Strange seas

Dolphins are pretty intelligent, and a lot like humans in many ways. They have finger bones and live in families, there's even evidence that they might have the rudimentary structures for music and language.

Here's something, though. Dolphins don't dream.

Humans dream while they're in REM sleep. Pigs, dogs, horses- they all go through REM sleep and give every impression of dreaming while doing so. Dolphins have never been observed to do this. Instead, the two hemispheres of their brain can sleep independant of each other. During the night, one half sleeps for an hour while the other keeps the dolphin swimming and returning to the surface for air, and then they swap. If you wake a dolphin up each time the halves swap over, you can make it sleep-deprived in one half but not the other. During this half-sleep, the dolphin is blind in one eye. It never enters a state like REM or gives any sign of dreaming.

There is a species of dolphin that lives in the river Indus that sleeps as much as a human does, but only in snatches of one minute in every three.

If we ever get to converse with an alien intelligence, we shouldn't assume that they're going to be enough like us to enjoy Bach and chocolate pie.