Sunday, 21 September 2014

Predator v Predator

Herbivores eat plants, carnivores eat herbivores. That, at it's simplest, is how food chains work. But, of course, food chains are almost never that simple in reality. For a start, it's surely obvious that, given the chance, big carnivores eat small carnivores. They generally aren't a high proportion of their diet, not least because the sort of parasites you might find inside a small carnivore may not be very healthy for the big ones, either (mammalian carnivores being fairly closely related, in evolutionary terms). But they'll certainly do it.

Carnivores eating other carnivores is called intraguild predation, which sounds like it ought to have something to do with World of Warcraft, but doesn't. (Unless your characters eat one another, which I'm fairly sure the game doesn't allow). Inevitably, such predation means that the lifestyle of a small carnivore is somewhat different from that large one. It's not just being eaten themselves that they have to worry about, either. There's also the risk of something larger coming along and pinching the dinner that you just spent so much time catching. Which, while we're on the technical terms, is called kleptoparasitism.

In some places, this sort of competition leads to a clear hierarchy of carnivores, with the larger ones, at the very least, chasing smaller ones off their prey, and quite possibly eating their young, if they can get away with it. In the African savannah, for instance, the order goes: lion > spotted hyena > leopard > cheetah. Cheetahs are superb predators, but they're built for speed, not strength, and speed doesn't count for a lot if you're trying to defend a heavy carcass or young cubs. Similarly, while leopards and hyenas would probably be fairly well matched in a one-on-one fight, hyenas travel in packs, and leopards don't.

While fit adult cheetahs don't get eaten by much, carnivores even further down the chain do. And we don't have to go as far as Africa to see similar principles in operation. Even Europe, for instance, has a number of wild mammalian predators, and the smaller ones certainly have to be wary of those that are larger. A fox may not be very scary to humans, but it is to most members of the weasel family.

Red foxes (Vulpes vulpes) and pine martens (Martes martes) are both European carnivores with broadly similar diets. Pine martens are, however, more likely to stay in forests, and its possible that at least part of the reason that they don't go out in the open more is that fear being attacked by things like foxes. With good cause, since red foxes will kill and eat pine martens, and apparently do so quite regularly where the two come into contact. (Americans may wish to note that while the European pine marten is a different animal to the one on the western side of the Atlantic, the red fox isn't).

So, while red foxes are not themselves free from threat - especially in areas with significant human populations - we might expect the behaviour of the two animals to be rather different, and the pine martens to generally be more cautious than their vulpine fellow predators. To test this, we need to see how they behave around food in the wild.

We don't, however, have to watch them as they go about their daily hunting - something that's probably harder with small pine martens in a forest than it is with lions out on the savannah. We tend to think of flesh-eating animals as either predators or scavengers, but, in reality, most are both. True, there are some animals, such as hyenas, that are particularly adapted to scavenging, but even hyenas are effective hunters, and animals such as dogs will scavenge relatively fresh kills if they find them - or can steal them from something smaller.

Foxes and martens are no exception. For the most part, they hunt, but they won't pass up a freshly killed carcass if they come across it. So Camilla Wikenros and co-workers decided to set up cameras next to moose carcasses in southern Sweden, to see what happened. The moose carcasses in question were ones that had been helpfully, if unwittingly, supplied by wolves, animals somewhat higher up the predatory totem pole than foxes, let alone martens. Wolf packs do not necessarily strip an entire carcass to the bone, and these ones had left at least some edible food behind, allowing smaller scavengers to move in once the wolves were safely out of the way.

Now, this was in Sweden, where it gets rather cold for much of the year. In the winter, at least, the dead moose might as well have been in a freezer, and they remained edible for months after death. Being large also helped, because even though the wolves ate over half the meat themselves, there was still plenty left for scavengers, and it took a very long time to strip them to the bone. As one might expect, foxes and martens were by no means the only animals to feed on the carcasses, either, but even so, the researchers recorded multiple visits by those animals each day.

Evidently a big, available supply of frozen meat was too tempting for them to miss out on. This was especially true in the early spring, when both foxes and martens are likely to have young mouths to feed. However, the two kinds of animal avoided one another. For instance, while both animals tended to visit the carcasses at night, the martens mostly did so before midnight, and the foxes afterwards.

It's worth pointing out, though, that there may be other things going on here, in terms of the animals' daily routines. That's not least because foxes aren't the only thing that will eat pine martens in the area, so the martens are likely to be more nervous in general, rather than specifically concerned about that one larger predator. On the other hand, at least some of the time, it appeared that martens had followed foxes to the site of a carcass, and waited until they'd gone before nipping in to grab some for themselves. So it could be that the foxes are unknowingly providing their smaller rivals with clues about where to eat.

The researchers expected that the martens would spend more time at the carcasses looking about nervously than the foxes did, but there wasn't really a lot of evidence for this. The main thing the martens seemed worried about was how visible they were - whether the carcass was out in the open, rather than under trees. However, when they did look about, they made more of an effort, even standing up on their hind legs (which is helpful, but risky), while the foxes were more likely to be looking about in a relatively nonchalant manner.

So, while the difference was less than expected, the foxes did act as if they were more confident. Which, given that there wolves around, might not have been the best strategy. But, as it happens, in this area of Sweden, wolves have only recently been re-introduced, so it could be that the foxes haven't really got used to them yet, and "forget" that they're there.

Which, when you think about it, may be good for the wolves in the short term, but in the longer term, should help the foxes and martens, too. Because there's no way they would have brought down something the size of a moose. Wolves, the deadly top predator of the European wilds, are actually helping their smaller rivals by providing them with long-lasting frozen meals to help them through the tough winter, and keep their young fed in the spring.

Big predators may be a danger for smaller ones, but the reality is they can help them, too.

[Photo by Malene Thyssen, from Wikimedia Commons]

No comments:

Post a Comment