Looking more like a creature out of a Steven Spielberg movie than one of our closer primate relatives, Callithrix saterei made its official debut as a new species of marmoset in 1996. The 50 known marmoset species, most of which weigh under a pound, are among the smallest of New World monkeys. Since 1990, six new species have turned up, most of them, like C. saterei, in the Amazon, where they dwell in the lower to middle levels of forest trees. The squirrel-size C. saterei eats insects and fruit but specializes in chewing little holes in trees to extract gum. C. saterei is bare-faced, with an orange-brown belly and dark red-brown fur on its back and limbs. Its most distinctive feature is its bright orange genitalia. Both males and females have long, fleshy, labia-like appendages framing the usual sex organs. It’s a really bizarre adaptation, says primatologist Russell Mittermeier, president of Conservation International. I have no idea whatsoever what its function may be.