a 2009 review of the scientific literature, University of California at Riverside biologists Nathan W. Bailey and Marlene Zuk, who advocate more study about the evolutionary impetus for homosexual behavior, state, “The variety and ubiquity of same-sex sexual behavior in animals is impressive; many thousands of instances of same-sex courtship, pair bonding and copulation have been observed in a wide range of species, including mammals, birds, reptiles, amphibians, insects, mollusks, and nematodes.”
