Sunday, 6 January 2019

Gerbil versus Rattlesnake

Deserts are harsh environments, and living in them poses animals a number of problems, not least of which are daytime heat and an absence of water. In order to survive in such a place, animals need to evolve suitable adaptations - extremely efficient kidneys to reduce water loss are one such example.

But the thing about deserts is that, while there are some geographic differences between them, the challenges of living in one are at least broadly similar regardless of which desert it happens to be. And while, say, the world's seas are all connected, the deserts aren't. So animals, including mammals, have had to evolve means of surviving in them several times over, re-developing the necessary features each time they encounter a new one.

But it's often the case that there are only so many ways to solve a given evolutionary problem. The exact details will vary depending on what the ancestral animal happened to look like - evolution has to work with what it's already got - but in many cases, we see similar adaptations arising again and again. And, if we get similar groups of species exhibiting similar adaptations in similar environments, we should expect that, broadly speaking, all desert ecologies should be fairly similar.

But that is, quite obviously, an oversimplification. For a start, not all deserts are that similar - some are cold, for instance. And, even if we ignore that, simple geography presents some limitations. The deserts of Australia for instance are, by virtue of the continent's isolation, home to different kinds of animals than the Sahara, giving evolution a different starting point to operate from. And thirdly, there's the fact that, sometimes, an animal in one location evolves a feature that's a different - and perhaps even a better - solution to the same problem as its relatives in a different part of the world.

Gerbils are an example of the second of those principles in operation. Gerbils are a subfamily of the wider mouse family, and they are notable for only ever living in dry environments. These aren't necessarily all deserts as such, and, when added to the fact that they have been around for millions of years, and have seen a fair few changes to world climate in that time, they can be found across a wide swathe of the world, including a number of 'un-connected' deserts.

But there are still limits to that expansion, and the Atlantic and Pacific Oceans are certainly among them. The Bering land bridge was much too far north for them to cross, so while they have also lived in Europe in the distant past, today they are found only in Africa (both north and south - the savannah of East Africa is dry enough for them) and southern Asia. And not, for example, in the American southwest, which at least some species would probably love.

Or would they? The climate certainly seems fitting, but, for the same reason that there are no wild gerbils in the Americas, there are some unique predators there that they'd have to deal with. But there are a lot of things that eat gerbils, as is typical for small rodents, so one might question how much difference that would really make. If you're going to be eaten, does it really matter which predator is doing the eating?

Well, perhaps.

Anderson's gerbil (Gerbillus andersoni) inhabits the north coast of Africa from Tunisia to Egypt and is also found in Israel and southern Jordan. Here, it lives in some remarkably dry deserts, avoiding the semi-arid, more 'Mediterranean', climates of places such as Tripoli. Still, it is along the coast, so it does avoid the ridiculous temperatures found further inland and can make use of partially solidified coastal dunes, rather than open, loose, vegetation-free sand.

Even so, it avoids the heat of the day, and is nocturnal, meaning that its main predators are likely owls and similarly nocturnal snakes. Both are good at seeing in the dark, but that doesn't mean that darker nights don't offer at least some protection for the gerbils so that, the darker the night, the more likely they are to be active.

Which works, to a point, so long as the predator you're trying to avoid doesn't have much better night vision than you do. There's nothing like that in the Sahara... but there is in the Mojave.

Because the Mojave has rattlesnakes. Rattlesnakes are a kind of pit viper, and pit vipers have the unusual ability to detect infrared radiation - in fact, that's what the "pits" on their faces are for. So good are they at this that they can home in on an animal by body heat alone, being able to "see" their targets even in the absolute absence of light. And, while there are some non-rattlesnake pit vipers in Asia, they aren't found in Africa, so Anderson's gerbil, at least, has never faced anything like them.

Of course, they do face other kinds of vipers on a regular basis, so perhaps that's enough. Here's where the third point that I made above enters the picture: sometimes one of two otherwise similar animals evolves a better way of doing what they both do, but they're so far apart that they never come into competition. In this case, those animals are rattlesnakes in the Americas, and regular vipers in North Africa... the rattlesnake is apparently a superior hunter at night, but just how much difference would that really make to the gerbils?

To find out, a group of researchers placed some gerbils into a large outdoor terrarium divided into four sections: one with local Saharan horned vipers (Cerastes cerastes), which they are used to, one with sidewinder rattlesnakes (Crotalus cerastes) imported from Arizona, one with both, and one with neither. The two types of snake are extremely similar - for example, they both move with a 'sidewinding' motion, and both have small horns on their heads ('cerastes' literally means 'horned', and also refers to a mythical horned snake from Greek legend). Apart from the fact that the rattlesnakes have the built-in IR sensors that let them see in the dark, they even hunt in the same manner.

Given the similarity, it's perhaps unsurprising that the first finding from this study was that, yes, Anderson's gerbils do understand that rattlesnakes are dangerous, even if they've never met them before. One snake is as scary as any other.

We know this from measuring something called the giving up density. This is the amount of food left in a foraging patch (specifically supplied ones in the case of the experiment) at the point that the animal using it gives up and goes somewhere else. If the animal perceives it as being too dangerous to hang around for long, the giving up density will be higher (that is, there's more food left) than they would otherwise be. And that's what happened here.

However, some interesting patterns emerge when we compare how the gerbils behaved based on the phase of the moon. On dark, moonless, nights, the gerbils were less active when there were snakes around than when they were not, as we might expect. But, on bright moonlit nights, they weren't so bothered by the snakes, unless both species were around at the same time.

This tells us a couple of things. For one, gerbils are good at knowing whether there are snakes about (they can probably smell them), and reasonably skilled at avoiding them when they are. Indeed, only one gerbil died during the 13 non-consecutive days of the experiment, when one of the rattlesnakes finally got lucky.

Secondly, it suggests that, while rattlesnakes have little advantage over horned vipers individually, especially on moonlit nights when both can hunt, the species are more effective when paired with each other than when they are paired with another of their own kind. On darker nights, the advantage of this pairing is reduced so that, if anything, the vipers are getting in the sidewinder's way when they can't see what's happening.

That's interesting information with respect to the snakes. If anyone did try to introduce rattlesnakes to the Negev or the Sahara, it would be bad news for the gerbils, but not for the other snakes that you might expect them to be in direct competition with. It's worth noting, on this front, that invasive snake species have rarely pushed out native snakes from their habitats, and are only truly successful where there is no equivalent native animal.

The implications for the gerbils are good, too. They can clearly adjust their behaviour rapidly in response to a change in circumstances, even when faced with a creature they have never encountered before. And it generally works... even when the new predator has infrared-sensing superpowers.

[Photo by Mark Bratton of the US Air Force. In the public domain.]

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