Viverrids: Rise and Fall of a Wastebasket

The carnivorous mammals have been recognised as a taxonomic order since the official dawn of biological classification in 1758. In that first publication, Systema Naturae, Carl Linnaeus listed 36 species. The concept of "families" was a later innovation, but Linnaeus used the rank of "genus" much as we would use families today, and, in the case of what we now call the carnivorans, there were six.

These were the cats, dogs, bears, weasels, seals, and a sixth group that he called (in Latin) "ferrets".

However, this last group did not include the animals that we call ferrets in English; he placed those (correctly) with the weasels. It instead included five species of small bitey animals that didn't really fit elsewhere, including mongooses and skunks. In 1821, John Edward Gray promoted this group to a family, the Viverridae. Because of the rules being established at the time for how to name families of animals, this literally translates from the Latin as "ferret-like", despite it not including the actual ferrets. Linnaeus' name for the group had long since stuck.

Gray's initial description of the family included just four species, fewer than the listing made over six decades earlier. That was partly because he was only listing one example species from each genus within the family, but also because of the removal of the skunks, which had since been placed with the weasel family. That would have cut it down to three, but Gray had also added the meerkat, which had first been described in 1776.

Gray provided a description of what he believed set the viverrids apart from other carnivorous mammals, and this broadly remained in place for over 150 years. The group blossomed as our knowledge grew; by the mid-20th century, we had named dozens of species. 

There was a general belief at the time that the group represented a very primitive form of carnivoran, relatively unchanged descendants of the original stock that led to the cats and hyenas. (Hyenas may seem odd here, but the structure of the bones in the ear placed them on that side of the family tree, not with the dogs). There was debate as to whether mongooses and meerkats were slightly more advanced than the others, and so belonged in a family of their own, but otherwise, we had a general consensus.

In the late 20th and early 21st centuries, molecular biology, as it so often did, came along and messed all that up. 

We now know that mongooses and meerkats are more closely related to hyenas than they are to the true viverrids. As such, they indisputably get their own family. That wasn't really a surprise, with the proposal to split them off going back to the 19th century, but it also became clear that many of the other animals we had been placing with the viverrids belonged elsewhere. All we had had was a collection of vaguely cat-like animals that we didn't have enough information to classify properly. The viverrids might as well have been a wastebasket.

Some of the viverrid species turned out to belong with the mongooses, despite visibly not being mongooses themselves. These eventually got their own family, representing the fact that they had evolved in isolation from their closest kin. Others turned out to be closely related to the cats, or to be the last representatives of very ancient branches that weren't especially related to anything. All of these had to be moved elsewhere.

But still, Gray had described a family of animals, based on Linnaeus' earlier description of the genus Viverra. Those animals still existed, and their name hadn't changed. Indeed, the molecular studies showed that they formed a distinct branch within the carnivoran family tree and quite an old one at that. The family had shrunk, but not vanished.

Today, we have to define the Viverridae partly on genetic grounds. There are no absolutely unique anatomical features that set them apart from everything else - if there were, we would probably not have included those other animals with them in the first place. But, nonetheless, we can make some general points.

Viverrids are typically smallish animals, no larger than a cat. They have slender bodies, enhancing the resemblance to ferrets, although their limbs are not quite so short. Unlike members of the weasel family, they typically have long, pointed snouts, with small ears and a long tail. They have five toes on each foot, unlike cats, which are missing the big toe on the hind feet. Although Gray originally said that they walk on tiptoe like a cat or dog, this turns out not to be true of all the species discovered since - some have flatter feet with a good grip, suitable for climbing trees.

Viverrids belong to the cat side of the carnivoran family tree, and have a number of features in common with their feline relatives. For example, they have long whiskers, and most have retractable claws, although this has not been fully evaluated for every species, so there may be exceptions. They are typically nocturnal and have correspondingly large eyes, their night vision enhanced by having the same reflective, eyeshine-creating layer on the inside that cats do.

Having a longer snout, they have many more teeth than cats (or humans), with most species having 40. These include three small clipping incisors at the front of each side of each jaw, just like cats, followed by large stabbing canine teeth. Behind each canine are four premolars, with multiple points for shearing flesh, and then two or three molars. Aside from the carnassial teeth - the main flesh-cutting teeth that typify carnivoran mammals in general - the molars are small and shrink further towards the rear. In many species, the third molar, which would be the wisdom tooth in humans, is missing altogether.

One feature that is unique to the true viverrids, although it is not present in every species, is the perineal scent gland. This is a paired structure opening out onto a bare patch of skin between the anus and the genitals, and is used by both sexes to leave scent marks for communication. In some species, it is a very simple structure, just a simple sac lined with fluid-secreting cells, but in others, it can be large, complex, and have lip-like folds around its entrance. The scent it produces is musky, strong, and long-lasting, and is a key feature in the economic value of these animals since it is a key ingredient in many perfumes. These days, we tend to harvest the scent from farmed animals, rather than simply killing them and removing the gland, and, in any event, vegetable-based and synthetic alternatives are becoming more common.

Viverrids also typically have anal scent glands, as many other carnivorans do. These are smaller and used to add scent to their droppings, rather than for smearing onto branches or other surfaces, as the perineal secretions are.

So what are we actually talking about here? Because they no longer include the mongooses and meerkats, most viverrids are fairly obscure to those of us in the West. While there is one species that lives in southern Europe, all the others are restricted to either Africa or southern Asia. Compared with tigers, wildebeest, and so on, they are small, secretive, and almost entirely restricted to forests where they are difficult to see. It doesn't help that they are nocturnal. 

So we just don't see them much, even on wildlife documentaries. Scientific knowledge is also often lacking, beyond which species live where, and what they look like. Even so, they aren't all a complete mystery, with the civets being perhaps the most familiar. In fact, nearly half of viverrid species have the word "civet" in their common name, although they are distributed among three of the four subfamilies, and thus aren't a real biological group of animals.

It therefore follows that many so-called civets are actually something else that we don't have a better word for in regular English. Next time, however, I am going to be looking at the real ones, including the original "perfumed ferret" that Linnaeus described all the way back in 1758.

[Photo by Brendan Herbert, from Wikimedia Commons. Cladogram adapted from Hassanin et al. 2021.]