Sunday, 14 April 2013

Freedom to Dive

Mediterranean water shrew
There are well over 5,000 different species of mammal in the world. From a human perspective, a great many of them are very similar - at least a thousand of them can reasonably be described as either 'mice' or 'rats'. Where they live in different continents or unconnected parts of the same continent, that's not really an issue: European beavers don't care what the American sort are up to. It's a different matter when they live together.

But, of course, they often do. For example, there are fifteen different species of insect-eating bat in Britain alone. But if you have two different kinds of animal living in the same place, in the same way, eating the same thing, they have no choice but to compete. Inevitably, one of them will be slightly better at it than the other, and, given enough time, one of them has to either die out or change what it's up to. So how come there are so many similar animals?

Generally speaking, the answer is that they're not all doing exactly the same thing: there's some subtle difference in what they're up to. In some way or another, they're partitioning the available resources. One simple possibility, for instance, is that they aren't eating the same thing after all. If (to pick a rather unlikely hypothetical example) one of you eats only raspberries and the other eats only blackberries, there's plenty for both, and you don't have to fight over who gets to eat what.

Another alternative is to find food in a slightly different way, or to live in a slightly different place. For instance, if you're a carnivore, you could hunt at different times of the day - one species hunting at night, one during the day. Inevitably, you're going to find different things out and about when you do that. Or one species could live by riverbanks, and the other in the woodlands.

In the case of bats, even if two species are both hoovering up whatever insects fly past, perhaps one has manoeuvrable flight that lets them flutter about in densely wooded areas, while the other has great straight-line speed and likes plenty of open terrain. Or one hunts close to the ground, and another higher up. All of these things do happen, and they explain why there's so many different species seemingly in the same place.

So, each animal often has a very specific thing that it likes to do, and this is often especially true of small species that don't have to wander about over a large and diverse area to find their dinner. But what if things change? You've got two species, partitioning their resources by being very precise about what they do - and then one of them goes away, for whatever reason. Suddenly, the one that survives has a lot more options. Sure, they might not be as good at whatever the other species did, but that doesn't mean they can't have a go at it. And there is, after all, food just there for the taking.

This is called ecological release. It's often most obvious where you have two species that live in overlapping, but not identical, regions. Where they're found together they behave differently, so that both survive (or, obviously, they wouldn't be found together), but where they're apart, they're a bit broader in what they get up to. Or, to look at it the other way round, where they live together, they have to focus on what they're really good at, even if they might like to do rather more.

There are, for example, two species of water shrew in Europe. The common water shrew (Neomys fodiens) is found in much of Europe, from Scandinavia and Britain as far south as Italy and northern Spain, and across the Balkans to the Black Sea Coast. The Mediterranean water shrew (Neomys anomalus) lives in much the same area, but generally further south. It is not found, for example, in Britain, northern France or Germany, the Low Countries, the Baltic States, or Scandinavia, all of which are home to its "common" relative. On the other hand, it is found in southern Spain, Portugal, Albania, Greece, and Turkey, which are a bit too warm for its cousin. That leaves plenty of places, from northern Spain and southern France, right across to Bulgaria, where you can find both.

There are incidentally, various alternative common names for these two species. For example, "Mediterranean water shrew", "southern water shrew" and "Miller's water shrew" are all the same thing. This is why we have scientific names: if I say "Neomys anomalus", then scientists, at least, know exactly what I'm talking about, and nobody has to be confused.

Since it's the first time Synapsida has really discussed shrews, I should probably briefly explain what they are. Superficially, they look rather like mice, if often somewhat smaller and darker in colour. Indeed, in German the word for shrew is 'spitzmaus' - literally something like 'pointy mouse', referring to the shape of the nose. But shrews are definitely not mice. Mice are rodents, with a fairly small number of teeth, including a huge pair of gnawing incisors, and they're mostly herbivorous, with the wild species feeding on things like seeds. Shrews have lots of sharp teeth, making them quite unlike rodents, and they feed on insects, earthworms, and the like. The two groups aren't even particularly closely related.

