Bats in the Belfry

Mammals, like other animals, need a safe place to sleep. For large animals living in herds on the open plains, safety in numbers may be the best they can do, with some keeping guard while the others perhaps try to hide in long grass. Hiding in trees or sleeping on rocks out in the ocean are valid options for those in the right habitat. For many others, however, especially the smaller ones, some kind of den, nest, or burrow provides just the ticket. In the case of bats, we have roosts.

When it comes to bat roosts, it's likely that most people initially think of caves. Caves can hold communities of thousands of bats, often with many different species sharing the same one. Caves are ideal roosts for bats, because they provide a stable environment safe from the weather, few predators will enter them, and, when you're nocturnal anyway, you don't care that it's dark. 

In reality, however, many bats do not roost in caves. Cavities in trees and cracks in cliff surfaces can suffice for some, while others simply shelter in dense foliage. Sometimes that's because they'll take whatever they can get, as is the case, for example, for the Jamaican fruit bat, which appears equally happy in caves and trees. Others are highly specialised, such as the Madagascan sucker-footed bat, which roosts only under the folded leaves of the traveller's palm tree. 

But even bats that prefer caves don't exclusively use them as roosts, because we're all familiar with stories of them using man-made structures instead. Sometimes these are close enough to being artificial caves, such as bats roosting in abandoned mines or old railway tunnels. But more often, they are buildings, intended for something other than digging into a hillside. We're talking attics, barns, and, yes, belfries. 

Clearly, bats cannot have evolved to live in attics, so we might ask why they do so now. The traditional explanation has been that, for the most part, it's all our fault. By destroying the natural habitat of bats, we have forced them to find alternatives and, if you can't find suitable caves, a building might be the next best option. In recent years, however, it has become clear that it may be a bit more complex than this. Bats may, under the right circumstances, actually prefer buildings to caves. It's less likely that there will be predators there, and an insulated building is probably warmer and more reliable than a small cave open to the elements. This might enable a bat species, for example, to expand further north than it otherwise would (although, in recent decades, climate change may also be a factor here). 

It's not necessarily easy to find out how often bats use buildings as roosts. Certainly, we can go to buildings that might host bats and see if there are any there. But this tends to result in you finding bats... well, where you expect to find bats. You can't know if there were any in places you didn't look, because small bat colonies can be quite difficult to spot, and you don't know what proportion of the population is using artificial, rather than natural, roosts.

Fortunately, that's no longer the problem it used to be. These days, we can capture bats out in the wild, tag them with radio transmitters and release them. Over the next few days or weeks, we can use radio telemetry to see where they go, giving us a much better picture of how (if at all) they use buildings rather than natural roosts. Among other things, this can give us a better idea of whether they are more likely to use buildings when their natural habitat has been destroyed, whether they really do use them to colonise colder parts of the world, and so on.

Such studies have been possible for long enough now that we can put them together to develop a broader picture of what is going on. In this light, a recent review of the literature was able to identify 239 studies on bat use of above-ground artificial structures across Europe, the US, and Canada and pool their results. True, this leaves open the possibility that things may be different on other continents, where climate and patterns of urbanisation and habitat destruction may vary, but one has to start somewhere, and there are far fewer studies to review in, say, Africa.

Overall, the review was able to cover studies looking at 31 species of European bat and 33 from America. Perhaps surprisingly, the difference between the two continents was stark. 38% of identified bat roosts in Europe were in buildings, with 15 of the species being more likely to use such places than natural habitats. In North America, it was just over 2%, almost entirely accounted for by just six species, only one of which used artificial roosts more than a quarter of the time. (That one is the Florida bonneted bat, a relatively rare species that lives only in southern Florida. It represents just four of the 6,795 examined roosts in America, so it could just be that the relevant researchers "got lucky" with their small sample size.)

One could point out that the two continents do, of course, have different species of bat - nothing lives in both. Although we can't rule this out, it seems unlikely, since some of the species are very closely related. Yet 47% of mouse-eared bats (Myotis spp.) and house bats (Eptesicus spp.) roost in buildings in Europe, while only 3% of bats belonging to the same genera do so in North America. 

This is not to say, of course, that bats are only rarely found in buildings in the US and Canada. Both the big brown bat (Eptesicus fuscus) and the little brown bat (Myotis lucifugus) are regularly reported to make their homes in American attics. It just turns out that there are far, far more of those bats out in the wild. And most species apparently avoid buildings altogether. 

Whereas in Europe, 80% of species use buildings to some extent, with around half of them actually preferring them to nature. Common examples here include Natterer's bat (Myotis nattereri), Geoffroy's bat (Myotis emarginatus), the serotine bat (Epesticus serotinus), and the grey long-eared bat (Plecotus austriacus), but there are many more. Even so, it isn't universal, with, for example, exactly zero of 160 identified roosts of the Alcathoe bat (Myotis alchathoe) being artificial.

There is, however a significant difference between the two continents, and that is how much wilderness they contain. Urban areas, and perhaps more importantly, cropland, cover about twice as much of Europe, in proportion to its size, than they do North America. There are vast stretches of open land and wild forests in the American West that simply have no counterpart in most of Europe. 

At first glance, this would support the idea that bats use buildings as roosts when we destroy their natural habitat. But it may not be quite so simple, because American bats are no more likely to use buildings in the densely populated parts of the continent than in, say, Idaho and Montana. Furthermore, in Europe, bats are more likely to use buildings in the north (Scotland, Scandinavia), where there genuinely is a fair amount of wilderness - the exact opposite of what we'd expect.

That latter fact lends support to the idea that bats are spreading north using comfortable, insulated buildings in places where natural caves or forest roots would be too cold. This has long been known of, for example, the greater mouse-eared bat (Myotis myotis), which has expanded from Portugal to Germany, and Savi's pipistrelle (Hypsugo savii), which is native to the Mediterranean and Middle East, but has moved 800 km (500 miles) north to southern Poland in the last 30 years or so. (This happens in North America, too, and is probably the only reason there are bats in northern Alaska at all).

The reasons behind the difference between bat usage of human-built structures on the two continents may be complex and due to multiple reasons. While the population density of Europe may be a factor, however, the researchers behind the review theorise that it isn't so much the sheer amount of urbanisation and cropland that matters, but how long it has been there.

By the time Europeans discovered America, their home continent already had a population of around 78 million, while the new one had less than 5 million. Moreover, the researchers argue, the sort of buildings constructed by Native Americans north of Mexico were hardly conducive to bat habitation, and their ecological footprint was far smaller than that of their European contemporaries. Europeans, however, had been building churches and other large multi-story brick and stone buildings for centuries. By some definitions, urbanisation and habitat destruction in North America were virtually non-existent before 1800, but you have to go back to the 9th century to find something similar in Europe.

If this is right, and it's merely a guess, bats have had a thousand years longer to adapt to the widespread presence of suitable buildings and the expansion of cropland, not to mention that we know they had been using them to some extent since at least Roman times. Bats have low reproductive rates and live longer than other mammals of their size, so maybe those out in the former colonies just haven't had enough generations yet to get used to the rapid change.

[Photo by Connor Long, from Wikimedia Commons.]