Sunday 9 November 2014

Figs and Pepper: the Diet of Fruit Bats

Carollia brevicauda, the silky short-tailed bat
The majority of bat species eat insects, often caught on the wing, using their remarkable sonar abilities. But there are a vast number of bat species, and by no means all of them have this diet. Probably the best known exceptions are the fruit bats and the vampire bats, of which the former are far more numerous.

In fact, fruit-eating has evolved at least twice among bats, both times in the tropics, but on opposite sides of the globe. As a result, there are two, quite different, kinds of fruit bat in the world. In the Old World - Africa, Asia, and Australasia - we have the flying foxes, the exceptionally large bats with long, almost dog-like faces. Indeed, these look so different from other bats that they were long thought to represent an entirely separate lineage within the bat family tree, although the truth turns out to be more complex. When the term "fruit bat" is used without qualification, it's more likely to refer to these, and they've been somewhat in the news lately as the likely origin of the Ebola virus before it spread to humans.

The other group are found in South and Central America, and look much more like typical bats. This group includes the tailless fruit bats (Artibeus spp.), the short-tailed bats (Carollia spp.) and the yellow-shouldered bats (Sturnira spp.). One of the problems with talking about bats is that, especially with species outside of Europe and North America, they don't have common English names. Well, technically, most of them do, names made up by scientists because they feel they probably ought to, but nobody outside of a specialist knows what they mean, and they tend to be rather cumbersome. So, for once, I'm going to stick to those scientific names in what follows.

Bats, because they fly, tend to be fairly wide-ranging, so many of these different kinds of fruit bat live in the same area. There have to be differences between them, or some of them would drive the others to extinction, at least locally. One of these differences is the particular type of fruit that they prefer to eat, and, as a result they have developed different strategies and bodily forms relevant to those fruit. Carollia species, for instance, feed mainly on peppers (the spice, not the vegetable), Sturnira mainly on plants related to tomatoes, and Artibeus on a mixture of figs and the rather similar-looking fruit of tropical Cecropia trees.

Primates detect ripe fruit primarily by their colour; this is probably why our colour vision has evolved to be rather better than that of most other mammals. Flying at night, it's unsurprising that fruit bats prefer to use scent to find ripe fruit of their preferred kind, although they do use echolocation as well, especially when searching for fruit in the first place. On the other hand, at least some fruit bats can see in colour, if not as well as we can, so it's not impossible that they use this, too.

In order to diversify, and eat different kinds of fruit, the different kinds of New World fruit bat have evolved subtly different physical forms, behaviours, and physiological adaptations. One of the more obvious physical differences is the size of the different kinds of bat. Even within the same genus, larger bats tend to eat larger fruit (rather than eating a larger amount of similarly-sized fruit), at least partly because they apparently like to carry it away to eat at their leisure, and they have to be big enough to fly while carrying it. Jaw strength is another factor, with the bite forces of various bats being matched to the strength they need to crack open their favourite food, and no more than that.

Behavioural adaptations include the habit of Jamaican fruit bats, a species of Artibeus, of squeezing fruit before eating it, to extract the maximum amount of juicy pulp; when they eat fruit with more seeds and less pulp, they leave the indigestible seeds behind, along with the rind, so that what they do eat is more nutritious. An example of a physiological adaptation, on the other hand, would be that Sturnira bats have less active digestive enzymes in their guts than their relatives, presumably because their diet is sufficiently high in sugar and protein that it isn't worth the trouble of breaking it down any further. Similarly, while equally good at digesting various kinds of sugar, they prefer to drink solutions of sucrose, which they expect to find in fruit, to glucose/fructose solutions, which they don't. It is, incidentally, less clear whether or not certain bats might be immune to unpleasant compounds found in particular fruit, although it does seem plausible.

These preferences for different foods help to keep the bats separated, ensuring that there's enough food to go round without them getting in each others' way. This is partly related to where favoured food types are found, with figs and Cecropia fruit being found in relatively high branches, and tomato-like plants and pepper down in the undergrowth. Thus, we find that fig-eating Artibeus species are more common in lowland habitats such as the Pantanal wetlands and dense, humid forests, and Sturnira bats more common in places like high altitude cloud forests. Going down to a smaller scale, coffee plantations, which have relatively little natural undergrowth (coffee being a shrub, and largely inedible to fruit bats), also discourage Sturnira bats, but are still frequented by others.

In general, it seems that these sorts of patterns are the best way to predict where you will find particular kinds of fruit bat. This is especially relevant where you're trying to conserve bat species, and trying to determine how disturbed habitats - such as coffee plantations - will affect the local wildlife. Since fruit bats distribute seeds throughout the forest, and, because of their flying ability, can do so over significant distances, this affects more than simply the bats themselves.

Indeed, the picture is likely more complex than I've described above. There are over forty different species in the three genera of fruit bat that I've talked about, and something has to keep them from competing with each other as well. In fact, new species are still being discovered, so the actual total is surely even higher than we know.

Bats may be often overlooked when considering ecology in the tropics, in favour of more obvious creatures such as monkeys, birds, and jaguars - or even tropical frogs, which at least have the advantage of looking pretty. They're often drab (although not always), only come out at night when you can't see them, and fly about where it's hard to study them. But they are as important a part of the network of ecology as anything else and, fruit bats are amongst the most common of all mammals in the American tropics. By extension, only a few of them are currently endangered, but it would be best to keep it that way. They may need jungle, but the jungle also needs them.

[Photo by Diego Lizcano]

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