Sunday 12 March 2023

Fruit Bats of Madagascar

While most attention tends to focus on large charismatic animals such as rhinos and tigers, it will probably come as no surprise to discover that many species of bat are endangered. Here in Britain, all bats are legally protected by the Wildlife and Countryside Act 1981, making it an offence to disturb their roots. That's perhaps an unusually robust example, but other protections and conservation efforts exist across the world.

On the other hand, it is true that the majority of bat species are not especially threatened, at least on a worldwide scale - although things may be different locally. Bearing in mind that around one in six bat species are so recently identified and so little studied that we simply don't know how common they are, only around another one in six are rare enough to be listed as "threatened" by the International Union for the Conservation of Nature. Still, that's not exactly a small proportion, and since there are somewhere around 1,400 named species of bat, it's not small in absolute terms, either.

Tha bats are a highly diverse group, representing no fewer than 21 taxonomic families, none of which are likely as familiar to the layman as terms such as "cat family" or "deer family". Some of these contain very few species, representing oddities that don't quite fit into any of the main subgroups, but there are still five families with over a hundred species each. Of these, the one that contains the highest proportion of threatened species is the fruit bat or "flying fox" family, the Pteropodidae.

In total, around a third of all flying fox species are considered "threatened", double the proportion among bats as a whole. There are probably two main reasons for this, with one being that many flying fox species are found only on specific, isolated, islands, where it doesn't take much to put them under threat, since their population probably isn't very high to start with. Another is that flying foxes are much larger than most bats - something that makes them worth hunting as a food source.

Yet, like pretty much any other animal, flying foxes can be important parts of their ecosystem. While they are by no means the only "fruit bats" (there are many much smaller fruit-eating species in the other families), fruit is a significant part of their diet and they not only help spread seeds in their dung, but can also play an important role in pollination. The fact that so many live on isolated islands is significant here, since many of those islands have few, if any, other fruit-eating animals, placing them in a unique position.

Even on larger islands, the role of flying foxes can be important. Take, for example, Madagascar, the world's fourth-largest island. Here, lemurs appear to be the primary dispersers of fruit seeds - rather than birds, as is typically the case elsewhere. But, even if lemurs take the number one slot, they are by no means alone.

There are three species of flying fox on Madagascar: the Madagascan flying fox (Pteropus rufus), the Madagascan rousette (Rousettus madagascarensis), and the Madagascan fruit bat (Eidolon dupreanum). So, yeah, not the most original common names ever, but there you go. All three are "vulnerable" or "threatened" species, although they fall short of being fully considered endangered species, and all three play a role in pollination and seed dispersal on the island. Indeed, the Madagascan fruit bat is the only known pollinator for a local endangered species of baobab tree. 

All three species are routinely hunted, especially the Madagascan flying fox (which, with a 120 cm [4-foot] wingspan, is the largest one), as well as being targetted to prevent them from eating fruit crops. Indeed, it's perfectly legal to do so, with an official hunting season in place between May and September. All of this makes them important subjects for conservation research. However, there's also a pragmatic reason to study the bats: while the Madagascan species obviously have nothing to do with COVID (wrong continent, wrong bat family) they do host other coronaviruses that could potentially make the leap to humans in the future. Understanding how and when those might spread is exactly the sort of thing we should be paying attention to.

Being in the tropics, the difference in temperature between winter and summer is not great in Madagascar - 14 °C (58 °F) and 21 °C (69 °F) are typical daily averages at the capital for July and January. But there are distinct dry and wet seasons over most of the island, leaving aside the perpetually rainy east coast. Thus, we might expect the bats to time their reproduction so that young are born when fruit is going to be most available and all of them to do so at the same time.

Well, the first part of that is true... but the second is not. The Madagascan flying fox times its breeding so that most births occur between September and October, just as the dry season is ending, while the rousette mostly gives birth in December as the wet season approaches its peak. The first study describing the timing of births in the third species, the Madagascan fruit bat, only came out late last year, but it placed that firmly in November, halfway between the other two. 

While the staggering of the births themselves between the three species might help to sustain infective viruses and make it easier for them to jump between species, this does, however, seem less likely in this particular case. That's because, while the rousette and the Madgascan fruit bat do often share the same cave system, something that's known to promote interspecies viruses in other bats, they seem to occupy different parts of the caves, and interact less than one might think. More importantly, the Madagascan flying fox does not roost in caves at all, but in trees, where the risk of exposure from the other species is obviously much lower.

The same study showed that, despite the different timings of their birth, all three species time the peak of their lactation - and thus, the most rapid growth of their infants - to the same time period, at the height of the rainy season. This is achieved by the Madagascan flying fox, which births first, growing more slowly as an infant, so that it takes it longer to reach maturity. Combined with the fact that the hunting season lines up perfectly with when they're most likely to be pregnant, this prolonged growth phase may make that species in particular more vulnerable than previously supposed, something that aligns with suspicion from earlier studies that its population is declining more rapidly than the IUCN estimate suggests.

The chances are that it was that sort of finding that the researchers were interested in when they started the study in 2013. The coronavirus implications probably came later when people started to get excited about such things and mentioning them gave a chance for your paper to be boosted. But, as the pandemic shows, we are connected with the natural world and it's probably a good idea to keep an eye on what's happening.

[Photo by Bernard DuPont, from Wikimedia Commons.]

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