Mouse Lemurs in the Mangroves

Well over 6,000 species of mammal have currently been described, many of which have only been confirmed as distinct species in the last few decades. There is undeniably a trend in the 21st century towards "splitting" of species along comparatively fine dividing lines, but it's also the case that this has been supported by advances in genetics and the like that have made it easier to tell when two groups of animals really are different, and not interbreeding.

But the corollary to this is that many of these species look extremely similar. It's often the case that, once a new species has been described on the basis of genetics, we can point to physical differences between it and whichever species we split it off from. But these are necessarily minor differences, perhaps previously thought sufficient to justify a designation as a subspecies, but more likely not noticed at all. Given the physical similarities, we'd expect that their diet, behaviour, and so on would also be similar - they are, after all, very close relatives.

And yet, many of these highly similar species live right alongside one another. You'd think that, with two creatures doing more or less the same thing in the same place and not interbreeding with one another, that one would, over time, prove marginally better at it than the other and outcompete it, driving it to extinction. For that matter, if they didn't, how did they separate in the first place?

When we look into such cases, we invariably find that there is some subtle difference between the behaviours or foraging styles of the two species that might not have been obvious at first glance. Perhaps, while they live in the same area, they use different parts of that area, one species using the tops of trees while the other forages lower down, or one hunting for insects in patches of short grass and the other in more unkempt pastures. Perhaps they operate at different times of day (although, at least among closely related species, this is unusual). Or perhaps which species has the upper hand changes with the seasons, so that neither has long enough to fully outcompete the other. Other possibilities also exist.

With regard to the first one, however, how closely related species might divide up the same broad patch of land between them can sometimes come down to one being more specialised than the other. That species is highly tuned to feeding off one particular resource, while the other is more generalist, being able to switch diet as necessary. In this case, we'd expect the generalist species to be more widespread, while the other is concentrated in smaller regions. In the long run, the generalist species will likely win out, being better able to adapt to environmental change and perhaps being the rootstock from which numerous, different, specialist species branch off, each in their own little niche. But, in the shorter term (which can still be a long time, historically speaking), the specialist species are better at doing whatever it is that they're doing.

One example of an animal that fits this description is the mouse lemur. This was first described in 1777, by John F. Miller, originally as a species of regular lemur, but was given its own genus in 1834. As the name suggests, it is a remarkably small lemur, about 13 cm (5 inches) in length, not counting the tail, and weighing about 60 g (2 oz.) They are no longer placed in the lemur family proper, instead being placed with the almost equally small dwarf lemurs, but they are close relatives and are strong contenders for the title of "world's smallest primate".

As was common at the time, they were given multiple different scientific names through the 19th century, as individual naturalists thought they'd found what appeared to be new species, or simply weren't aware that they'd "discovered" something that had already been named. In 1931, a major survey confirmed that there was, indeed, only one species. 

Which, as you might have guessed, is no longer considered to be the case.

In 1977, two hundred years after the original species was named, it was confirmed that one 19th century subspecies was, after all, a wholly separate species. The original became the grey mouse lemur (Microcebus murinus) and the second one the brown mouse lemur. It didn't stop there.

As trends in scientific naming began to change in the 1990s, more species were split off, some of which turned out to fit the old 19th-century names but many of which did not. By 2001, there were eight recognised species and, at least at the time of writing, there are currently 25. All of them similar enough that, for much of the 20th century, nobody had noticed they weren't all the same thing.

The grey mouse lemur is the most widespread of these many species, being found across almost the whole of the west coast of Madagascar. Not coincidentally, it is one of only two species of mouse lemur that aren't currently considered threatened, and even then, its potential habitat is expected to shrink over the next few decades. Compared with other mouse lemurs it seems to be something of a generalist, able to tolerate a range of different forest and shrub habitats, perhaps helped in part by an ability to hibernate through tough times and store fat in its tail.

Compare this with the golden-brown mouse lemur (Microcebus ravelobensis). This was first described as a distinct species in 1998 on the grounds of its physical appearance - for example, it turns to have a smaller body and a longer tail, as well as being a different colour. Like most mouse lemur species other than the grey, it lives in only one specific area which, in its case consists of the Ankarafantsika and Mariarano forests in northwest Madagascar, together covering about 3,300 km² (1,300 square miles). Although it can enter a state of daily torpor that comes close to hibernation, it can't store fat in its tail and there's evidently some reason why it has never spread beyond these comparatively small areas of woodland when the otherwise similar grey mouse lemur clearly can.

Almost since it became apparent that the two species were distinct, there have been studies trying to figure out quite what the differences are in their habits that lead to them living alongside one another without significant competition. These have primarily been conducted in Ankarafantsika, due to its larger size and its status as a protected national park. From these, researchers have concluded that while the two species do, indeed, seem to occupy different parts of the forest, the pattern isn't as simple as we might expect. Specifically, the grey mouse lemurs living there seem to focus on particular types of sleeping place more than the golden-brown ones do, making the latter appear to be the generalists. Perhaps while the grey species are generalist more broadly, in this particular area, they change their habits to reduce competition.

Ankarafantsika forest is best described as a tropical deciduous lowland forest; it's comparatively dry and about 50 km (30 miles) inland. It's here that Lake Ravelobe, from which the species takes its scientific name, is located, since an animal found close to that lake was used as the type specimen that originally defined the species. But the Mariarana Forest is different, an area of mangrove swamp on the coast lying around the mouth of a short river.

Mangrove swamps are a distinctive environment, not least because of their regular flooding. Compared with other forest types, they tend to have fewer species of plant, and hence probably of animal. In fact, this may be doubly true of Madagascan mangrove forests, as compared with those on continental Africa. In general, lemurs tend to avoid such places, although there are a few exceptions and the same is not necessarily true of monkeys. So, if the supposedly specialist golden-brown mouse lemurs can live here as well as in the drier woodlands inland, maybe they're more adaptable than we thought, given the right circumstances?

A recently published survey of the mouse lemurs in Mariarana showed that both species are found there, but not necessarily living side by side. It turns out that while the grey mouse lemurs are found across the forest, the golden-brown species live almost entirely in the drier areas away from the river banks where the mangrove trees are at their densest. So the supposition that they aren't highly specialised doesn't hold; they really are restricted to a very specific type of dry tropical forest and probably have difficulty spreading to other parts of Madagascar because of the lack of suitable corridors down which to do so.

The grey mouse lemurs, on the other hand, are willing to tolerate the mangroves, which often have the sort of large cavities in their trunks that they like to sleep in. It probably isn't their preferred habitat, but, if other mouse lemurs aren't there, they can locally prosper, since their typical diet - insects and tree gum - is still there to be eaten. Quite what it is that the golden-brown mouse lemurs dislike so much about mangroves (or, indeed, any sort of forest other than their preferred one) isn't clear, although it might be to do with the complexity of the flora or the relative lack of lianas to climb on. 

Which is useful to know if we want to help preserve these species. The grey mouse lemur seems reasonably secure, but the golden-brown species is not, and most of the other highly localised species are far more endangered than it is. We typically know a lot less about these other species, but they tend to be restricted to even smaller patches of land. The most extreme example is probably the Bemanasy mouse lemur (Microcebus manitrata), inhabiting two tiny patches of surviving woodland on the south coast with a combined area of about 50 km² (19 square miles). 

Other species may be able to share the same habitat, by using different parts of it, but if others are deeply tied to very particular needs, conservation efforts are needed more than ever.

[Photo by Charles J. Sharp, from Wikimedia Commons.]