It's a known fact that whales and birds sing. Whales may use their songs to communicate with other whales and to find out where their fellow pod members are. Birds use songs to communicate a variety of things—to greet other birds, to "say" they're hungry, to warn other birds of danger, and to attract mates, for example. The gibbon, a species of primate, is also known to sing. Pairs or families of gibbons often sing together, and pairs who have been together for a long time harmonize (make pretty music together) well. The purpose of their singing might be to protect their territories or strengthen connections with their mates. And recently, scientists have discovered evidence that mice sing too.