The animal kingdom’s ‘sweet transvestite’: The Female Spotted Hyena

Female spotted Hyenas are a strange. While the anatomy of striped hyenas (Hyaena hyaena) and brown hyenas (Hyaena brunnea) tend to be relatively standard, the female spotted hyena (Crocuta crocuta) throws conformity and normality out the window with their convincingly masculine genitalia (Muller & Wrangham 2002).

The female spotted hyena internal anatomy
(Source: Curious Cox. Image by Unknown)

Female spotted hyenas have a clitoris which has the same size, length and erectile ability of their male counterparts’ penises. Strangely enough, they have no external vagina and instead mate, give birth and urinate from the urogenital canal that their clitoris encompasses (proving giving birth to be very difficult indeed!) In addition to their pseudopenis, female hyenas have developed a pseudoscrotum that appears to be the fusion of the labia.

Female spotted hyenas have what is referred to as sexual monomorphism. Sexual monomorphism is rare in the animal kingdom and even rarer in species where females imitate males (Muller & Wrangham 2002). Female spotted hyenas’ genitals mimic male genitalia so well that even researchers of the species are unable to determine between the sexes of cubs until they are around three months old. Juveniles, subadults and adults are also impossible to sex unless you have an extensive understanding of the species (Muller & Wrangham 2002).

It was initially thought that this genital masculinisation was the non-functional result of unusually high levels of androgens (hormones such as testosterone and androsterone) while the cubs are developing in the womb. Although this still thought to play a role in the masculinisation of the female genitalia, it was discovered by Licht et al (1998) that initial masculinisation occurres before these hormones are activated and therefore cannot be the sole cause of masculinisation.

While it is still uncertain what has produced such an unusual trait, there are many speculations about how natural selection has played a role in producing females with such large and realistic male genitalia and how this seemingly hindering adaptation might be somehow advantageous. Muller and Wrangham (2002) suggest that it could be to defend against sibliside in the early stages of life where it is observed that females are most likely to be killed by their siblings (Frank et al 1991). The genital masculinisation may also help to reduce the chances of females becoming the target of infanticide by dominant females when they kill the cubs of lowerranking females. Yet another possible reason is to defend against intergroup aggression as it has been witnessed that females that are unknown to the clan are dealt with considerably harsher than unknown males (Holekamp et al. 1993). Each of these hypothesis are yet to be definitavely proven yet.

References

Female spotted hyena and her "Penis", n.d. photograph, viewed 17 March 2014, <http://curiouscox.wordpress.com/2012/01/11/the-spotted-hyenas-she-penis/>

Frank, LG, Glickman, SE & Licht, P 1991, 'Fatal sibling aggression, precocial development, and androgens in neonatal spotted hyenas', Science, vol 252, no. 1, pp. 702-704.

Holekamp, KE, Ogutu, JO, Dublin, HT, Frank, LG & Smale, L 1993, 'Fission of a spotted hyena clan: consequences of prolonged female absenteeism and causes of female emigration', Ethology, vol 93, no. 1, pp. 285-299.

Licht, P, Hayes, T, Tsai, P, Cunha, G, Kim, H, Golbus, H, Hayward, S, Martin, MC, Jaffe, RB & Glickman, SE 1998, 'Androgens and masculinization of genitalia in the spotted hyena (Crocuta crocuta). 1. Urogenital morphology and placental androgen production during fetal life', Journal of Reproduction and Fertility, vol 113, no. 3, pp. 105-116.

Muller, MN & Wrangham, R 2002, 'Sexual Mimicry in Hyenas', The Quarterly Review of Biology, vol 77, no. 1, pp. 3-16.