Mammals fascinate people with their perpetual grin

The 32 species that make up the dolphin family are closely related to whales. They include killer whales, which grow up to nine me tres long and are famous for their aggressive hunting pods.

The rest are smaller. The common dolphin is up to 2.1 metres long, and the bottlenose is up to 3.6 metres.

Many dolphins have a pronounced beak and all have a central dorsal fin and sharp teeth for feeding, mainly on squid and fish.

Significantly for humans, the mouths of many species of dol phin turn up in a perpetual grin. As a result, the animals have long fascinated people.

Ancient Greeks reported altruism in dolphins, saying they had rescued lost seafarers, and often depicted the creatures in art. Ex perts say dolphins are smarter than dogs and similar in intelli gence to chimpanzees.