Biological Exuberance: Animal Homosexuality and Natural Diversity

The types of sex change that are found, the number and fluidity of genders, and the overall social organization of these species are so complex that a detailed terminology has been developed by scientists to describe all the variations. In some species, females turn into males (this is called protogynous sex change), while in others males become females (called protandrous sex change). In some fish, sex change is maturational; that is, it happens automatically to all individuals when they reach a certain age or size or else occurs spontaneously at different times for each individual. In other species, sex change appears to be triggered by factors in the social environment of the fish, such as the size, sex, or number of neighboring fish. In female-to-male fish species, many different gender profiles are found. In some cases, all fish are born female, and males result only from sex change (such a system is called monandric). In other cases, both genetic males (born male) and sex-changed males are found (this arrangement is called diandric). In these fishes, genetic males are sometimes referred to as primary males while sex-changed males are called secondary males. Often these two types of males differ in their coloration, behavior, and social organization so that transsexual males form a distinct and clearly visible “gender” in the population.

Homosexuality as a “masculine” activity: a male Bighorn ram mounts another ram. Males who mimic females in this species (behavioral transvestites) do not generally permit other males to mount them, unlike nontransvestite rams.

Things get even more complicated in some species. Among secondary males, some change sex before they mature as females (prematurational secondary males), while others change sex only after they live part of their adult life as females (post-maturational secondary males). Many species also have two distinct color phases: fish often begin life with a dull color and drab patterning, then change into the more brilliant hues typically associated with tropical fish as they get older. Which individuals change color, when they change, and their gender at the time of the color change can yield further variations. Many species of parrot fishes, for example, have multiple “genders” or categories of individuals based on these distinctions. In fact, in some families of fishes, transsexuality is so much the norm that biologists have coined a term to refer to those “unusual” species that don’t change sex—gonochoristic animals are those with two distinct sexes in which males always remain male and females always remain female.