Sexual mimicry

Sexual mimicry is where one sex takes the characteristics of the other within a species. Examples of sexual mimicry in animals are the spotted hyena, bonobo chimp, spider monkey, lemur, european mole. It also occurs in plants, where flowers of one sex may mimic those of another.

Sexual mimicry can have adverse reproductive costs, for example the spotted hyena has to copulate and give birth through the long peniform clitoris. The umbilical cord is 12-18 cm long whilst the journey from uterus to the clitoris end is 60cm, resulting in death by anoxia of many young.

Clitoromegaly is an example of sexual mimicry and occurs in the Spotted Hyena as first noted by Aristotle. It results from natural high levels of testosterone in the placenta causing priming of the secondary sexual characteristics of the female foetus. In this case female external genitalia resembles that of the male, having a false penis from the clitoris and false scrotum from the vulva, this is a result of extreme hypertrophy. The females are also 10% larger, more aggressive and dominant, a trait that is normally observed in males. More specifically, the unusual features are due to an increased production of androstenedione in the maternal ovaries during pregnancy. Androstenedione is an androgen and converted to testosterone by the enzyme 17β hydroxysteroid dehydrogenase in the placenta, then transported to the fetus. In most mammals, the placenta converts androstendione to oestrogen via the enzyme aromatase. Aromatase is deficient in the hyena placenta.

The european mole is another good example of sexual mimicry. The male testes are intraabdominal and the female ovaries are bilateral meaning the bipolar gonads have both male and female features; this makes the mole very difficult to sex. The females also appear to have an enlarged peniform clitoris, these characteristics are also due to testosterone. During the breeding season, female testosterone levels decline and the pole containing interstitial ovarian tissue regresses, whilst the pole containing follicles grows. This is prevalent in Old World moles but has not been found in New World moles, suggesting it has arisen from high selective pressure.

Disorders do exist in humans that cause one sex to take on characteristics of the other, for instance girls suffering from congenital adrenal hyperplasia in most extreme cases are born with external male genitalia. This is not a case of sexual mimicry though, as it is not adaptive and only survives due to recessive refuge.

References

*Place NJ, Glickman SE. 2003 Masculinization of female mammals: lessons from nature. [http://www.vet.cornell.edu/labs/place/publications/16Place&Glickman.2004.pdf]