offshore to feed. They travel along the shore during foraging trips, and the individuals that occupy the same bay may change daily. Some individual dolphins do not always go to a bay to rest; however, in Hawaii, dolphins do seem to return to the same site each trip.
Spinner dolphins live in an open and loose social organization. The spinner dolphins of Hawaii live in family groups, but also have associations with others beyond their group. Mothers and calves form strong social bonds. Spinner dolphins seem to have a promiscuous mating system, with individuals changing partners for up to some weeks. A dozen adult males may gather into coalitions.
Vocalizations of spinner dolphins include whistles, which may be used to organize the school, burst-pulse signals, and echolocation clicks. The spinner dolphin has a 10-month gestation period, and mothers nurse their young for one to two years. Females are sexually mature at four to seven years, with three-year calving intervals, while males are sexually mature at seven to 10 years. Breeding is seasonal, more so in certain regions than others.