|The common, European, weasel, Mustela nivalis|
The weasel family is also, arguably, the most diverse of the carnivoran families. Nobody would doubt that a tiger is a kind of cat, and its hardly surprising to learn that foxes are members of the dog family, but the majority of members of the weasel family are not animals that, in everyday speech, we would call weasels. True, ferrets, for example, do look rather like out-sized weasels, but its probably less obvious that the family includes such animals as badgers, otters, and wolverines.
But what, exactly is a family of animals? The latest edition of Mammal Species of the World lists 144 families of mammal, 121 of which are placentals, but such a list can never be truly definitive. The modern rules for defining any natural group of animals, whether it be a family, subfamily, order, or anything else, is that all the species in that group must be more closely related to each other than to anything outside the group. This means that a family, like any other meaningful group of species, includes a single common ancestor and all of its living descendants.
This is why otters, for example, have to be included in the weasel family. They are actually quite closely related to the weasels themselves, which means that to give them their own family we would have to split the existing weasel family into seven or eight different groups, and even to do that, we would have to put the badger-like animals into at least four different families. Far easier, then, to keep it as it is.
Still, there's no inherent reason why you couldn't do this if you really wanted. There is no scientific definition of what constitutes a 'family', and what makes it different from, say, a mere subfamily of something else. Its just a term of convenience, something that feels about right, but that is always subject to change, especially if we learn more about how the animals in the family relate to each other, or to species outside the family.
Considering that there is no clear definition of what a family is, the rules on how to name them are quite specific. Every named family must have a type genus. A 'genus' is a group of very closely related species, and it forms the first part of every species name. For example, lions (Panthera leo), tigers (Panthera tigris) and jaguars (Panthera onca), among others, all belong to the genus Panthera. The type genus is the one that gets to define what the family is; no matter what changes are made to the membership of the family, the type genus must always remain a part of it.
This did, in fact, happen with the weasel family. Skunks used to be considered to belong to the family, until it turned out that weasels were more closely related to raccoons than they were to skunks. Under the rule that all the members of a family have to be more closely related to each other than to anything else, and since nobody was going to abolish the raccoon family, skunks had to be given a new family, all to themselves. The type genus for the weasel family is Mustela, the genus that includes the actual weasels, so it was the weasel family that got to keep its existing scientific name, and the skunk family that had to be given a new one.
Although the rules are a little different in botany, in zoology, the name of any family is derived by taking the name of the type genus, and adding "-idae" to the ending. Thus, the correct scientific name for the weasel family is Mustelidae, and its members are technically called mustelids. For subfamilies, the rule is the same, except that the ending is "-inae". This means that, if the family has any subfamilies at all, one of them is going to have a very similar name. In this case, the Mustelinae, or mustelines, is that branch of the Mustelidae that includes the true weasels and not, say, the otters.
Given the great variety among the mustelids, what do they have in common? They are all small to medium-sized carnivorous animals, with relatively short legs; males are generally larger than females. Most have an elongated, slender body, although badgers, for example, have a heavier, stockier build. They have five toes on each foot, usually ending in curved, non-retractile claws, although these can vary from the strong digging claws of badgers to the minute and useless claws of some types of otter.
One of the most useful features for identifying different kinds of mammal from their skeleton is the shape and number of the teeth. This is particularly useful in fossils, where teeth tend to preserve well, and the great variety of different forms that teeth can take in mammals means that they can be highly distinctive in a way that those of, for example, reptiles, are not. Mustelids have the typical pattern of teeth found in all carnivorans, with large, sharp canine teeth, and powerful meat-shearing carnassials towards the back of the jaw.
However, even by the standards of carnivorans, mustelids tend to have a highly carnivorous diet; they touch vegetable matter less than dogs, let alone bears or raccoons. This means that they have less need of the cheek teeth that normally grind up tough food, and can have a more specialised dentition. The end result is that mustelids have less teeth than most other land-based carnivorans - with the exception of cats, which have a similarly high-meat diet. Because they don't need to fit so many teeth into their jaws, the snouts of mustelids are usually short (again, like cats), giving many of them a somewhat "cute" appearance. Indeed, the badger, with its relatively long snout, is also the mustelid that has the most teeth.
Carnivorans tend to have four premolar teeth on each side of the upper and lower jaw, where mustelids often only have three. More distinctive, perhaps, is that, having no need to chew up plant material, there are very few teeth behind the great flesh-shearing carnassials. There is only a single molar behind the carnassial in the upper jaw, which is itself somewhat adapted for tearing meat, while the only tooth behind the carnassial in the lower jaw is vestigial, no more than a small peg, and is missing altogether in the honey badger.
Most mustelids have anal scent glands, which can discharge unpleasant-smelling musk when they are alarmed; it was this feature, among others, that led scientists to initially assume a closer relationship with skunks than later turned out to be the case. They also have particularly enthusiastic sex lives, copulating for literally hours on end, apparently to ensure that the female, aroused by all the attention, ovulates while she is actually mating - something that virtually guarantees conception.
Beyond that, mustelids display the great variety that has led to us giving them so many different common names. They inhabit every continent, except Australia and Antarctica, and are adapted to almost every habitat imaginable. There are mustelids in the snowfields of the Arctic tundra, in forests, grasslands, tropical jungles, and all but the hottest of deserts. Many have adapted to living in rivers and lakes, or even in the open waters of the sea.
Over the coming months, in between my more usual posts, I intend to take a closer look at all of the different kinds of mustelid, taking a wander through the interesting, yet often-overlooked, world of weasels.
[Image from Wikimedia Commons. Cladogram adapted from Koepfli et al. 2008.]