Sunday, 12 February 2017

Scaring Off Snakes

Animals would, on the whole, prefer not to be eaten. As a result, they have evolved a number of ways of avoiding this fate. Being particularly large and fearsome is one tactic - very little eats lions, after all - but that obviously won't work for more most creatures. Many other defensive measures are passive, such as camouflage, or involve hiding or only coming out at a time of day when the local predators aren't around much.

An approach that's essentially the exact opposite of camouflage is the "aposematic display", in which the animal has stark, highly visible, colour markings that warn predators it is dangerous. Of course, you really need something to back this up, or the predators will eat you anyway, and, moreover, find you quite easily. Among mammals, among the clearest example of this are the skunks, with their dramatic black-and-white colouring that warns potential predators that they might get a face full of stink if they try anything.

For this to work, predators need to know what "dramatically black-and-white" (or whatever) is supposed to mean, so there's incentive not to change it too much over the course of evolution, and for otherwise unrelated groups of animals to pick similar patterns, a phenomenon known as Mullerian mimicry. This is, to be fair, more commonly seen in non-mammalian groups (Muller himself was studying nasty-tasting butterflies when he came up with the idea in 1878), but the principle remains the same.

Many animals, even if they aren't as visible as skunks, also make threat displays when they see a potential predator. They may actually approach the intruder, or at least make some highly visible, or audible, indication that they have seen it. This may help to indicate that they're physically fit, and too much trouble to chase, or that, being aware of the predator, they'll get a head start if it comes for them. Such signals can take a variety of different forms, and have evolved in many different sorts of mammal.

But, again, as with the visible markings, this works best if the predator already knows what the signal means. So we might expect such signals to be similar, even when the animals making them are otherwise unrelated. The problem is, this doesn't really seem to be the case, with the signals from different animals seeming to be quite variable. On the other hand, while there has been extensive study of Mullerian mimicry with regards to physical traits such as colouration, there has been much less when it comes to behavioural traits such as warning signals. So maybe it's there, but we're not really seeing it.

This isn't to say that warning signals themselves have not been studied, sometimes in considerable depth, including the ability of some animals to tailor their responses to the specific predators they happen to have spotted. For instance, we know quite a lot about how rodents, of many different kinds, use such signals to deter snakes. That's easily enough to make it obvious that the signals used by different groups of rodent are, well... different. But these rodents may be living in different environments, and facing different threats, so it doesn't mean that, where circumstances are similar, the threat displays of otherwise unrelated animals don't at least come to resemble each other a little more.

If we're going to see this anywhere, it would be among two different types of rodent that nonetheless face the same predator in the same geographic area. If anything would benefit from the predator only having to learn one set of warning signs, those would be the ones. In this regard, it's interesting to note the recent observation that a particular species of ground squirrel makes drumming noises with its feet as part of its threat display towards venomous snakes. That's significant because other species ground squirrels don't normally do this, but another species of rodent living in the exact same area regularly does. So is this mimicry, or just a coincidence?

To find out just how similar the threat displays were, researchers took animals of both rodent species and watched how they behaved when faced with deadly snakes.

The two species in question are the round-tailed ground squirrel (Xerospermophilus tereticaudatus) and the desert kangaroo rat (Dipodomys deserti), both of which live in the Sonoran and Mojave Deserts of the south-western US. Aside from both being rodents, they really aren't that closely related at all. Ground squirrels are, of course, squirrels, and more specifically, are related to marmots. Kangaroo rats, on the other hand, are members of the pocket mouse family, which is in turn, related to gophers. In fact, the two species last shared a common ancestor somewhere around 50 million years ago, over three quarters of the way back to the time of the dinosaurs. Heck, beavers are more closely related to kangaroo rats than squirrels are.

But, both being relatively small rodents living in the same area, they both face the same predators. In this case, the researchers chose to use sidewinder rattlesnakes (Crotalus cerastes), which are common in that part of the world. In order to stop the snakes eating the experimental subjects, the researchers ensured that they were well fed (it honestly doesn't take very much... one lizard a fortnight apparently did the trick) and, moreover, that they were gently but firmly tethered to the ground so that they could move the front part of their body, but not actually slither after anything.

Ignoring the foot-drumming for the moment, there was little doubt that the two kinds of animal did respond differently to the snakes when placed close to them. Kangaroo rats kicked sand in the snake's faces, and went right up to them, even touching them with their noses, before rapidly jumping away. Both of these actions make use of their powerful hind legs, and jumping away at high speed is also, of course, how they respond if a snake really does attack them. So it could well be that, since in this case they were doing it purposefully when they didn't really have to, they were demonstrating their physical fitness and ability to escape from danger. "Don't bother coming after me," they seem to be saying, "it'll be wasted effort."

The round-tailed ground squirrels, on the other hand, were far more cautious. Indeed, they were more cautious than other species of ground squirrel that have been studied previously are known to be. Many of those will approach snakes, to at least within striking distance, and do sometimes kick sand or dirt at them, but the round-tailed species kept out of range, raising and wagging their tails vigorously. This would be a clear indication to the snake that it has been seen, as well as giving the squirrel a head start if it has to run, all the while minimising risk.

The researchers suggest that the round-tailed species might be more cautious towards snakes than other ground squirrels because it is somewhat smaller, and thus may be less able to defend itself, and can be killed by a smaller dose of venom if it does get bitten. But this is something that we don't really know.

But what about the foot-drumming? Both species used this technique about as often as one another, and for roughly the same amount of time. But closer analysis showed that this might all be misleading, because the detailed methods they used were not the same. The kangaroo rats drummed with one hind foot, thumping the ground four or five times a second for about four seconds or so. But the ground squirrels while also using their hind feet, alternated between them, delivering a short, rapid, drum beat of 20 strikes per second for just a single second at a time.

These would clearly sound different to the snake, and while both presumably serve the same purpose of letting the predator know that it has been seen by an animal that is physically fit and capable of fleeing, it would surely identify them as being distinct. Learning one, in other words, probably wouldn't make much difference to the snake's ability to learn the other.

That the squirrels do it at all, when related species don't, may be because the sand on which they live is particularly good at transmitting these sorts of vibrations. But it's something that they've likely developed entirely on their own, with no good evidence that they're imitating their neighbours, the kangaroo rats. The hypothesis is, in this case, busted.

 Maybe, given a million years or so, the two species might synchronise their drumming sounds to appear more similar. The kangaroo rats did, after all, intersperse their slower drumbeats with short bursts of much faster drumming that did, vaguely, resemble those of the squirrels. While they'd likely still be hard to mistake for one another, it's possible that that could change, as the animals continue to co-evolve.

But we don't have that long to wait, and it isn't something that's happened so far, so maybe not. Perhaps there are better example of mimicry out there, amongst the many, many species of rodent, let alone other mammals, but this isn't one of them. Instead, we have two different species of prey animal developing two different strategies for dealing with the same threat. Even when they seem to be similar, that can turn out to be misleading.

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

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