Homosexual sexual behavior occurs in the animal kingdom, especially in social species, particularly in marine birds and mammals, monkeys, and the great apes. Homosexual behavior has been observed among 1,500 species, and in 500 of those it is well documented. This discovery constitutes a major argument against those calling into question the biological legitimacy or naturalness of homosexuality, or those regarding it as a meditated social decision. For example, male penguin couples have been documented to mate for life, build nests together, and to use a stone as a surrogate egg in nesting and brooding. In a well-publicized story from 2004, the Central Park Zoo in the United States replaced one male couple's stone with a fertile egg, which the couple then raised as their own offspring. The genetic basis of animal homosexuality has been studied in the fly Drosophila melanogaster. Here, multiple genes have been identified that can cause homosexual courtship and mating. These genes are thought to control behavior through pheromones as well as altering the structure of the animal's brains. These studies have also investigated the influence of environment on the likelihood of flies displaying homosexual behavior. Georgetown University professor Janet Mann has specifically theorized that homosexual behavior, at least in dolphins, is an evolutionary advantage that minimizes intraspecies aggression, especially among males. Studies indicating prenatal homosexuality in certain animal s
