When a female fossa is born she has a small clitoris and vulva, as might be expected. Then, at around seven months of age, something odd starts to happen. The fossa's clitoris enlarges, grows an internal bone and acquires spines to become a facsimile of the male's penis. It even exudes yellow liquid on its underside, like an adult male's. The female fossa sports her penile clitoris for a year or two, until she becomes reproductively active, when it magically disappears. Authors of a scientific paper on fossa genitalia postulated that this might protect the adolescent female from the unwanted attention of sexually pushy males or aggressively territorial female fossas.

The female fossa's transitory flirtation with a lookalike penis might, of course, serve no function at all. Not all traits do. Much like the redundant human appendix, it could simply be a relic from the fossa's evolutionary past that was sufficiently benign to avoid being selected against. Or be a side effect of another trait that evolution has selected for. Deciphering the ultimate evolutionary cause of a novel characteristic is a speculative sport. But decades of study into a close relative of the fossa has provided valuable evidence for the mechanics underlying such 'masculinized' genitalia. These insights have challenged a long-standing scientific prejudice concerning the 'passive' nature of female sexual development and gendered stereotypes of the hormones involved.

The genitals of the spotted hyena, Crocuta crocuta, have been causing a stir since the time of Aristotle. Ancient naturalists believed the hyena to be a hermaphrodite on account of the female's pudenda, which are the most sexually ambiguous of any known mammal's. Not only does the female spotted hyena have an eight-inch clitoris that's shaped and positioned exactly like the male's penis but she also gets erections. Both female and male spotted hyenas display and inspect one another's sexual tumescence during 'greeting ceremonies'. Crowning all this female virility is what appears to be a prominent pair of furry testicles.

This scrotum is in fact false: the hyena's labia have fused and filled with fatty tissue and merely resemble male gonads. This means that the female spotted hyena is the only mammal with no external vaginal opening at all. Instead she must urinate, copulate and even give birth through her curious multi-tasking clitoris – hence the antiquated hermaphrodite rumour. In more recent years, scientists have noted that males and females are so similar that they can be differentiated only by 'palpation of the scrotum' – something of a last resort, one assumes, when sexing an animal famous for its bone-crunching bite.

The female spotted hyena's sexual transgression doesn't stop at her genitals. Scientists have also been fascinated by her similarly 'masculinized' body and behaviour. Females can be up to 10 per cent heavier than males in the wild (20 per cent in captivity). This is unusual, as amongst mammals males are generally larger in size.2 In the rest of the animal kingdom, and thus the majority of animals, sexual size dimorphism is however generally the reverse. Fatter, more fecund females produce more eggs, so amongst most invertebrates and many fish, amphibians and reptiles it is the females that often outsize the males.3

Female spotted hyenas are also more aggressive than males. These highly intelligent, social carnivores live in matrilineal clans of up to eighty individuals governed by an alpha female. Males tend to be the sex that disperses from the natal matriline and, as such, the lowest rung of hyena society: submissive outcasts begging for acceptance, food and sex. Females are considered dominant in most situations, engage in rough play and vigorous scent-marking as well as leading the territorial defence – all behaviours more commonly associated with the opposite sex.

The radical gender-bending life of the female spotted hyena was originally assumed to be the result of an excess of testosterone circulating in her blood. Androgens, the group of sex steroid hormones that includes testosterone, have been unambiguously branded as male: andro meaning 'man' and gen a 'thing that produces or causes'. So the obvious assumption was that these big, belligerent female hyenas, much like the mole we met earlier, must be swilling with the stuff. But much to everyone's surprise, the circulating levels of testosterone in adult female spotted hyenas do not rival those in males.

Excerpted from Bitch by Lucy Cooke. Copyright © 2022 by Lucy Cooke. Excerpted by permission of Basic Books. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.