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Roy and Silo, two male chinstrap penguins at New York's Central Park Zoo have been inseparable for six years now. They display classic pair-bonding behavior--entwining of necks, mutual preening, flipper flapping, and the rest. They also have sex, while ignoring potential female mates.

The bonobo, an African ape closely related to humans, has an even bigger sexual appetite. Studies suggest 75 percent of bonobo sex is nonreproductive and that nearly all bonobos are bisexual. Frans de Waal, author of Bonobo: The Forgotten Ape, calls the species a "make love, not war" primate. He believes bonobos use sex to resolve conflicts between individuals.