Sexual mimicry

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European Mole

Sexual mimicry occurs when one sex displays the characteristics of the opposite sex within a species. It is believed to be an adaptive trait. Examples of sexual mimicry in animals include the spotted hyena, bonobo, European mole and some species of insect. Sexual mimicry can also occur in some species of plant, including orchids.

Perhaps the most studied example of sexual mimicry occurs in the female spotted hyena. The external genitalia of female spotted hyenas resembles that of the male. Females have a peniform clitoris, resembling a penis, and fused labia, resembling a scrotum. The cause of this masculinization of external genitalia has been attributed to androgen secretion via the fetal ovary and/or fetal adrenal cortex.[1] There has been additional evidence to suggest that placental circulation of androgens from the mother provide a major source of testosterone for female fetuses. When pregnant spotted hyenas were treated with anti-androgens, many feminizing effects on their young were observed for at least 6 months following birth. For males, the penis resembled the external genitalia of a normal female spotted hyena. The common sex differences in genitalia were exaggerated in females.[2]

Female spotted hyenas are generally larger and more aggressive than males as they live in a social system where females are dominant over males. It is believed that this sexual mimicry acts as protection based on this highly aggressive, female dominant societal structure.[3] The specific androgen mainly responsible for the high aggression and external masculinized genitalia of females is androstenedione. Additionally, the enzyme responsible for conversion of androstenedione to testosterone, 17β-hydroxysteroid dehydrogenase is abundant in the placenta of spotted hyenas. When androstenedione from the mother reaches the placenta, 17β-hydroxysteroid dehydrogenase converts it to testosterone which reaches the fetus. This results in masculinization of the female.[4]

Another example of sexual mimicry occurs in flat lizards. While females mimicked males in the spotted hyena, males mimic females in the flat lizard. Flat lizard males tend to be territorial and aggressive with other males. Therefore, it is adaptive for males to mimic females in order to gain access to females on the territories of resident males while avoiding aggressive encounters. There are two types of males in this population; she-males, who mimic females, and he-males, who appear male. It has been observed that she-males are able to fool he-males into believing they are female visually, but not chemically through scent. The she-males who were successful in gaining access to females were those who avoided close contact with other males, thereby reducing the chances of detection through chemical signals.[5]

Finally, the European mole has been shown to display sexual mimicry similar to the spotted hyena. The external genitalia of many species of female moles is masculinized by the presence of a peniform clitoris. The ovaries contain a discrete interstitial gland, which accounts for the secretion of androgens, including testosterone, in the fetus. While this is the main mode of external masculinization for Old World females, it has been found that some species of New World females do not contain an ovarian interstitial gland, but still display masculinized external genitalia. This suggests that female moles of certain species can develop masculinized genitalia in the presence or absence of the ovarian interstitial gland.[6]

Sexual mimicry can have adverse reproductive costs. For example, the female spotted hyena has to copulate and give birth through her long peniform clitoris. The umbilical cord is 12–18 cm long, while the journey from uterus to clitoris end is 40 cm. The umbilical cord often will break before the cub emerges often leading to death by anoxia for many young. This journey is not only harmful for cubs, but also for the mother. The tissue of the clitoris often will rip open when giving birth for the first time. This can be fatal.[7]

While disorders do exist in humans that cause one sex to take on the characteristics of the other, these are not cases of sexual mimicry as they are not adaptive and only survive due to recessive refuge. These can include Turner syndrome, Klinefelter syndrome, or congenital adrenal hyperplasia.

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  1. ^ Lindeque, M., Skinner, J.D. (1982). Fetal androgens and sexual mimicry in spotted hyaenas (Crocuta crocuta). Journal of Reproduction and Fertility. Vol. 65, 405-410.
  2. ^ Drea, C.M., Weldele, M.L., Forger, N.G., Coscia, E.M., Frank, L.G., Licht, P. Glickman, S.E. (1998). Androgens and masculinization of genitalia in the spotted hyaena (Crocuta crocuta). 2. Effects of prenatal anti-androgens. Journal of Reproduction and Fertility. Vol. 113, 117-127.
  3. ^ Frank, L. G. (1997). Evolution of genital masculinization: why do female haenas have such a large ‘penis’? Trends in Ecology & Evolution, Vol. 12, Issue 2, 58-62. Accessed October 2013. doi:10.1016/S0169-5347(96)10063-X.
  4. ^ Glickman, S.E., Cunha, G.R., Drea, C.M., Conley, A.J., Place, N.J. (2006). Mammalian sexual differentiation: lessons from the spotted hyena. TRENDS in Endocrinology and Metabolism. Vol.17 No.9, 349-356. Accessed October 2013. Doi:10.1016/j.tem.2006.09.005.
  5. ^ Whiting, M.J., Webb, J.K., Keogh, S.J. (2009). Flat lizard female mimics use sexual deception in visual but not chemical signals. Proceedings of The Royal Society B. Vol. 276, 1585-1591. Accessed October 2013. Doi:10.1098/rspb.2008.1822.
  6. ^ Rubenstein, N.M., Cunha, G.R., Wang, Y.Z, Campbell, K.L., Conley, A.J., Catania, K.C., Glickman, S.E., Place, N.J. (2003). Society for Reproduction and Fertility. Vol. 126, 713-719. Accessed October 2013.
  7. ^ Frank, L. G. (1997). Evolution of genital masculinization: why do female haenas have such a large ‘penis’? Trends in Ecology & Evolution, Vol. 12, Issue 2, 58-62. Accessed October 2013. doi:10.1016/S0169-5347(96)10063-X.