Sunday, 3 November 2019

Bat Poo and Fig Trees

Seba's short-tailed bat (Carollia perspicillata)
feeding on wild pepper
If the presence of plenty of carnivores is generally bad news for herbivores, it may seem that the presence of plenty of herbivores is bad news for plants. Clearly, such problems as over-grazing do exist, but the reality can often be more complex. Fruit are a particular case in point.

Many fruits are tasty specifically to encourage herbivores to eat them, containing highly resistant seeds that pass through the herbivore's digestive tract, land in a nice pile of manure, and germinate to create more plants in future. It has been estimated that, in most wild forest environments, anything from 45% to 90% of tree species bear edible fruit of this kind. (Of course, a number of the fruits we see in supermarkets are even tastier, because we've bred them that way, with the banana being perhaps the most extreme example. But it's not as if wild apples and oranges, for example, don't exist and aren't attractive to animals).

Not all herbivores eat these kinds of fruit, as opposed to other things that botanists would call "fruit", such as grass seeds. Primates are one of the more obvious examples of mammals that do, and it's theorised that our unusually good colour vision arose in part so that we could easily tell which ones were ripe. But another group of mammals that eat a strongly fruit-based diet are, unsurprisingly, the fruit bats.

The term "fruit bat" probably conjures up the image of the large long-snouted bats of Africa, tropical Asia, Australia and the Pacific. But these are by no means the only bats to eat fruit. The Americas are home to another group of frugivorous bats, sometimes called "fruit-eating bats" in a possibly rather desperate attempt to avoid confusion with the Old World sort. There are a great many of them, and they look much more like the other bats we are familiar with in the west,. Indeed, they belong to the same family as a number of insect-eating species, not to mention, among other things, the infamous vampire bats.

Despite this, compared with seed dispersal by birds, and even by ground-dwelling mammals, such as bears and raccoons, fruit bats (of both kinds) have received relatively little attention. This is not to say, however, that such studies don't exist. Indeed, especially over the last couple of decades, there have been enough that we can now form a reasonable overall picture of how beneficial the bats are to the plants they are feeding on.

From the plant's perspective, it is not enough for their fruit to include juicy, nutrient-filled flesh around the seeds, or to be visibly attractive to potential frugivores. All of this would be a pointless waste if the seeds could not subsequently survive being eaten and digested. So, when we look at how fruit seeds are affected after being eaten by bats, it's not surprising that, on average, the answer is "very little" - seeds of this type are just as likely to germinate after being eaten as they would be if they simply dropped off the tree.

But that "on average" conceals rather more complexity than one might think. There are some fruits, such as mulberries, that are regularly eaten by bats but which still suffer from the resulting digestive processes. Perhaps these are more commonly eaten by other frugivores, or perhaps the benefit of being carried to some new place outweighs the disadvantage of being slightly less likely to germinate once the seeds get there. It's not as if plants produce one seed at a time, after all; they get plenty of attempts.

But if some seeds are inevitably damaged by animals trying to eat them, there are others that actively benefit from the experience. These seeds are actually more likely to germinate if they have previously passed through a bat's digestive tract than if a human simply removes the pips and drops them on the ground (the usual method of comparison). It's not certain why this is, although it's probably something to do with the seed's tough outer coat being partially stripped away by digestive juices.

Figs seem to be one of the best examples of this, and it's noteworthy that these are often a favourite fruit of bats. Indeed, it has been shown that some kinds of ripe fig produce scents that are especially attractive to bats, but less so to birds, while the reverse is true of other species.

The same is true, to a lesser extent, of the species of bat that is doing the eating. That is, some bats have a slight positive effect on the seeds they eat (as measured by their likelihood of subsequent germination), while others either make little difference or make things worse.

An example here comes from pepper (as in peppercorns, not bell peppers) which seem to benefit more from consumption by short-tailed bats (Carollia spp.) than they do from being eaten by some other fruit bats (Artibeus spp.) that live in the same area. The reverse seems to be true of some cecropia seeds, suggesting that some fruits are adapted to the digestive systems of particular kinds of bat.

Quite how that would work isn't entirely clear, but there are some possibilities. In particular, short-tailed bats are typically smaller than Artibeus species, which means they have a shorter digestive tract, and food passes out the back end rather more quickly. In general, that's probably a good thing for the seeds, but cecropia has a slimy pulp around the seeds, the partial removal of which might make them more likely to germinate. Short transit times in smaller bats might, therefore, not give the digestive juices long enough to do their work.

Having said all of which, the fact remains that, in most cases, passing through a bat's colon does nothing either particularly good or particularly bad to the sort of seeds that they are likely to eat. Generally, having its seeds be carried away to somewhere new is enough of a benefit for the plant. Bats are just one group of creatures responsible for dispersing seeds through a forest or other green environment and, in general, birds play a larger role. But, for some plants at least they can be particularly important, and there can be a real benefit to being in pat poo.

[Photo by "Desmodus", from Wikimedia Commons.]

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