Sunday 18 December 2011

Grazers and Browsers - and how to tell them apart without watching

Wildebeest are grazers - note the squarish muzzle
One advantage of studying the fossils of prehistoric mammals, as opposed to dinosaurs, is that mammals are still around today, while non-avian dinosaurs aren't. That gives us the ability to compare fossil species with living ones, and be fairly confident that our comparisons make sense. That's not to say, of course, that we can't infer quite a lot from the shape and structure of dinosaur bones, and work out details of their lifestyle and habits. But there's nothing much like non-avian dinosaurs around today, so there will inevitably be some guesswork involved when we do - educated guesswork, to be sure, but guesswork none the less.

Although the same can be said of fossil mammals - especially the stranger ones - in many cases, we can be more confident that our educated guesses are likely to be accurate. For example, sabre-tooth cats were, well... cats. So we can look at, for example, the proportions of their limbs, compare them with living cats, and deduce whether they were more like, say, jaguars, than they were like  leopards. Because leopards, jaguars, and sabre-tooths are, in many respects, quite similar, it's pretty likely that inferences drawn from the first two will apply to the third, unless there's some good reason to suppose otherwise. We know what cats are like, and sabre-tooths were cats, so that tells us a lot.

And what about herbivores? Herbivory includes a range of different diets, such as animals that feed mainly on seeds, or fruit. But large mammalian herbivores tend to have two possible feeding strategies: grazing and browsing. The best way to tell the two apart would be by examining their dung, and, failing that, the structure of their digestive systems could well be helpful. Neither, of course, are possible, if all you have is a fossil skeleton, but, fortunately, there are other clues we can examine.

Grazing animals feed primarily on grass. Grass can be difficult to digest, which is why some animals have four-chambered stomachs, and is also wearing on the teeth, not just because grass itself is relatively tough, but because it's hard to eat it off the ground without getting at least some grit or soil in with it. But it does have the advantage of being fairly plentiful, and the fact that it's all much of a muchness - one mouthful of grass is more or less like any other. Grazing animals, such as cows and sheep, therefore tend to have relatively broad muzzles, so that they can chomp up large clumps of grass in one go.

A browsing animal, on the other hand, feeds on leaves, buds, and the like, be they from shrubs, bushes, trees, or whatever. Such food is easier to digest, so that the digestive system does not need to be quite so big (although still, generally speaking, larger and more complex than that of carnivores). However, while suitable browse is common enough in the right sort of environment, it isn't distributed in quite the same way. Leaves and so on are found in bushes and trees, not so easy to get at as grass and, crucially, mixed in with things like twigs.

The problem with twigs is that, while they certainly contain nutritious material, they also contain a lot of wood, and wood is completely indigestible. At the chemical level, wood is composed of a complex aromatic polymer, called lignin, that is neither carbohydrate, nor protein, nor fat, and, apart from termites, there are pretty much no members of the animal kingdom that can digest it. So, from a nutritional standpoint, any wood that you eat is wasted effort. Of course, some animals will eat smaller twigs and so on, to get at the sap, or other nutritious material, inside, but, generally speaking, browsing animals want to eat as little of the stuff as possible.

Which means that they don't just want to scoop up large quantities of plant matter, like grazers do - they want to be picky, aiming for the tasty looking leaves, buds and other green bits that don't have wood in them. All of which means they tend to have relatively slender snouts, that they can poke into bushes with fine precision. So, a deer, for example, tends to have a narrower head than, say, a bison.

We can extend that principle - an animal with a wide snout is more likely to be a grazer than a browser - to fossil animals in general, such as dinosaurs, but it's perhaps easier to do so when there is a direct  analogue alive today, and we can make a direct comparison. There are, however, a couple of complications. Firstly, many large herbivores are both grazers and browsers, so that there are no hard and fast lines between the two. Secondly, in science we want to be precise, so we need a numerical measure of the shape of the jaw that helps us decide whether a particular fossil belonged to one or the other (or both).

There have been a number of proposals made as to how this can best be done. Most of these rely on the shape of the lower jaw, which is apparently more likely to be preserved intact than the upper. The lower jaw in large herbivorous mammals generally includes a row of incisor teeth at the front, for clipping off plant material, followed by a long, toothless gap, and then the chewing teeth at the rear, behind the snout proper, and in a part of the head more shaped by the attachments for the jaw muscles. Methods for quantifying the shape of the forward part of the jaw - the bit that's relevant here - include measuring the width just behind the row of incisors, how curved that row is, and even how the different incisors compare to each other.

A new method, proposed by Danielle Fraser and Jessica Theodore of the University of Calgary, instead compares the width of the jaw with its depth. By 'depth', I'm referring to the distance from the tip to what would be the base of the chin on a human, but which is much further back, on the underside of the snout, in the sort of animals they're looking at. And, of course, that's where the existence of close living relatives comes in useful, because we can look specifically at ruminants, which we know are broadly similar in all other respects, and ignore more distantly related herbivores, such as horses or kangaroos, and have a reasonable chance that all other factors are going to be equal.

What this means is that we can collect skulls from a range of living animals, and see how good the different measures are at predicting what they eat. It may seem logical that browsers have longer, narrower snouts, but is it really true, and if so, is it true enough to allow us to make a reasonable guess as to the diet of an animal we know only from the skeleton? Or are there other factors we need to take into account? For instance, a bison is a lot larger than a deer, and that certainly affects other aspects of their body shape, so might it affect the shape of the head, too?

Well, apparently not. Indeed, the authors conclude that, in their survey of 34 species of ruminant, the ratio of muzzle length to width was the best predictor of how the animal ate. Most of the older methods, while not useless, gave only a vague indication, and made the wrong prediction almost as often as the correct one. The only exception was a method that evaluates the shape, not of the lower jaw, but the upper one - and even that was less useful than their new method. The upper jaw of ruminants has no teeth at the front, just a tough pad against which the incisors chomp to crop food, which may make the shape of it less important than the tooth-bearing lower jaw. But, since it obviously can't be a completely different shape from the lower jaw, there would have to be some correlation even by default.

So does this method allow us, with certainty, to say that any newly discovered fossil ruminant belonged to a grazer or a browser? Hardly, even if it is the best method we have found so far. That's because of the wide range of animals that are somewhere in between, and the fact that not everything follows the general trend. For example, it turns out that hartebeest (a kind of antelope) have a narrower muzzle than you would expect, bearing in mind that they are, like cattle, grazers. Perhaps there's something different about the way they graze, but it seems as likely to me that there's some other confounding factor we haven't thought of.

What we can say, however, is that, if we have a lower jaw that shows a relatively extreme shape, similar to a wildebeest (grazer) or moose (browser), for example, that we can be fairly sure what it isn't. Given that many animals are somewhere in between, with jaw shapes correspondingly varied, we can never rule out the possibility that an animal isn't a bit of both, although perhaps with a preference in one particular direction. But, given the muddiness of real-world nature, that isn't all that bad.

So we can say something about how an animal ate, without ever having to watch it feed, or examine its dung, just from the shape of its jaws and teeth.

[Picture from Wikimedia Commons]

Synapsida will be taking a break over Christmas, but will be back in the New Year.

1 comment:

  1. Leaves are grass aren't they? If so by your definition a grazer is also a browser. And why not? It sounds reasonable to me.

    Richard Mullins