Some, like clownfish, swap when there’s a gap in the social order. Others switch when they get to a certain size, when they’re looking for a mate, when the weather is hot, when the environment is ...

Text by Nancy Pick, photographs by Mark Sloan (HarperCollins, $22.95) Neuroscientist Joseph LeDoux first encountered the brain’s “soft mushy mass” while extracting bullets from cows’ brains in his ...