Monkeys aren't that into human music. But monkey music? They can get into monkey music. When researchers composed arrangements for cello and voice based on cottontop tamarins' calls and then played the music for the monkeys, the animals responded to the emotions of the music: threatening pieces  made them anxious; soothing pieces  caused them to lounge around and eat chow. Peaceful human music had no effect on the monkeys, the team reports  online 2 September in Biology Letters, although some hard-rockin' songs by the band Metallica mellowed them out a bit. Whether the tamarins were actually appreciating the music the way we do is still an open question, however. Maybe to them, the monkey music just sounds like monkeys.