Things you always wanted to know about fossa sex (but were afraid to ask)

Fossa - Rob Gordon Photography — Adult fossa in Madagascar, courtesy of Rob Gordon ((c) Rob Gordon Photography)

[Disclaimer: there will be many repetitions of the word “clitoris”. Don’t worry, we’ll get through it together.] Fossas are the largest carnivores in Madagascar. For those unfamiliar with them, they look a bit like a giant weasel crossed with a cougar. Their scientific name is Cryptoprocta ferox, which translates as “hidden anus, ferocious” – their anus may not be particularly hidden or ferocious, but the rest of “down there” can certainly be pretty cryptic.

Foooooosa. No genitalia are apparent, but then it is a children’s film.

In 1902, a Swedish naturalist called Einar Lönnberg published a report on the anatomy of their genitalia. In it he described two individuals who, while female, had some more typically male characteristics as well. 100 years later, a team from the university of Aberdeen decided to look into it a little more closely, and produced my new favourite paper “Transient Masculinization in the Fossa”. It has some insights into how truly weird fossa anatomy is, and how we can make sense of it by studying their behaviour. First, the males. Male fossas have large testicles but otherwise their anatomy isn’t unusual. They have a baculum – a bone in the penis – which is a feature widespread among carnivore species. They also have spines on the surface of the penis to stimulate the vaginal wall during sex – again, this is something seen in a wide variety of animals including domestic cats and lemurs, and often means the female’s ovulation is triggered by mating.

Figure from the 2002 Hawkins study, showing genitalia of adult male, adult female and juvenile female. Click to enlarge.

Baby female fossa start out life looking like typical female carnivores, with a vulva and a clitoris that’s small and unobtrusive. But something odd starts to happen at around seven months of age. Their clitoris enlarges, grows an internal bone and acquires spines. They even produce a yellow secretion similar to that seen on adult males. This “masculinisation” lasts for about two or three years and reverses when they get older. By the time they’re ready to breed the masculine changes have disappeared. So what’s the point of all this? The authors of the paper think that the answer may lie in the fossa’s social life. In fossa society, the females have smaller territories and are more fiercely territorial than the males. Each area of fossa forest has what I’ll call the Fossa Sex Tree, used by all the fossa nearby. When a female fossa is ready to mate, she will climb up the Sex Tree and yell loudly. Males gather below, and she chooses three or four from the assembled masses. They then mate repeatedly over a couple of days. Mating itself involves clambering around on thin branches, and fossas sometimes inflict wounds on each other by biting or hanging on with claws.

fossa Fi Fern enrichment — Juvenile fossa playing with rope. Photo by Fi Fern.

Once you know this, it makes sense for adolescent females to pretend to be male for a little while when they first leave their mothers. That way other females won’t see them as territorial invaders, and they can avoid potentially injurious mating attempts until they’re ready to breed. And because the females choose to mate with more than one male, the males evolved large testicles to produce enough sperm to increase their chances of being the father of her litter. Fossa sex is a perfect example of how traits that at first seem completely bizarre can be explained by understanding the behaviour that evolved alongside them. – @pakasuchus P.S. Here is the end product of fossa sex, running off with a camera: