Sunday, 6 January 2013

Caprines: Goats, Horns, and Antlers

Over the last year, I completed a survey of the weasel family, the largest, and most diverse, family of carnivoran mammals. When it comes to the big herbivores, the largest and most diverse family is the Bovidae - the cattle family. This includes a wide range of animals, from the true bovines (that is, bison, yak, and so forth) to slender gazelles. In fact, there are well over a hundred species in the cattle family, and covering them in the same way that I covered the weasels would take me a couple of years or so.

So, instead, I am going to look at just one subfamily within the group. After the gazelles, it's the second largest cattle subfamily, with the bovines down in third place. These, as you'll have guessed from the title of the post, are the goats.

Or, more accurately, "caprines". Just as many members of the weasel family aren't literally weasels, many members of the goat subfamily aren't goats in the usual sense of the word. If you've never really looked at mammalian taxonomy, there will probably be at least a couple of species I'm going to cover over the coming months that will surprise you, and likely a few you've never heard of. One collective term for all of these animals is "goat-antelopes", but it's hardly common usage, and I don't actually find it very useful - not least because most of them don't look much like the regular image of an antelope, either. So I'll stick with the technical term of "caprines", or sometimes just call them goats, and you'll have to accept that I mean that in an unusually broad sense.

But let's begin by putting them in context. Goats are members of the cattle family. But what does that mean?

For all their diversity, there are a number of features that all members of the family have in common. For instance, they are all herbivores, they have cloven hooves, and four-chambered stomachs. Those features, however, are by no means unique. Many close relatives of the cattle family have the same attributes - most notably, perhaps, the deer family. So what can we find that's more distinctive?

Buffalo skull
One thing that's often used in telling different mammal families apart is the exact shape and number of their teeth. These are handy, not just because mammalian teeth are very variable, but because they preserve well, making them especially useful for trying to work out what a fossil specimen actually is. Fully grown cattle, as it turns out, have 30 or 32 teeth, and they have a pattern that's quite different from that in humans (who also have 32 teeth... just not the same ones).

At the front of the mouth are the incisors, the clipping teeth that, in cattle, are used to cut off bits of plant to eat. There are three of these in each side of the lower jaw, and exact shape of them, and of the front of the jaw, can help us tell whether an animal is a grazer or a browser. However, if you look closely at the image of the skull to the right you might be able to see that it appears to have not three, but four, pairs of incisors. The fourth one is actually the canine tooth, normally used to bite into tough meat. With no need to do that, but plenty of need to crop plants, cattle evolved so that the lower canine tooth became, in effect, an extra incisor.

You'll also notice straight away that there are no incisors, or canines, in the upper jaw at all. This is a feature of all members of the cattle family; the teeth are replaced with a hard pad that the animal can use like a chopping board as it cuts off leaves. Further back, there's a long gap without any teeth, allowing the animal to hold food in place with its cheeks as it chews it up.

Then we get to the cheek teeth, which are specially adapted to chewing up tough vegetation - and quite different, therefore, from the chopping, almost knife-like, cheek teeth of carnivores. There are usually three premolars, and three molars, on each side of each jaw, and they're all pretty much the same shape. About the only difference is that they tend to become slightly larger the further you go back in the jaw, and in some species, the first premolar in the upper jaw is missing altogether, leaving them with just 30 teeth. Like many other herbivores, the cheek teeth of cattle have sharp enamel ridges, with an outline shaped like a crescent moon.

All of this will certainly enable a zoologist to tell the difference between a goat and, say, a pig. But it turns out that the teeth of deer have almost exactly the same pattern. Compare the buffalo skull above with the picture of the moose skull in the post I linked to in the last paragraph, for instance. Given that deer and cattle have a lot of other physical similarities, then, aside from genetic evidence that they are different (of which there is plenty, incidentally) is there anything obvious that tells them apart? Or, to put it another way, why do goats belong to the cattle family, not the deer family?

To put it simply: cattle have horns, deer have antlers.

That's not a semantic point: horns and antlers are quite different things, and goats very clearly have horns. Of course, both horns and antlers are head ornamentation, and they share a similar purpose. They can be used for display, and, if that fails, for ritual combat in order to decide something important, such as access to mates. While there are plenty of exceptions, they're often found only on the male of the species, because they're the ones that do the most fighting, and when females do have them, they tend to be smaller. Come to that, by acting as giant radiator fins, they also help to keep the head cool, although it's hard to believe that's a major reason for having them.

How caprines relate to the rest of the cattle family
Beyond that, though, antlers and horns really aren't all that similar, and, so far as we can tell, they evolved quite separately. One immediately obvious difference is that antlers are usually branched, while horns are never branched. (I'm deliberately ignoring the pronghorn "antelope" here; despite the name, it isn't a member of the cattle family, and its horns are weird in more ways than just having a short spike on the back). Of course, that's not an infallible guide, since some small species of deer do have tiny un-branched antlers, never mind that not all animals are fully-grown adults. But, even so, there's no way you'd mistake the antlers of a large stag for the horns of a goat or buffalo.

But there's more to it than just the shape. Antlers are made of bone, and, when they first grow, they are covered with a layer of soft skin called "velvet". Once the antlers are fully grown, the velvet dies and the animal rubs it off, leaving nothing but dead bone behind. At the end of the year, the antler falls off, and the stag grows a new, larger, set the following spring.

Horns, in contrast, are permanent structures. Once they start growing, you have them for life, barring accident, and you never get a second set. That alone is a pretty significant difference from the ever re-growing antlers, and it means that the animal doesn't have to divert a huge amount of calories into growing the things again every year. But they're also structurally different. The horns of cattle have a core made of bone, but that bone is encased in a sheath of... well... horn.

Horn (the substance) is made of keratin, and is therefore essentially the same material as hooves, claws, or fingernails. It's often ridged, and coloured, giving horns a distinctive appearance that antlers lack. In many cases, the horny sheath is much larger than the bony core on the inside, making horns spectacularly curved or pointed. This is helped further by the fact that, unlike antlers, the bone in horns is still alive, with a blood supply, which means that it can continue to grow throughout the animal's life.

Horns are, strictly speaking, not unique to the cattle family, or even to mammals. There are such things as horned lizards, for instance, and there isn't really another word besides "horn" for the thing on a rhino's nose. Similarly, there are some members of the cattle family - and not just females - that don't have horns. For that matter, there are some deer that don't have antlers. But nonetheless, if it does have true, permanent, horns on the top of the head, and it's a living species of mammal, it has to belong to the cattle family.

And goats, of course, do. That's how we knew, even before we had the genetic and biochemical data that we now possess, that goats were more closely related to cows than they were to deer. At the same time, they are obviously different in some respects. Caprines, essentially, evolved to exploit marginal habitats, surviving on relatively low quality food, and often using rugged terrain to hide from predators. Many of them today live in mountains, but they were more widespread during the Ice Ages, allowing them to spread across the northern hemisphere, where they are now found in Europe, Asia, North America, and northern Africa. Unsuited to the lush tropics, however, they never reached southern Africa, South America, or Australia - unless we ourselves brought them.

There are over thirty species of caprine, and next month I'll begin by looking at some of those species in more detail. And I'll start with one that is, at least to many of us, probably quite familiar...

[Pictures by "Ladydragonflycc" and "Jebulon", from Wikimedia Commons. Cladogram adapted from Hernandez-Fernandez & Vrba, 2004]

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