Sunday, 19 April 2015

How the Lemurs Reached Madagascar

Pygmy mouse lemur
Madagascar is an island. It's quite a large island, to be sure - at nearly 227,000 square miles, it's the fourth largest island in the world, after Greenland, New Guinea, and Borneo. That's nearly twice the size of the UK, or, if you prefer, almost 90% the size of Texas. Even at that size, however, the fact that it's an island has been highly significant for the sorts of animals that live there.

When it comes to islands, the problem for land-dwelling mammals is how they're supposed to get there in the first place. Sure, it's not a problem for bats, which can cross all but the most ridiculous amounts of sea (although not necessarily on purpose!) But, for everything else, it's something of an issue.

Of course, one possibility is that the ancestors of whatever animals we're talking about "were there already". Britain, for example, has been an island since around 6,000 BC, so whatever animals could wander there from the continent before that were already part of the native fauna. True, many of them have been killed off since, which is why we no longer have wild wolves or bears in the UK... but they were there once. And it clearly explains why the various small mammals found on Britain, from red squirrels to harvest mice, badgers, and hedgehogs, are also native to continental Europe. We haven't been an island for that long.

Madagascar is a different matter. It has been an island for somewhere in the general region of 88 million years. It is separated from Africa by a body of water that is, at its minimum, 6,500 feet (2,000 metres) deep and 260 miles (420 km) wide. That's a lot harder to swim across than the English Channel, and, even at the height of the Ice Ages, sea levels have never been anywhere near low enough for it to be high and dry.

So Madagascar has been entirely isolated from the rest of the world for essentially the whole of those 88 million years. That's one heck of a long time ago, and crucially, when we're talking about mammals, its before the extinction of the non-avian dinosaurs. That doesn't mean that, when the island first split away (from India, as it happens - it's so long ago that the continents were not where they are now) it was entirely devoid of mammals. There may well have been small creatures hiding in the foliage trying to avoid being eaten by the big scary reptiles. But 88 million years would mean that the descendants of those animals, if they were still around, would be nothing like those that evolved elsewhere.

So let's take a look at Madagascan mammals. What do we have? Okay, yes, bats, but we expected them. And... primates, rodents, a number of things vaguely related to mongooses, and a whole bunch of tenrecs. Tenrecs are probably the oddest ones of those, being small insect-eating mammals most of which look a bit like overgrown shrews, but even they are related to other animals on the African mainland.

Taken as a whole, the animals of Madagascar are undeniably odd, and there are several families of mammals absolutely unique to the island - lemurs being perhaps the most well known example. But not 88-million-years-of-separate-evolution odd. At some point, then, land-dwelling, non-flying, mammals crossed over from Africa to Madagascar. They did it millions of years ago, giving time for their descendants to evolve into unique species, and they didn't necessarily do it very often. But they did it.

Which raises the question: how?

One possibility that has been raised is that the island hasn't really been as isolated as we think, and that it may have been joined to Africa around 30 million years or so ago. There are, however, a couple of problems with this. For one, if there was a land bridge connecting the two, it appears to have been one that only small animals bothered to cross, which seems a little strange. Secondly, and more seriously for the theory, our current understanding of geology suggests that it's probably not true.

Way back in 1915, however, another possibility was suggested: that, on rare occasions, animals got trapped on bits of floating vegetation that drifted away from the shore and accidentally rafted across the Mozambique Channel to Madagascar, where they bred and colonised the island with their descendants. Over the decades since, this has become by far the favoured explanation, but it's not without its problems.

Perhaps the most obvious counter-argument is: "What, seriously? This actually happens?" Well, yes, it does. For instance, this does seem to be exactly how green iguanas got to Anguilla. Now, Anguilla isn't as remote as Madgascar, and there are other reasons (which I'll get to shortly) as to why iguanas would have an easier time of this than ancestral lemurs or the like, but remember, it doesn't have to happen very often. Even if we knew that it had to have happened within a single million year time span (and it's actually longer than that) that's ten thousand centuries worth of attempts to get it right.

