Saturday 30 January 2016

Bovids and Bovines - What Makes a Cow?

The cattle family, Bovidae, is the largest and most diverse of the various families of big herbivorous mammals, with only the deer family coming close to it in number of species. The majority of its members are antelopes of one kind or another, and it also includes goats and sheep, as well, of course, as the cattle themselves. Given the number of species within it, and the fact that a bison, say, has a number of obvious differences from a gazelle, there have been various attempts down the years to group its members into subfamilies or other smaller groupings.

However, opinions have differed as to quite how this should be done. How many significant sub-groups are there, which of them are deserving of the rank of "subfamily", and which animals go with which? Even with modern techniques for analysing genetics and coming up with computer-generated metrics of relatedness, the answers to these questions are not entirely clear. Partly that's because the first two questions at least are essentially arbitrary, and partly because we haven't really got enough samples from some of the more obscure species to really nail the family tree down. There could be anything up to sixteen subfamilies, or just two, or somewhere in between.

But one thing that everyone has agreed on for over a hundred years, no matter how they subdivide the family, is that one of those subfamilies should be the Bovinae. As the name implies, this is the subgroup that includes domestic cattle, and other species particularly closely related to them. Not only that, but, at least since G. G. Simpson's five-subfamily scheme of 1945, we've also agreed on pretty much exactly which species belong within it.

For the sake of convenience, I am going to refer to these animals collectively as "bovines". This word can, of course, have other meanings. It could equally well be used to just refer to those members of the Bovinae that are particularly close to the actual cattle, just to cattle themselves, or, in general parlance, to anything that looks kind of like a cow. But, for simplicity, and since I'm going to need a word to refer to the subfamily as a whole, that's how I'm going to use it here.

So what is it that's so distinctive about the bovines? Modern genetic studies show us that the bovines are the oldest branch on the cattle family tree, treading a separate evolutionary path before the ancestors of goats separated from impalas, or reedbucks from gazelles. We have good grounds to suppose that they really are an evolutionary group, and one that's distinct from everything else. Even if we bundle absolutely all the others into one big subfamily, the bovines stand apart.

But that's not obvious from just looking at them. The cattle family as a whole is distinguished, broadly speaking, by being composed of cloven-hoofed ruminants, most of which have true horns, rather than antlers. The collective term there is "bovid", but what distinguishes bovines from the other bovids?

Perhaps the most obvious feature is that they do tend to be big. Not as large, of course, as elephants, rhinos, or hippos, but, giraffes aside, bigger than pretty much anything else with cloven hooves. And, unlike giraffes, that size comes in the form of a squat, heavy build, with powerful muscles and a short, blunt snout ideal for scooping up swathes of grass.

But there are is a problem with using this as an identifying feature. If you bulk up an antelope enough, adding muscles and body mass, it's eventually going to look somewhat like a cow. Take, for example, wildebeest. These are really big antelopes, and they do look somewhat cow-like, but they aren't, in terms of their evolutionary history, "bovines". Musk oxen are, perhaps, an even clearer example, looking quite obviously cow-like, despite being more accurately described as a sort of giant goat.

A modern, ten-subfamily, scheme
for classifying bovids
Not only that, but it does cut both ways. Slim a cow down and give it the ability to run fast, and you basically have an antelope. And some members of the bovine subfamily are antelopes, by any reasonable definition, and can get quite small. So, while it's true that bovines tend to be big, the operative word is "tend"; they're not all that size, and it isn't a unique feature anyway.

In fact, while bovines share a number of anatomical features, it's difficult to find one that's true of absolutely all of them, and never of anything else. Perhaps the closest you'll get is in the structure of the horns. The horns of bovines grow in a sort of curving, spiral pattern, although how obvious this is depends a lot on which animal you're looking at. In the most cow-like forms, for example, the horns grow out sideways, curving in an arc, and they're so large that the animal can only carry them because they're partly hollow, with empty sinuses stretching into them from the skull.

In other bovines, the horns may be so short that the spiral pattern is difficult to see, while, in others, it forms a tight corkscrew that you really can't miss. Inside the horn, the bony core bears a pointed keel along one edge. Often this is all but invisible, even in a skeleton, but in other species, it's prominent enough to be readily apparent while the animal is alive. Another oddity is that many antelopes have rings around the horns, giving them a ribbed appearance, while those of bovines are always smooth.

There's also the matter of the sac-like scent glands. These glands are found in various parts of the bovid body, depending on the species; typically in front of the eyes, in the groin region, or in various positions on the limbs and feet. They are used to scent-mark territory, singal sexual status, and the like, informing other animals of their status, often by depositing a pungent-smelling goo on twigs or visible outcrops. Every species in the cattle family has at least one pair of them somewhere on their bodies... except for bovines, in which they're completely missing in all but two species.

Which is why bulls don't smell like billy-goats.

Which isn't to say that scent is irrelevant to them, or that they don't have scent glands at all. It's merely the large, fluid-secreting glands that they lack, and, just as human beings secrete scent from their armpits and certain other bits of their anatomy, so do bovines. But, also as with humans, it's relatively normal skin glands that do the scenting, not highly specialised structures.

For the most part, bovines are forest-dwelling animals, although a few of them do, of course, live out on the grassy plains or in other, relatively desolate, habitats. In the wild they are (or, at least were historically) found across much of the Northern Hemisphere and throughout most of the more habitable bits of Africa. They never reached South America, where the role of "large herbivorous quadruped" is really only taken by deer.

Today, of course, there are a great many bovines in South America, because we humans have brought them. The group includes a number of domesticated species, and one of them in particular has been spread around the world as a beef and dairy animal. In total, however, there are something like thirty species, although the exact number may depend on which expert you talk to. Over the coming year, I'm going to try and look at all of them.

I'm going to begin close to home, with the one animal that nobody can possibly dispute is bovine: the domestic cow.

[Photo by Keith Weller of the US Department of Agriculture, in the public domain. Cladogram adapted from Hernandez-Fernandez & Vrba, 2004]

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