Rabbit v. Rabbit

Mountain cottontail

There are literally thousands of different mammal species currently recognised. Most of them are small, and a great many of them can be quite hard to tell apart at first glance. Or even second glance, quite frankly.

The reason that they look so similar is that some body plans have been particularly effective, allowing the creatures possessing them to survive and multiply across the globe. But, if they're so similar, why doesn't one species, over the course of time, drive the other to extinction? Even if one isn't even just marginally better at doing whatever it does than the other (and, since they aren't literally identical, that's unlikely) then sheer bad luck ought to hit one or the other of them eventually.

A Side Order of Flies

Most species of bat eat insects, whether caught on the wing or plucked from leaves or other surfaces. But this is by no means true of all species, with the second most common diet - perhaps representing around a quarter of known species - being one based on fruit. Indeed, fruit-eating had evolved more than once among bats, with the giant fruit bats of the Old World not being especially close relatives of the much smaller ones found in the Americas.

While a great many herbivorous and omnivorous mammals include fruit as part of their diet, one estimate is that only around 10% of mammalian species rely on it as their primary source of food - most of them bats or primates. While that's not exactly a tiny proportion, given the number of mammalian herbivores in general, it isn't a huge one either. This, it has been suggested, is because, while fruit are great as a source of calories, they tend not to be high in protein, and a healthy animal needs a supply of both.

In the case of humans, eating plenty of fruit is well known to be a good thing, but trying to eat nothing but fruit for any extended period of time is likely to be a problem. Not only are you likely to suffer from lack of protein in your diet, but you will also suffer deficiencies in certain minerals and vitamins. That one of the main vitamins you would be short of is vitamin D, which promotes calcium absorption in the gut, is a particular problem, bearing in mind that there isn't much calcium in fruit to start with. For this reason, fruit-only diets can be a real problem for children, who need that calcium to grow their bones.

Too Many Monkeys

You can never have too many herbivores.

Well, actually, yes you can, but it doesn't happen very often in the wild. Populations of animals are naturally limited by a number of factors. For large predators, for example, the primary limiting factor is how many other animals there are around that they can eat. If the population of predators grows too large some of them will starve, and the population drops.

For small to medium herbivores, however, the limiting factor is rarely food supply, at least in the long term. (In the shorter term, something like a summer drought may be a different matter). For them, the main thing that prevents their population growing too large is that any excess population will get eaten by the predators.

This is, of course, an oversimplification. Large herbivores, such as elephants and rhinos, have relatively little to fear from predators, especially once they reach adulthood. Small predators, such as weasels, have just as much to fear from large ones as herbivores do. And there can be all kinds of other factors coming into play, especially where mankind has mucked up a long-standing ecosystem. But it's not entirely false either, and the upshot is that we rarely get to see what would happen if herbivores multiplied as fast as they'd obviously like to.

It's Tough Being a Herbivore

A horse skull

The majority of mammalian species are, to a greater or lesser degree, herbivorous. This shouldn't be too surprising: there is always more plant matter around than there is fresh meat, and it's a lot easier to catch. While the earliest mammals may well have fed on things like insects, a great many have since taken advantage of the widespread presence of plants to become herbivores.

Strictly speaking, most won't absolutely refuse to eat animal matter. For one thing, if you're munching down on leaves, you're going to eat the odd insect now and then, if only by accident. And most mammalian herbivores will, in reality, eat the occasional bit of - mostly invertebrate - flesh, or tuck into things like eggs if they find them lying around. But these are a minor part of their diet at best, allowing us to draw a useful, if hazy, line between them and true omnivores.

The largest order of mammals are the rodents. While many are genuinely omnivorous, most use their great gnawing teeth to feed on things like nuts and hard seeds. This illustrates one of the problems with herbivory: while your food might be easy to catch, it's not necessarily very easy to eat. Many wild plants are hard or thorny, and they're often not as packed with nutrients as meat is. As a result, herbivores usually spend a lot of their time feeding, and they require special adaptations to make the most of their meals.

Why Rabbits Aren't Rodents

Beaver skull

Over one third of all mammal species are rodents. By almost any reasonable measure, they are the most successful of all the mammalian orders. There are rodents everywhere, in every environment, and on every continent except Antarctica. There are even rodents native to Australia; they're the only placentals of which this is true, other than dingos and bats. They have even adapted, better than any other non-domesticated mammal, to life in urban environments. Indeed, the natural habitat of the house mouse is, pretty much, houses - presumably they used to live somewhere else at some point (river banks, probably), but not really any more.

However, that's going by the scientific meaning of the word 'rodent'. In everyday speech, I've heard the term applied to a even wider range of animals, including such things as shrews, but, most commonly, rabbits. Yet rabbits, along with hares and pikas, are not rodents. On close examination, it doesn't take much to demonstrate that, despite their size and shape, shrews aren't rodents, but rabbits... well, rabbits are pretty rodent-like. So what's the difference?

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.