Another problem, however, is that is you actually stick a raft of floating vegetation into the Mozambique Channel, it's unlikely to end up in Madagascar, because the currents don't go in that direction. Instead, what would happen is that it would likely end up being pushed back towards Africa again, and hitting land somewhere to the south of where it had started, or else end up being swept into the vast expanse of the southern Indian Ocean.

Or, at least, that's what would happen if you tried it now. An analysis in 2010 provides evidence that, at around the time we're talking about (when, remember, the island wasn't in the place it is now) the prevailing sea currents went in the opposite direction, heading towards, not away from, the west coast of Madagascar. According to the analysis, the currents would have switched to their current pattern around 20 million years ago or so, which might explain why none of the unique animals on the island seem to have been there for less time than that.

But the same analysis confirmed that another counter-argument may be justified: it estimated that the journey would have taken around four weeks. That's four weeks of clinging to a mat of vegetation with little food and no fresh water. And, of course, the animal has to get to the other side in a sufficiently healthy condition to breed with its compatriots - or to give birth, if it's a female that's already pregnant.

Now, this is where the problem isn't so bad for green iguanas. Being reptiles, they are "cold blooded", and don't need so much in the way of energy reserves, so long as they aren't doing a lot. But warm blooded mammals need a certain amount of energy just to keep going, so prolonged starvation is much more of a problem for them. Could they really have survived such a journey?

One possibility is suggested by something that a lot of mammals do when food is going to be in short supply for a while: they enter torpor. Superficially similar to sleep, torpor is actually a far more extreme state, in which the body shuts down most of its functions, and reduces body temperature to the point that, in the most dramatic cases, it's essentially cold-blooded.

We're perhaps most familiar with torpor in the context of hibernation, in which some animals can avoid eating or drinking for months, never mind weeks. Hibernation, however is, by its nature, seasonal, and generally requires quite a lot of preparation in the form of bulking up on fat reserves and so on. It's unlikely to have much to do with the colonisation of Madagascar, unless the animal happened to be trapped at exactly the right time.

But torpor is a continuum, with true hibernation at one end, and something not far from regular sleep at the other. In between, we have daily torpor, in which an animal can partially shut down its metabolism for a short time each day, in order to get by on, if not zero food reserves, at least far less than normal. Animals capable of it often enter daily torpor whenever survival is difficult. For example, sugar gliders, a kind of marsupial, go into daily torpor whenever the weather is bad, and they have difficulty foraging for food.

Clearly, it would be advantageous for any animal finding itself on a floating raft of vegetation in the middle of the sea to be able to enter torpor from time to time, and thus maximise its chances of making it to shore alive. But that doesn't mean that it's what happened. Can the sorts of animals found on Madagascar actually do this, or is it just a red herring?

A recently published review of the literature suggests that they can. Members of the dwarf lemur family, the Cheirogaleidae, can almost all enter daily torpor, and some of them even hibernate. While other lemurs don't seem to do so, if they're all descended from the same ancestral stock, it could be that the common ancestor did, and that the later, larger, species lost the ability when it ceased to be necessary, but that dwarf lemurs remain closer to the original condition. At least some tenrec species also hibernate, or enter shorter periods of torpor, as well, and it's thought that this may be a primitive trait, held over from their distant evolutionary past.

For other animals, the situation is less clear. Madagascan carnivores, such as the fossa, don't seem to enter torpor at all, and nobody has studied the rodents in this particular respect. There are other members of both groups elsewhere in the world that at least can do this, however - badgers and dormice being examples in each case - so, again, it's not inherently unreasonable.

We can't say from this that the ancestors of Madagascan mammals slept their way across the sea from Africa to reach their new home. The carnivores, in particular, may not have been able to. But it is possible, and combined with the other evidence we have, it does seem as if such creatures made it across to the island from the mainland somehow. The details of quite how they survived the journey may never be known for certain, but it's something they needed to succeed at only a few times in several million years of trying.

However they did it, such journeys may be responsible for the unique and fascinating animal life found on the world's fourth largest island.

[Photo by "Bikeadventure" from Wikimedia Commons]

No comments:

Post a Comment