Don't Sleep with Your Sister

Conservation of mammals (or other animals) isn't simply a matter of providing enough suitable habitat for them to live in. One other consideration is that that habitat should not be overly fragmented - and this can be a problem as we build roads or other transport networks that cross otherwise wild terrain. The problem with fragmented habitats is that, even if there is enough space and food to support a small population of the animal in question, that population cannot reach and interact with other populations. And this leads to inbreeding.

We have known that inbreeding is a bad thing in animals since... well, probably at least since we started domesticating livestock. In our own species, there's a natural revulsion against incest, something that's reflected in moral teachings that go back at least as far as the Old Testament, and similar codes in other cultures. Animals too, avoid inbreeding when they can, perhaps finding the scent marks left by genetically similar individuals to be unpleasant.

Small British Mammals: Water Shrews

Two of the three species of shrew found in Britain are very similar to one another in appearance and habits. This is hardly surprising, given how closely related they are. The odd one out, however, is not quite such a close relative, and is rather more distinctive.

This is the Eurasian water shrew (Neomys fodiens), which unsurprisingly, is simply called the "water shrew" in Britain. Despite its differences, even at first glance, it's pretty obvious that it is a shrew: it's a small, long-tailed animal with short fur, tiny ears, small eyes, and a narrow, pointed, snout filled with sharp teeth. However, by the standards of shrews, it's unusually large. Fully grown adults can reach as much as 10cm (4 inches) in length, and weigh up to 25g (0.9 oz.), closer in size to a typical mouse than to other native shrews.

Bat Poo and Fig Trees

Seba's short-tailed bat (Carollia perspicillata)
feeding on wild pepper

If the presence of plenty of carnivores is generally bad news for herbivores, it may seem that the presence of plenty of herbivores is bad news for plants. Clearly, such problems as over-grazing do exist, but the reality can often be more complex. Fruit are a particular case in point.

Many fruits are tasty specifically to encourage herbivores to eat them, containing highly resistant seeds that pass through the herbivore's digestive tract, land in a nice pile of manure, and germinate to create more plants in future. It has been estimated that, in most wild forest environments, anything from 45% to 90% of tree species bear edible fruit of this kind. (Of course, a number of the fruits we see in supermarkets are even tastier, because we've bred them that way, with the banana being perhaps the most extreme example. But it's not as if wild apples and oranges, for example, don't exist and aren't attractive to animals).

Not all herbivores eat these kinds of fruit, as opposed to other things that botanists would call "fruit", such as grass seeds. Primates are one of the more obvious examples of mammals that do, and it's theorised that our unusually good colour vision arose in part so that we could easily tell which ones were ripe. But another group of mammals that eat a strongly fruit-based diet are, unsurprisingly, the fruit bats.

How Sloths Turned Upside Down

One of the things that sloths are noted for is spending a lot of time hanging upside down. This isn't, of course, something they literally spend their entire lives doing; they come down out of the trees to defecate, and they do sometimes travel over the ground - or swim across rivers - to move from one tree to another. They have also been observed, especially in zoos, casually sitting about in what we'd consider a fairly normal posture. But even so, being upside down is how they spend most of their time.

This is obviously pretty odd, from the perspective of a mammal (or, indeed, most other animals). You might think that what must have happened is that, at some point in the distant past, some ancestral tree sloth first headed up into the branches, decided to do so by hanging from them rather than by the more obvious method favoured by (say) primates, and that all of its descendants simply happened to do the same thing. In other words, it's a one-off quirk of evolution.

Except that can't be right.

The Smallest British Mammals: Shrews

Common shrew

For some reason that I no longer recall, a picture of a shrew cropped up while I was talking with a friend on the internet. "That's a mouse," she insisted. "No," I said, "it's a shrew." "Mouse!"

It was only later that I remembered that the German word for "shrew" is spitzmaus (literally "sharp/pointed mouse") and, since my friend was Austrian, she wasn't necessarily entirely wrong. From a certain perspective, perhaps to German-speakers, all these small, furry, long-tailed things are mäusen.

And it's certainly true that shrews do look, superficially, quite like mice. They tend to be smaller, with narrower snouts, and very small ears and eyes, but there is something quite mouse-like about them. Nonetheless, shrews (unlike, say, voles) are not rodents. In fact, as placental mammals go, they aren't even particularly related to rodents - humans are more closely related to mice than shrews are.

The Curiosity of Lemmings

The reason that there are so many different animal species in the world is that they all have different living requirements; it's no secret that a tiger has different needs from a hedgehog. In most land-based habitats, we can look around and see a fair variety of mammal species and it's obvious that, while they may compete in other ways (by eating each other, for example), they can all survive because they are using their chosen habitat in different ways.

But, in some cases, it's quite hard to find more than a handful of species - especially if we're going to restrict ourselves to mammals, and ignore all the birds, insects, and so on. This is often true of city centres, for example, where you may not have much aside from rats, mice, and urban foxes. But in the wild, however, we are typically talking about particularly marginal habitats, such as deserts, high mountains and other desolate wastes.

A Hole of Your Own

Many animals dig burrows for shelter, whether from the weather or from predators. In some cases these are complex or extensive burrow systems, such as we find with rabbits or gophers, and some animals, such as moles, try not to leave their burrows at all, adapting to a subterranean life. Most are much simpler than this, a basic hole in the ground in which the animal can rest securely at night - or during the day, as the case may be.

At the opposite extreme to the specialised diggers, however, are those animals that don't dig burrows at all, but still find it useful to seek shelter in this manner. These are creatures that will either use natural cavities or take over an abandoned burrow originally dug by something else. If they can't find one, it's usually not a disaster, although it may make life a little uncomfortable. But that's not necessarily true when it comes to time to breed.