The term "gender bender" has often been applied to pop stars because of their ambiguous style of dress. But one species of mammal has taken this further, temporarily developing physical characteristics of the opposite sex, writes Caroline Davis.
Research led by Paul Racey at Aberdeen University found females of the largest breed of the mongoose family undergo a temporary sex change as part of growing up.
The young female fossa, found only in Madagascar, begins to develop masculine sexual characteristics at about one year old. As the animal approaches maturity at about three to four years old, it loses the male characteristics.
Paul Fowler, an Aberdeen endocrinologist involved in the study, said: "This is a very unusual occurrence. The fossa is the only mammal known to do this."
Claire Hawkins and a team of researchers captured 43 wild fossas. They measured the penis bone in males, the clitoris bone in females and other sexual characteristics. The animals' genetic sex was confirmed by DNA testing. They found that the masculine features were most pronounced in the animals aged between one and three years and the clitoris bone length got shorter as the animal got bigger.
Fossas are solitary creatures, and mothers are known to become aggressive to their young.
In other species, females are thought to be masculinised because of high levels of male sex hormones, androgens, but this was not the case in the fossas examined.
The researchers suggested two reasons why this transient sex change occurs. Masculinisation could protect the juvenile females from sexual harassment by adult males. As fossas are rare and females have very short annual fertile periods, males try to mate with any female they encounter, leaving small inexperienced fossas especially vulnerable.
Masculinisation could also protect young females from the very territorial older females.
Dr Fowler said studying how the change occurs, especially how the masculine features are lost, could provide insight into defective reproductive development in humans, whether by natural causes, disease or exposure to environmental pollution.
The research was published in the journal Biology of Reproduction .