How Not to be Eaten

This picture is far less cute if you're a mouse

Animals, on the whole, want to avoid being eaten. There are a number of ways of achieving this, including being well armed or armoured, or just being too large for most predators to bother with. For relatively small and inoffensive animals, however, we're left with being difficult to find, and being good at escaping when you are detected.

But even this can cover a range of different strategies, or combinations of strategies, not least because there are a lot of different predators out there, and what may be good for escaping from one may be less effective against another. Moreover, if every animal evolved to avoid, say, the open grasslands in favour of forest cover, then there'd be an awful lot of potential food out there going begging. Inevitably, something would evolve to take that risk. There's always a pay-off involved.

It's as a result of things like this that there are so many species of small mammal, with subtle differences between them that are not always obvious to the casual observer. To understand the predator-prey dynamics of a particular habitat, we really need to look at a whole range of creatures. So many, perhaps, that it's beyond our ability to easily analyse in its entirety, but we can at least take a stab at a manageable group of species in one particular place, trying to see what makes them different, and why what works for some does not always work for another.

The cerrado is the savannah habitat of South America, lying broadly between the Amazon Forest in the northwest and the coastal Atlantic Forest in the southeast. Like the African savannah, it is a fairly rich habitat, with strongly seasonal rainfall producing a mix of woodland and grassland, much of it open, but with surprisingly dense tree cover in places. Taken as a whole, it is home to over well a hundred of species of mammal, so that when a couple of Brazilian researchers decided to conduct a study of how the local rodents try to avoid being eaten, they had to narrow it down a bit.

So, first of all, figure out what's around. To do this, they created a number of pitfall traps - essentially open buckets buried in the ground - to see what fell into them. This may sound a bit crude, but it has been shown to be more effective for capturing small mammals than at least some of the more sophisticated alternatives. They found eleven different species of small mammal in the area, but only three were at all common.

By far the most numerous were delicate vesper mice (Calomys tener). Despite the name, these, like many American "mice", are not members of the mouse family as we would think of it in Europe. They instead belong to the same family as the hamsters, but are so incredibly mouse-like that, honestly, you'd have to be a real expert to tell the difference. This particular species is the size of a relatively small house mouse, with a brownish body and pale grey underparts. It's nocturnal, and prefers open terrain to thick woodland.

Coming in a fairly distant second, we have the black-footed pygmy rice rat (Oligoryzomys nigripes), another species that's pretty damn obscure to anyone who isn't really into their South American rodents. Another member of the "hamster family", it also looks almost exactly like a mouse (and a small one at that - it isn't rat-sized), but is fairly good at climbing, and is therefore more likely to be found in woodland than is the vesper mouse.

To get a bit of variety, the researchers also added the hairy-tailed bolo mouse (Necromys lasiurus) into the study, despite it not being very common in the area. Another member of the same family, it's notably larger than the other two (although still basically a "mouse", not a rat), is generally active during the day, where the others are nocturnal, and is more inclined to dig burrows.

The species that did place third in abundance, however, was Bishop's fossorial spiny rat (Clyomys bishopi). The spiny rats are a family of their own, so this one isn't so closely related to the others. It also really is rat-sized (albeit a fairly small rat), and not only digs tunnels, but, unlike the other three, lives in communal groups rather than on its own. So it's clearly the most distinctive of the four.

Now that we know what animals we're looking at, and how common they are, the next thing is to figure out how much they get eaten, and then compare the two numbers. Given how common they are, it's pretty much certain that the vesper mice will form a large part of the diets of whatever local animals eat rodents, but if some predators find them difficult to catch, they might not be as common as we'd expect. To find out, we need to check the predators' poo.

The researchers examined the dung and pellets of three local predators: maned wolves (which I'll discuss as part of the dog family in November or December) and two kinds of owl. One of the owls is actually the barn owl (Tyto alba), an astonishingly widespread bird that can be found on every continent except Antarctica. It is, of course, a nocturnal, aerial hunter, but in contrast, the other owl species examined, the burrowing owl (Athene cunicularis) sits on a perch waiting for unsuspecting prey to pass by, and is unusually active during the day. Plus, they dig burrows.

 It turned out that bolo mice were particularly vulnerable to the barn owls, being eaten in greater numbers than their simple abundance would suggest. On the other hand, the maned wolves ate significantly more spiny rats, which the owls avoided, and far less of the vesper mice. It might well be that the spiny rats don't get attacked by the owls simply because they're too big, but some of the other differences aren't so easy to explain.

One possibility the researchers considered is that some of the rodents were just better at hearing, and could spot when something was coming. This, after all, seems to be a key method used by desert-dwelling rodents to avoid their predators, so why not here, too? It's hard to tell just how good the hearing of a mouse is without some fairly detailed research, so in this case, they measured the volume of the animals' auditory bulla. This is the bony hollow that encloses the middle and inner ear, and the larger it is, relative to the rest of the animal, the better its hearing is likely to be.

The spiny rat turned out to have the best hearing, although that may well be because it's a social animal that needs to communicate with its own kind. However, none of the species had particularly good hearing, which makes it unlikely that that had much to do with it - perhaps other senses, such as vision, were of more use.

But even if you do spot a predator coming, that's of little use if you can't escape from it. Of the four species, the one that, judging from the length of its legs, can run the fastest is the pygmy rice rat - which also has the advantage of being able to run up trees. That may not be much defence against being dive-bombed by an owl, but it ought to help against the maned wolves, which chase creatures along the ground. And, indeed, the rice rats were eaten less than expected by the wolves.

But then, the wolves also avoided eating the vesper mice, and to an even greater extent. It's not that they won't eat them, because they did, but they did so much less than you'd expect given how common they are. The vesper mice don't run particularly fast, but it seems that, unlike the pygmy rice rats, they do try and avoid coming out when the moon is full. This may be an effective strategy, sticking to the very darkest parts of the night so that the wolves can't find them... although it doesn't seem to be much use against the barn owls.

So, four different species, four different strategies. Vesper mice avoid being eaten by hiding, and make up for the fact that this doesn't seem to work very well against the owls by breeding like crazy - which is why there's so many of them. Pygmy rice rats probably aren't so good at avoiding detection, but are good at running away once they notice something's coming for them.

The bolo mice seem to have the worst time of it, and are particularly rubbish at avoiding the barn owls. This may explain why they are less common than the other three species, at least in this particular habitat; perhaps they're just better at living somewhere else, such as deeper woodland. Finally, the spiny rats are not often attacked by the owls, perhaps due to their size, although their social habits may also make it easier for them to keep a look out, but apparently they don't run fast enough to escape a hungry wolf.

That running and hiding seem to be the most effective strategies overall likely has to do with the lush habitat and dense undergrowth in the cerrado. Head out into the desert, where there's nowhere to hide, and all of the priorities would change...

[Photo by "travelwayoflife", from Wikimedia Commons.]