Dolphin

The dolphin is a highly intelligent marine mammal. Because it has to come to the surface to breathe and keep an eye out for danger, a dolphin cannot fall fully asleep. Instead it sleeps with one half of its brain at a time, keeping the other half awake, often with one eye open.^[1] A sleeping dolphin is, therefore, also an awake dolphin, and can hold both positions without seeming confused about it.

Names

Each bottlenose dolphin develops its own distinctive whistle, called a signature whistle, early in life. Other dolphins copy a dolphin's whistle to call it, and the dolphin responds to its own whistle much as a person responds to their name.^[2] Dolphins, in short, have names. Nobody has yet found out whether any of them are embarrassing.

The names last for life. A dolphin in a group that hears its signature whistle from a friend after many years apart has been shown to recognise it.

Sleep

After a couple of hours, the dolphin switches over and rests the other half of its brain. Over a day, each half gets its sleep and the dolphin is never fully off duty. Researchers have proposed this system for meetings, where it has been in informal use for years.

• Left half asleep: right eye shut.
• Right half asleep: left eye shut.
• Both halves asleep: not observed.

See also

• Ocean – its home
• Echo – which it also uses to find things
• Meeting – where half-sleeping is also practised
• Shark – the other fin in the water
• Hippopotamus – a surprisingly close relative

References

1. Hemisphere, H. (2000). Half Asleep. Current Press.
2. Whistle, W. (2013). “What's Your Name?” Marine Behaviour 2: 1.
3. Doze, D. (1998). One Eye Open. Reef Books.