16 April: Dolphin Day

It's Dolphin Day. Time for ten things you didn't know about dolphins:

1. There are about 40 different species of dolphin. Killer whales are actually a type of dolphin rather than a type of whale, and are the largest species. They can reach 9.5 metres (31 ft) in length and weigh 10 metric tons. The smallest is Maui's dolphin - 1.7 metres (5.6 ft) long and 50 kilograms (110 lb).
2. The closest living relative of the dolphin is the Hippopotamus.
3. The word "Dolphin" derives from the Greek word for "womb" so the literal translation would be "fish with a womb". An archaic term for a dolphin is mereswine ("sea pig").
4. Dolphins can hear frequencies 10 times the upper limit of adult humans. Their hearing is so good that a dolphin could survive even if it is blind. They also have good eyesight, both in and out of water, and a well-developed sense of touch - but no sense of smell.
5. Bottlenose dolphins have been found to have signature whistles, unique to specific individuals, rather like a human name. These whistles develop during their first year of life and stay with the dolphin for as long as it lives. The signature whistle of a male bottlenose dolphin tends to be similar to that of its mother, while the signature whistle of a female bottlenose dolphin tends to be more unique.
6. As well as whistles they also emit clicking sounds for echolocation, from an organ known as a melon, a mass of adipose tissue found in the forehead of all toothed whales.
7. Dolphins are highly social. They live in groups, known as pods, of around twelve individuals, which may, in areas where there is a lot of food, merge and become "superpods" of over a thousand dolphins. If one dolphin is sick or injured others will help it, bringing it to the surface to breathe. They have also been observed helping other species, such as sperm whales, and humans.
8. Dolphin calves are born tail first.
9. Male dolphins have been known to masturbate by wrapping a live Eel around their penis.
10. Four countries – Chile, Costa Rica, Hungary, and India – have declared dolphins to be "non-human persons" and have banned the capture and import of live dolphins for entertainment.