Species identification

Grampus griseus adults obtain lenghts of 2.6-4 m. Newborns are approximately 1 m in length. The species only shows small sexual dimorphism: adult males are on average 8 cm longer.
Adults weigh 300-500 kilo’s. Risso’s dolphins have a small melon (bulbous forehead), a tall, slender dorsal fin and a distinctive V-shaped groove on the front of the head.
The animals have an anchor-shaped white marking on the ventral side of their body.
Risso’s dolphins have 2-7 pairs of strong, oval teeth in the lower jaw, while the upper jaw usually does not contain functional teeth.

Colouration

Risso’s dolphins are born uniformly grey and become brownish-black as juveniles.
The newborn calves have vertical, grey birth-stripes (featal fows) upon the side of the body. Skin colouration varies considerably between individuals and fades with age to a very pale grey or white.
Fins, flippers and flukes remain dark. Colouration probably varies between males and females and it seems that the males are more white. As the colour of the species fades with age, however, this does not provide a very strong key to determine the gender of the observed individual. During their lifetime Risso’s dolphins obtain distinct white markings: scars probably caused by the teeth of other individuals or by confrontations with cephalopods (squid). Scarification patterns are individually unique and the pattern on the dorsal fin provides a very strong identification key for individuals.

Social structure

Social structure seemed to be stratified based on age and sex classes with strong associations between adult males and between adult females. The type of social organization, based on highly associated social units differ from the fission-fusion and matrilineal society models found in other cetacean species.