Sunday, 31 October 2021

Flesh-eating Bats

Greater noctule bat
(Well, it is Halloween...)

At least in the parts of the world with a temperate climate, the great majority of bat species are insectivorous. Of course, there are a number of vegetarian bats, especially in the tropics, including the large fruit bats of the Old World as well as smaller, often fig-eating, species in the Americas. But insect-eating does seem the default, even though there is general agreement these days that bats are more closely related to the larger carnivores and to the hoofed mammals than they are to the other small insectivores, such as shrews and hedgehogs.

From an ecological, trophic web, perspective, insectivores are a type of carnivore - after all, they eat other animals, even if they're small ones. But, when talking about mammals (and similarly sized vertebrates) most researchers tend to draw a distinction between those that hunt invertebrate prey and those that eat comparatively large vertebrates. So there's a distinction between leopards and killer whales on the one hand, and shrews and anteaters on the other.

In this sense, animals such as lions are "true" carnivores, while bats, for example, are something slightly different. Except... some bats do kill and eat warm-blooded prey.

Typically, the reason for this is that insects are in short supply during the winter. The fact that many bats hibernate means that this is less of a problem than it might be, but some of the larger, tropical (and hence, non-hibernating) bats do switch to eating flesh at certain times of the year.

One example is the greater false vampire bat (Macroderma lyra) of India and Southeast Asia. For most of the year, this feeds primarily on insects although, being a moderately large bat, it tends to go for big insects - something like half of its diet consists of cockroaches, and another quarter is composed of beetles of various kinds. Which is fine during the monsoon and surrounding wet season, when insects are everywhere, but during the dry season, while they don't necessarily vanish, they're less common. So, at those times of the year, the bat instead catches and eats small rodents.

The ghost bat (Megaderma gigas) is a close relative of the false vampire, and has a similarly appropriate name for the date this post is going out. It's even bigger, being something like 12 cm (4½ inches) in length, and lives along the tropical north coast of Australia. This is far enough north for there to be a significant wet season and, at such times, the bat feeds on large insects - in its case cicadas, grasshoppers, and beetles. But, when it gets drier, it starts to eat mice, birds such as honeyeaters and owlet-nightjars, and even other bats. The more distantly related large slit-faced bat (Nycteris grandis) of Zimbabwe has a similarly seasonal diet, eating insects in summer and frogs and smaller bats in winter.

There are, broadly speaking, two different strategies used by bats while hunting insects: gleaning and hawking. 'Gleaning' involves capturing prey while it is resting, often sitting on a branch or leaf, but possibly on the ground or the surface of a pond. The three species mentioned above all hunt their larger prey in this manner, although they are capable of using other strategies. The large slit-faced bat, for instance, catches over 90% of its prey - large or small - by gleaning it, using a mixture of echolocation and simply listening for the sounds it makes.

The greater noctule (Nyctalus lasiopterus), however, behaves differently. This is the largest bat in Europe, with a wingspan of around 45 cm (18 inches), and is found across the southern and eastern parts of the continent. During the summer, like any reasonable European bat, it feeds almost entirely on insects - mostly moths

But, as insects become scarcer in the autumn, it isn't only bats that feel the pinch but also insect-eating birds, which migrate in huge flocks to the warmth of Africa where plentiful food is still to be found. And the greater noctule bat is one of the few nocturnal animals to really take advantage of this, shifting its diet to almost nothing but birds at the peak of their migrations. Not only that, but while we've never actually seen it doing so (it would be hard to observe) there's every indication from the timing and the bat's own anatomical adaptations that it does so by 'hawking' - that is to say, it catches and kills birds in mid-flight.

Elsewhere, there is some evidence that the even larger great evening bat (Ia io) of Southeast Asia does something similar. (And, yes, when Oldfield Thomas gave it that scientific name in 1902, he did so deliberately to make it the shortest scientific name of any animal in the literature). The birdlike noctule (Nyctalus aviator) of Japan and Korea doesn't seem to focus on migrating birds specifically, but it does eat birds in the winter, and probably catches them in the same way as its larger cousin.

But not all bats that eat vertebrate prey do so only when insects are in short supply; some do so all year round. In most cases, these are large bats that opportunistically kill small mammals or birds but for which such animals form only a small and occasional part of their diet. Perhaps the best-known example that fits this description is the greater spear-nosed bat (Phyllostomus hastatus), which is large enough to kill vertebrates, and sometimes does so, but is really an omnivore that mostly eats insects and fruit. But some bats really seem to prefer eating fresh meat and do so on a regular basis.

Admittedly, there are no bats that are true specialist carnivores in the way that, say, cats are - they'll always eat insects, or even some vegetation, from time to time. But some do get fairly close. 

For instance, the fringe-lipped bat (Trachops cirrhosus) of the American tropics does eat a fair amount of insects, but, at least in Costa Rica, its preferred food seems to be lizards, and it has also been reported to eat a large number of frogs, along with some birds and other warm-blooded prey.

The single largest bat species in the New World, and the largest bat overall other than the vegetarian "flying foxes", is the mighty spectral bat (Vampyrum spectrum) with a wingspan of up to one metre (3' 3"). Both this and its closest living relative, the somewhat less atmospherically named big-eared woolly bat (Chrotopterus auritus), eat a significant number of both birds and other bats, topped up with small rodents. 

In fact, the spectral bat is probably the closest thing there is to a purely carnivorous bat since, while it's technically an omnivore that even eats the odd bit of plant matter every now and then, something like 90% of its year-round diet consists of birds and small mammals. It isn't eating particularly small birds, either, some of them being roughly the same size as itself. It mostly seems to target those that nest in trees at night (rather than, say, burrows) and may hunt using scent as much as echolocation or hearing.

It may be relevant that the fringe-lipped bat belongs to the same taxonomic "tribe" of bats as the spectral and big-eared woolly bats, meaning that all three fall into the same relatively narrow corner of the larger bat family tree. We don't know much about the other species that fall into this particular phylogenetic branch but while some are clearly insect-eaters, there are reports of at least one other species in the group feeding on birds - although how often it does so is unclear. The group is marked by relatively powerful jaw muscles and large, sharp, teeth, so this may, along with overall body size, be relevant to their diet. It's notable, however, that we can't say the same of the seasonal flesh-eaters even though some, such as the great evening bat, aren't much smaller.

Another feature that these highly carnivorous bats have in common is that, by the standards of bats, they have unusually large brains and moreover, ones with more complex folding on the surface. Although this is obviously a crude measure, and would need support from actual behavioural studies, it might mean that they are more intelligent than other bats, something often seen in other carnivorous mammals. Perhaps it takes more brains to kill something that at least has a chance of fighting back...

[Photo from Popa-Lisseanu et al. 2007, available under the CC-BY-2.5 attribution license.]


  1. For the seasonal flesh-eaters, is it known why they prefer insects when available? More calories per effort spent hunting them?

    1. Most likely. They don't tend to have the anatomical adaptations to carnivory that the full-time meat-eaters do, so they're probably just better adapted to catching insects, and it takes less effort.

  2. Considering that the definition of a hypercarnivore is an animal where over 75% of its diet is made out of other animals, the spectral bat legitimately would qualify as a hypercarnivorous bat.