Chewed bones and bird-eating microbats

One of the several children I was looking after yesterday knocked over a tray of rabbit bones I’d left to bleach in the sun. The bones came from a fox’s cache I’d discovered in a supermarket car-park, and as I picked them up off the ground the gnaw marks and missing ends of one of the long bones reminded me of a fascinating subject that I wish more people knew about. Not fox predation on rabbits, no, but bat predation on birds.

I’m planning to post various entries on bats at some stage (including on New Zealand’s mystacinids, recently discovered European bats, and on megabat evolution), but haven’t gotten round to it yet. As is arguably also the case with rodents, lizards or passerines, there’s enough interesting stuff to say about bats for one to become occupied with them full-time, such are their numbers (over 1000 species) and diversity. And among that diversity are a couple of lineages that have experimented with predation on other tetrapods, and I’m not talking about vampires (but I will another time).

Megadermatids, the false vampires or yellow-winged bats, are a group of the Old World and Australasian tropics. The best known members are the Australian false vampire or Ghost bat Macroderma gigas and the two Asian false vampires of the genus Megaderma. Long thought to be blood-feeders, megadermatids are odd in lacking upper incisors, and in having strongly reduced, thread-like premaxillae. They’re big, with Macroderma weighing up to 150 g and having a wingspan of 60 cm. Nycterids, the mostly African slit-faced bats, will also prey on tetrapods (frogs, birds and smaller bats). Phyllostomids, the New World leaf-nosed bats, include the tetrapod-eating genera Vampyrum, Trachops (famous for being a specialist predator of singing frogs) and Chrotopterus. They’re superficially like megadermatids, and even bigger, with Linnaeus’ false vampire Vampyrum spectrum having a wingspan of about 1 m (making it the largest microbat).

A few other tetrapod-eating bats can be found in other microbat clades, like Vespertilionidae (more on them in next post). A few pteropodids (fruit bats), notably the Hammer-headed fruit bat Hypsignathus montrosus, have been reported to feed from carrion and even attack tethered chickens (Hill & Smith 1984), and the New Zealand mystacinids are also known to sometimes scavenge from carcasses.

The foraging and predation techniques used by megadermatids have been reasonably well studied. They do eat invertebrates, but a proportion of their diet is made up of mammals (including rodents and other bats) and birds, and they also eat frogs and fish. Of all bats, megadermatids perhaps have the most flexible foraging style. They use low intensity, broadband FM echolocation calls, but also listen acutely with their massive ears for prey-generated noises, and they may also hunt prey by sight. Altringham (1999) drew attention to this possibility, noting that the eyes of some species ‘are almost owl-like’ (p. 219).

Using steath and darkness while hunting, they fly slowly among trees and shrubs and rocky areas, often close to the ground. Their attack on a tetrapod is culminated as the bat drops onto the prey, encases it in its wings and bites it hard on the head or neck (Kulzer et al. 1984). The prey is then carried back to the roost where it is eaten, and they can carry prey weighing about 60% of their own body weight. Mammal prey are usually eaten entirely, with just bits of the skull and parts of the gut, legs and tail being dropped to the floor. The situation with birds is somewhat different however, and went mostly unstudied until Walter Boles (an Australian palaeornithologist) produced a detailed paper on bird predation as practiced by the Ghost bat Macroderma gigas (Boles 1999).

The documented bird prey of Macroderma includes some 50+ species, most of which are ground-foraging passerines. Owlet-nightjars seem to be important prey items. In contrast to what happens with mammal prey, birds aren’t eaten whole. Damage to the posterior part of the sternum shows that the bats eat tissue from the abdominal region, and leave the area around the anterior part of the sternum and the coracoids alone. The distal parts of the wings and legs get dropped intact and undamaged, as do remiges and rectrices. Humeri seem to get chewed up and eaten (Boles 1999).

Given that Boles is a palaeornithologist, you should now be wondering why a specialist on fossil birds was so interested in the behaviour of an extant bat species. One of the most famous Cenozoic fossil sites in the world is Riversleigh in north-west Queensland (see Archer et al. 1996 for outstanding coverage of the whole fauna). Numerous bird fossils are known from Riversleigh, including ratites, dromornithids, storks, rails, raptors*, parrots, kingfishers, swifts and passerines (including logrunners and lyrebirds).

* Pengana robertbolesi, a Riversleigh raptor named by Boles (after his father), is yet another raptor that convergently evolved a hyper-mobile tarsal joint like that of Polyboroides (Boles 1993). If this interests you go here.

The bat fauna discovered at Riversleigh is also diverse: literally millions (according to Archer et al. 1996) of bat bones are known from there, representing (as of 1996) ‘more than 35 different kinds of bats’ (p. 135). Interestingly, at least five of them are megadermatids. And what’s really interesting is that many of the Riversleigh bird bones represent those same skeletal elements that megadermatids drop from the bird carcasses they eat, and possess damage matching that caused by megadermatids. It therefore seems that megadermatids were important accumulators of the avian remains discovered at Riversleigh (Boles 1999, p. 88).

It’s well known that owls are important accumulators of vertebrate remains, particularly in caves, but this is the first study showing that bats play this role as well. Given that, as discussed above, there are other tetrapod-eating bats elsewhere in the world, it would be interesting to know if these species also produce piles of bones beneath their roosts. I wonder if anyone has looked.

Megadermatids are well known as predators of smaller tetrapods, so no one would be really surprised to hear about any of the stuff I’ve just discussed above. But you might be surprised to learn, as I was, that an inoffensive little vespertilionid is also an awesome tetrapod predator. That’s what we’ll be looking at in the next post. PS - for the latest news on Tetrapod Zoology - including posts on vampire bats - do go here.

The Macroderma image was borrowed from Perth Zoo's site.

Refs - -

Altringham, J. D. 1999. Bats: Biology and Behaviour. Oxford University Press, Oxford.

Archer, M., Hand, S. J. & Godthelp, H. 1996. Riversleigh: The Story of Animals in Ancient Rainforests of Inland Australia. Reed Books, Kew, Victoria.

Boles, W. E. 1993. Pengana robertbolesi, a peculiar bird of prey from the Tertiary of Riversleigh, northwestern Queensland, Australia. Alcheringa 17, 19-25.

- . 1999. Avian prey of the Australian ghost bat Macroderma gigas (Microchiroptera: Megadermatidae): prey characteristics and damage from predation. Australian Zoologist 31, 82-91.

Hill, J. E. & Smith, J. D. 1984. Bats: a Natural History. British Museum (Natural History), London.

Kulzer, E., Nelson, J. E., McKean, J. L. & Moehres, F. P. 1984. Prey-catching behaviour and echolocation in the Australian ghost bat, Macroderma gigas (Microchiroptera, Megadermatidae). Australian Mammalogy 7, 37-50.