Water shrews, specifically, tend to live along the edges of small rivers, streams, and ponds, in marshland, and so on. Which means that not only do the two species of European water shrew live in the same geographic area, but in the same sorts of places within that area. So why is there such a large overlap between their respective homelands? If they're trying to do exactly the same thing in the same places, why doesn't one of them simply drive the other one out?

Well, we'd expect that there has to be some sort of subtle distinction in what they're up to. And, indeed, there is. It turns out that, although they live in the same habitats where they're apart, in places where both are found, common water shrews tend to stick to rivers and lakes, and Mediterranean water shrews to marshes and shallow streams. That is, common water shrews prefer deeper water, and that's how the two stay apart.

We can actually see this in their diet. Where they live together, about 95% of the food of common water shrews consists of things that live underwater, while that's true of only about 15% of the Mediterranean water shrew's diet.

The next question we might ask is why they do this. What is it about common water shrews that gives them an advantage in deeper water? I mean, one shrew is pretty much like another, isn't it?

Well, no, not quite.

If you look really closely at a common water shrew, you will see that it has quite large hind feet, and that there is a fringe of hair around the edges of the feet that, in effect, make them even larger and flatter. This is how the animal propels itself through the water, using its feet as paddles. The tail is also flattened from side-to-side, like the keel of a boat, with a mane of stiff hair running along the underside that, again, further emphasises this shape. The tail is thus literally both a keel and a rudder, keeping the animal stable in the water, just as it would on a boat.

If, on the other hand, we take a similarly close look at the Mediterranean water shrew, we see that these features are much less extreme, and there may not even be a mane of hair on the tail at all. All of this adds up to making the common water shrew better at swimming than the Mediterranean one. Where they're apart, that's not a problem, but where they're found together, the Mediterranean species sticks to paddling in shallow water, leaving all the difficult diving and deep-water swimming to its relative.

Which brings us back to ecological release. Where they're apart, the Mediterranean water shrew should get more practice at swimming in deep water. Still not as good as the common species, of course, but, then, they don't need to be. However, this might put different evolutionary pressures on the populations that need to swim. If you never swim in deep water, being rubbish at it doesn't make any difference, but if you do... well maybe those that are even slightly better at it will out-compete those that aren't.

That's what a recently published study decided to check out. They captured some Mediterranean water shrews in Portugal, where the common species isn't found, and tested their swimming abilities in an aquarium. They then placed them in a more complex terrarium, with artificial islands and deep ponds, to see how they looked for food. They then compared all of this to the results of similar experiments on common water shrews, and on members of the Mediterranean species captured in Poland, where the two live alongside one another.

They conclude that the Portuguese animals were better at swimming than their Polish counterparts. They weren't any faster - the shape of their feet and tails may limit that - but they did use a more efficient stroke, similar to that of the "common" species. They also spent more time floating about on the surface, something that common water shrews do, and that not only indicates that they were comfortable with the idea of deep water, but that also gives them more time to rest between dives.

As for foraging, the Portuguese animals did at least attempt to dive into deep water to look for food, although they weren't very successful when they did. Still, in similar experiments, Polish animals hadn't even bothered to make the effort. As to why they didn't find much food when they dived, it seems to me that they may well just be due to their physical limitations - they aren't built for it. The authors, however, while acknowledging that that's possible, also point out that, while there are no other kinds of water shrews in the area, there are Pyrenean desmans (Galeyms pyrenaicus). While these are semi-aquatic members of the mole family, and not shrews, there is a fair similarity in their lifestyle, and they just might be partially taking on the role of the common water shrews where the latter are absent.

Nonetheless, it does seem that Portuguese Mediterranean water shrews are more diverse in their habits, and better at swimming, than their Polish counterparts, and this does lend support to the idea of ecological release. They may not swim much in deep water, but when they get the chance to do so, over the generations, those that are better at it tend to survive over those that aren't. Interestingly, a 2007 study showed that there is a significant genetic difference between the two populations, perhaps enough to qualify them as separate subspecies. That's not been officially adopted, but it could be that greater opportunities for water shrews in Portugal are sending them down a different evolutionary path.

Which, over a sufficiently long period of time, is how we get new species in the first place.

[Photo by David Perez, from Wikimedia Commons]

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