\" \" Flickr These dolphins appear to a viewer to be simply slowing down, but are actually half asleep. They are using half their brains to swim, while shutting off the other side for sleep. Meanwhile, the other half of brain sleeps and the opposite eye is closed. Dolphins have a voluntary respiration system, which is why they have to keep part of their brain alert to breathe. Male bottlenose dolphins have been seen engaging in \"calf tossing\" to kill young dolphins so their mothers will become ready to mate again.