Monkey Music - for Monkeys

On io9, I saw an item about specially designed music that can sooth or agitate monkeys.

Monkeys Get a Groove On,
but Only to Monkey Music

Despite the fact that they are generally immune to human music, they respond to music based on vocal calls of their own species.

One weird thing that's mentioned is that the only clips of human music they reacted to was being calmed by Metallica!?!

*A great song about the horrors of
primitive monkey warfare