Sunday 14 October 2018

Bats That Eat Fish

Greater bulldog bat
With over 1,300 different species of bat known, it shouldn't really be surprising that there's a wide range of different dietary habits among them. Of course, a lot of that is on a fine scale, as I discussed just last month, but there are more significant differences in the kinds of things that bats feed upon. Insects, fruit, nectar, even blood all form the core diet of at least some kinds of bat.

One of the more unlikey food sources, however, is fish. Yet, when you think about it, fish is a really common food source for birds - there are a huge number of bird species that feed primarily on fish, including seagulls, kingfishers, and ospreys, among many others. Clearly, there's no inherent reason why a flying mammal couldn't eat fish, yet very few of them do.

As a recent review makes clear, however, "very few" is not "none".

But how many is 'few'? We have to begin by saying that a significant proportion of those 1,300 species of bat have never been looked at very closely. While many of them don't live anywhere that they're very likely to eat fish, or else have adaptations that clearly rule it out, there's a fair bunch that might, but we just don't know for sure.

Added to that, there are some species where we have examined the animals' dung or stomach contents, and found fish scales or other evidence that they must eat fish at least occasionally. But we've never seen them actually doing so, and it's at least possible that they, for example, scavenged on a dead fish that happened to wash up on a river bank or whatever.

There are four exceptions, most of which are "mouse-eared" bats or "myotises". Rickett's big-footed bat (Myotis pilosus) from China and Vietnam is often said to eat fish, although only one population has been spotted doing so regularly. Similarly, we know that the long-fingered bat (Myotis capaccinii) of southern Europe and north Africa eats fish on occasion, but, again only some populations do this on a regular basis.

The primary diet of the fish-eating myotis (Myotis vivesi) is, in fact, crustaceans, but the name isn't entirely inaccurate, and there's at least one place where the local population eats fish surprisingly often. It's particularly impressive by virtue of the fact that it catches them at sea - the other two myotises mentioned find them in ponds and rivers.

And, finally, there's the greater bulldog bat (Noctilio leporinus) of Central and South America, which seems to eat little else but fish - it's favourites appear to be anchovies, thread herring, and silversides.

The three myotis species are closely related, but live on different continents, and so it's unsurprising that they aren't each others' closest relatives - they belong to different branches within the vast myotis family tree. This suggests that their habits evolved relatively recently in evolutionary terms, perhaps in response to local conditions.

The greater bulldog bat, however, seems to have been around for about two million years, and presumably has been catching fish for at least that long. It has only one relative that's at all close (the "lesser bulldog bat", unsurprisingly), and that mainly eats water beetles.

The reason that more bats don't do the same is likely a combination of the fact that insects aren't in short enough supply for it to be worth seeking out new food sources, and more importantly, that it's just plain difficult. So how do fish-eating bats manage it?

First, find your fish. This is an issue for bats, since their sonar simply bounces off the water surface, and they are quite unable to 'see' what's swimming beneath it. In the case of the myotis species, which use the kind of broad-band FM calls that are ideal for short-range detection of insects, this means that the fish has to break the surface before the bats can detect it, and they have to be close by to do so.

Bulldog bats, however, use a different kind of call that is more suitable for detecting weak echoes at long distances, which means that they can home in on ripples on the water surface that imply a fish is swimming just beneath it. Because they have larger wings and more powerful flight than the myotis bats, they can swoop in from a long distance, precisely targeting fish that remain below the water line.

As they approach the target, fishing bats continue to use sonar, even if the fish begins to dive - even if they can't detect the fish, they still need to know exactly where the water surface is. At this point, if they were hunting insects, myotises would normally switch to a wider angle, lower bandwidth call, which helps them locate targets trying to dodge out of the way, but they don't always bother when hunting fish, presumably because this is useless once the fish is fully underwater again. Bulldog bats instead switch to FM calls, to home on the water ripples from close range.

Bats use their feet to catch the fish, in much the same way as they would snatch insects resting on the water surface. Once again, bulldog bats, with their strongly fish-based diet, are more effectively adapted for this than the myotises. They have unusually large feet, with long, narrow toes and sharp claws, and they can raise their tail out of the way, keeping it mostly dry even as their feet rake through the water to grab the submerged target. Even the soles of their feet are smooth and hairless in comparison with those of other bats, making them more streamlined, and so reducing the drag that the bat feels as it swoops towards its prey.

The myotises, being primarily adapted for catching insects, and only adding fish as a supplement to their main diet, have similar, but less striking adaptations - for example, they do get their tail wet when they grab fish, since it isn't flexible enough to hold out of the water. Their claws are also a lot smaller, so, while a bulldog bat can simply stab the fish anywhere it likes to haul it out of the water, the myotises have to try and stick their toes into its gill slits to give them something to grab onto.

Fish are, of course, far bigger than insects, requiring a lot more effort to lift and handle. Here again, the bulldog bats have the advantage: they are at least double the size of the myotises, and can take larger prey. If the fish is really small, the bat can simply eat it on the wing, as they would with an insect, but more typically, they have to carry it somewhere and eat it while perching, much as an osprey might.

Interestingly, bulldog bats can even echolocate with their mouths full, since they have cheek pouches that can store the food while leaving the rest of the mouth free. Myotises can't do this so far as we know, although, since they also carry the fish in their mouths after grabbing it (presumably because they need their feet in order to land) this likely poses something of a problem for them.

All of these difficulties explain why it is that so few species of bat catch fish. Those that do do so have likely adapted the techniques they originally used for catching aquatic insects to hunt for larger prey. It's a testament to the diversity and adaptability of mammals that they're really quite good at it.

[Photo by Susan Ellis, of Available under Creative Commons Licence 3.0.]

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