Sunday, 3 July 2022

Bats in the Fynbos

Lesser horseshoe bat
From a human perspective, the ability of bats to navigate in pitch darkness through the use of biosonar is undoubtedly remarkable. Not only do they have to be able to avoid obstacles, but also, in the case of the insect-eating species, to locate and identify tiny prey items against the background. Their ability to do this is affected by a number of environmental factors, and the exact nature of the echolocation calls, and how the bat interprets them, may affect which of those factors have the most influence.

For example, the distance that a biosonar ping can travel is affected by both the temperature and humidity of the air through which it travels. Since there are bats in almost every habitat suitable for insects to live, we might expect the way that they use their sonar to vary depending on the local climate.

Bergmann's Rule, proposed all the way back in 1847, is a general principle that larger animals tend to live in colder environments than smaller, but otherwise very similar, animals - so that, say, polar bears are bigger than tropical bear species. But there are many exceptions, and in 1970, ecologist Frances James proposed a modification now known as James' Rule, which zoomed in on individual species and added humidity into the mix. This states that animals living in hot humid environments are generally smaller than members of the same species living in colder, drier, ones. 

She was able to demonstrate this by looking at the variation in body sizes of woodpeckers, nuthatches, and other individual bird species across the eastern and central United States. But when we try to apply the same rule to bats, it just doesn't work. That's probably because smaller bodies would lead to higher frequency echolocation calls, which don't travel so well in hot, humid air, giving the bats a powerful evolutionary incentive to buck the trend. Which, incidentally, could lead to the sort of ecological consequences of climate change that we might not immediately think of.

Another significant factor affecting a bat's biosonar is the concept of "clutter": the more clutter there is in the background, the harder it will be to distinguish an insect at a distance. This clutter is primarily in the form of vegetation, so this tends to reinforce the effects of climate since hot, humid environments also tend to have more plants growing in them. But, even where that is not the case, it still makes sense that bats would have to adjust their echolocation calls depending on how cluttered, or how empty, their local environment is.

Bats, like dolphins and toothed whales, typically use a "low duty cycle" form of echolocation, where they pause between sonar pings so that they have time to listen for the echo coming back before sending out the next call. Since this means that they can't send out signals more rapidly, the solution is instead to use higher-pitched calls, which increase image resolution at the expense of losing long-range detection. Should they need to detect prey at a longer distance... well, they basically have to shout louder

This leads to what's termed the "foraging habitat hypothesis" which basically says that bats flying in cluttered environments should increase the frequency (and volume) of their echolocation calls to make it easier to pick out the small signals of an insect from the crowded background. The black vesper bat (Myotis nigricans) of Latin America is one species that has been shown to do this, being able to vary the nature of its calls depending on where it is searching for food.

This approach is evidently effective, but a few bat species are able to use more subtle methods to achieve the same ends. These so-called high duty cycle bats are able to analyse the Doppler shift in returning echolocation signals to precisely identify the wingbeats of flying insects. This allows them to easily distinguish their targets from background vegetation without the need to further modify the outgoing signal. And, since the Doppler shift also allows them to tell the difference between outgoing and incoming calls, they don't have to wait for the returning ping before sending out the next one. They achieve this impressive feat using specialised brain structures that are somewhat analogous to those used elsewhere in the brain to allow acute vision.

Partly because there are not as many bat species that use this technique and partly just because it's more complicated, less is known about how high duty cycle bats vary their calls in different environments. For instance, there could still be an advantage to using lower-pitched calls in open habitats, to get the increased range, even though this wouldn't make any difference to their ability to find prey against the background.

Among the bats known to use this technique are the horseshoe bats of the genus Rhinolophus, which are distinctive enough to be placed in their own family, distinct from those of all other bats. There are over 100 recognised species, which are present across much of the Old World. Although they are absent from extreme environments such as Siberia and the Sahara, and some are found in only very small localities, others are widespread. The greater horseshoe bat (Rhinolophus ferrumequinum) for example, lives in both Japan and the UK, and over a fair proportion of what's in between, covering a good range of different habitats in the process.

The Cape horseshoe bat (Rhinolophus capensis) is native to southern South Africa, where it primarily inhabits the fynbos, an area of shrubland and heath similar to the scrubs of the Mediterranean and coastal California. It feeds primarily on moths and beetles, which it plucks from vegetation or captures on the wing. However, at the extremes of its range, it can also be found on the southern edge of the Namib Desert, where it overlaps with the more desert-adapted species the Damaraland horseshoe bat (Rhinolophus damarensis) which had been less studied, but probably has a similar diet.

Using microphone recording arrays positioned near caves known to be inhabited by the relevant species, a recent study compared the calls of Cape horseshoe bats in the fynbos and on the desert margins with those of nearby Damaraland bats. These showed that the Cape bats living in the desert used lower-pitched calls than those in the fynbos, but that this wasn't enough to allow the sound to carry much further, as we'd expect it to. Nonetheless, they compensated for this by the simple tactic of increasing the volume, meaning that, in the absence of clutter in the open desert, they could search a larger volume of space for tasty moths.

We'd expect the Damaraland bats to do the same, compensating for the fact that their calls are inherently higher-pitched by upping the volume, but it turns out that they didn't, meaning that they could not detect insects at the same range as the more southerly species could. It's not that they aren't capable of shouting more loudly, since on a couple of occasions they really screamed it out, easily beating the Cape bats for volume. But both these occasions were on them leaving their cave, and were very much not what they typically do. It could be that the effort of constantly emitting really loud calls isn't worth the advantage gained in finding prey more quickly... it's all just too exhausting to keep up for long.

This leaves open the question of why the Cape horseshoe bats bother to lower the pitch of their calls when foraging in the desert compared with their preferred habitat of the fynbos. It's not getting them much in the way of increasing their prey detection range, because they achieve that by using a higher volume and, in any event, the Damaraland bats, which are better suited to the desert than they are, apparently don't need the extra range.

It might be that they eat different prey since we don't know much about exactly what Damaraland horseshoe bats eat beyond it being "insects". Perhaps more likely, where the two bats live together they might find it useful to use different calls to distinguish each other - we do know, for instance, that the two species can tell each other apart based solely on the sound of their calls.

But the reality is that we don't know. There may be some complication of their lifestyles, or some quirk of their evolutionary histories involved but, either way, there's something going on here that we wouldn't predict based simply on our understanding of the acoustics.

[Photo by F.C. Robiller, from Wikimedia Commons.]

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