Having said which, it's not especially big compared with predators elsewhere. There is little need to be a large carnivore when the island does not have, for example, any native antelopes. On average, they are about 75 cm (30 inches) in body length, not counting a cat-like tail that adds about the same length again. Adult females weigh around 6 kg (13 lbs), while the largest males can reach 8.5 kg (19 lbs), about the same as a West Highland terrier.
Exactly where they fit within the mammalian family tree was long a source of confusion, given a mix of features resembling a range of other animals. When English zoologist Edward Turner Bennett first described the animal in 1833, he considered it to be a type of unusually large civet. So it remained for a hundred until it was moved to the cats in the 1930s, back to the civets in the 1940s, and then to the mongooses in the 1990s. Over the next ten years or so, arguments continued back and forth between all three of these options until genetic studies finally settled the matter in 2003.
As has been confirmed by other studies since, the fossa is a euplerid, descended from the same original ancestor as all other Madagascan carnivorans. It isn't even a separate subfamily within that group, as had been suspected, being more closely related to the falanoucs and fanalokas than to the vontsiras.
A lot of the confusion, in the days before genetic studies, comes from its odd appearance. While it may be much smaller, it looks a lot like a puma/cougar/mountain lion, albeit with a slightly longer, more pointed face. The nose is prominent, like that of civets and mongooses, but the ears are catlike. The paws are large, with hairless soles and partially retractile claws; like many tree-climbing animals, they can twist their ankles backwards to make it easier to move down head-first.
A more bizarre adaptation, unique to fossas so far as we know, is that young females have male-like anatomy, but this fades as they reach full adulthood. This includes the presence of a secretory gland on the abdomen that is otherwise only functional in males, and an enlarged, penis-like clitoris, complete with foreskin and containing a spiky internal bone. (The presence of a clitoral bone, or baubellum, is not unusual in mammals, although it's absent in apes and humans and is usually very small). How this happens, and why, remains a mystery, although it doesn't seem to have anything to do with the presence of male hormones.
Fossas are forest-dwelling animals, but they seem equally at home in the semi-arid forests of the west as in the damp tropical jungles of the east, with the result that they are found essentially across all of the coastal regions of the island, with a few reports from the scattered patches of woodland that still remain in the interior - most of which have otherwise been cut down for farmland. They are, however, present at lower population densities than we would predict based on similar animals elsewhere, perhaps as a result of the breakup of larger patches of woodland.
Unusually, they appear to be equally active day or night, perhaps changing their habits based on the presence of humans, but otherwise sleeping whenever they feel like it, regardless of the position of the sun.
Although active on the ground, they are also adept at climbing trees, a useful trait when so much of their potential prey is arboreal. In captivity, they will eat almost any meat provided to them, and their diet seems similarly opportunistic in the wild. For most of their range, this inevitably means that their main prey is lemurs, which have essentially no other mammalian predators. In at least some areas, half of the animals eaten by fossas are lemurs and, since lemurs are larger than most of the alternatives, that would be a much higher proportion of their food by weight. Their largest prey are sifakas, a type of lemur that can weigh about 90% as much as the fossa itself.
Their second favoured prey seems to be the shrew-like and hedgehog-like tenrecs native to the island, but they will also eat other animals, including snakes and crabs, if those are what happen to be around. Although they don't like to travel far from trees, they will venture into farmland, where they eat chickens and even young goats. Although it's difficult to make such an assessment objectively, this has led to their reputation as the deadliest mammalian predator on Earth, relative to their size.
Like their relatives, fossas are solitary animals, although their larger size allows them to range across wider areas of forest. The typical home range of any given individual likely varies depending on where on the island it lives, and how far it will have to travel to keep fed, but some studies give surprisingly large figures of 55 km² (21 square miles) or more, with males travelling further than females. While they do scent mark their territories, using similar glands to those on other euplerids, individual ranges can often overlap, so they are likely not as territorial as one might expect. There are also some reports of pairs of males cooperating where their ranges overlap, even though fossas are normally solitary except for females travelling with young.
Mating is seasonal, taking place between September and December. Receptive females are often surrounded by groups of up to eight males, fighting for access in one of the few instances where they will make any noise towards one another. Even so, it seems to be the female who picks her partner, although on what basis is unclear.
Mating often, but not always, takes place up a tree to limit the chance of interruption. This is important, because the mating act takes at least an hour, during which the partners are locked together in a copulatory tie, as happens with dogs, which would obviously make it difficult for them to do much else. You would think that an hour would be quite long enough to satisfy both partners, but apparently not, as they will engage in several bouts over a period of up to 14 hours in total. Even then, the female may pick a different partner on the following day and continue for up to a week.
Exhausting, one would think.
Most offspring are twins, although singletons and larger litters also occur. Like other euplerids, the young are born blind and helpless, in their case with white fur. They grow relatively slowly to begin with, not opening their eyes for two to three weeks, by which time their fur has reached a pearl-grey colour. They only begin to become independent at four to five months, finally leaving home as the following year's breeding season approaches.
The number of fossas is declining due to a reduction in forested land in Madagascar and active persecution by farmers concerned about their livestock. Population estimates can be as high as 8,000 but the current number is likely quite a lot lower than this, and the animal is officially listed as vulnerable, although not quite endangered.
However, it is now thought that there were once two species of fossa on Madagascar. The other was the "giant fossa" (Cryptoprocta spelea), identified from subfossil remains in 1902. For a long time, it was not clear whether this was really a distinct species, and the debate still is not entirely settled, but the current consensus is that it genuinely was. With no true fossil sites on the island that might give us more of a picture, we can only say that it must have survived the Ice Ages, and we don't even know when it went extinct, other than that it must have been before Europeans reached the island in 1500. Given the age of the only known specimens, it might even have been before the native humans reached the island around 550 BC, but, again, we don't know.
One of the reasons for regarding the giant fossa as a distinct species is that, based on the remains we have, it seems to have been a bit too large to simply represent big specimens of the living species. They were about 30% longer than the living animal, giving weight estimates of at least 17 kg (37 lbs). It would have preyed on large lemurs, as some crocodiles and large birds of prey do on the island today. While its remains have not been found where we would expect crocodiles to be common, suggesting some degree of justifiable fear on its part, it may have eaten sufficiently different prey to share its territory with the modern species, allowing the two to live alongside one another.
[Photo by Chad Teer, from Wikimedia Commons. Cladogram adapted from Yoder et al. 2003.]
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