So in the previous post – required reading before you get through this one, sorry – we looked at the various hypotheses that have been published on the origin of sanguivory (blood feeding) in vampire bats. We saw that only two hypotheses matched with the phylogenetic pattern of feeding styles seen in phyllostomid bats and their relatives, and of those two theories – one proposing that vampires evolved from oxpecker-like ectoparasite-eaters, and the other proposing that vampires originated as insectivores that switched to wound-feeding – both have shortcomings. However, one final hypothesis is hinted at by the phylogenetic distribution of feeding types, but has yet to be proposed so far as I know….
Before I get started, what do you all think of the banners? They are not necessarily going to be there for ever – things will change – but they’ll do for the time being. A prize* for whoever gets the most enlightenment out of the assorted objects shown in the banner above. Every object there has some sort of personal significance.
* Subject to availability.
Back to the bats… One thing that makes phyllostomids different from most other microbat clades is the widespread habit of nectarivory. While specialised nectarivory is restricted to glossophagines and their close relatives (the long-nosed bats, long-tongued bats and flower bats), part-time or complimentary nectarivory is tremendously more widespread; so much so that its phylogenetic distribution strongly indicates that nectarivory is primitive for the clade that includes all phyllostomids except vampires (Wetterer et al. 2000, p. 167). Given this predominance of nectarivory, it is very tempting to take its origin just one step back to the origin of the entire phyllostomid clade, though there is no evidence from fossils nor from vampire diet that to do so would be correct. It is, however, not an unreasonable speculation [adjacent image shows the nectarivore Glossophaga; from the Tropical Bats site].
Is it conceivable that sanguivory might evolve from nectarivory? The part-time nectarivores among Phyllostomidae eat insects as well as nectar and pollen, and it is thought that this so-called food source duality encouraged, in some clades, the development of dedicated use of pollen, nectar or fruit (Gillette 1975, Ferrarezzi & Gimenez 1996). Could it also be that a food source dualist, used to feeding on both insects and nectar, was attracted to animal wounds, and eventually able to make the switch to sanguivory? Nectarivores are generally* arboreal or aerial feeders rather than terrestrial ones, so any bat that did make this switch would be assumed to have done so in arboreal habitats; a point which agrees with the fact that sanguivory in vampires apparently first arose among taxa that fed on bird blood [adjacent image, from here, shows a White-winged vampire Diaemus youngi, feeding on a chcken].
* Exceptions include some lizards and the New Zealand bat Mystacina.
All in all this is highly similar to Fenton’s wound-feeding hypothesis, but differs in that the vampire ancestors were not strict insectivores, but also used to lapping at viscous fluids. It is also supported by the fact that some food source dualists among phyllostomids have been reported to very occasionally feed on blood.
If a preadaptation to food source duality involving nectarivory was integral to the evolution of sanguivory, it would explain why sanguivory hasn’t evolved among any of the many other bat clades of the world: there are other nectarivorous bats (notably, many of the fruit bats), but none of them are also part-time insectivores. The other hypotheses also fail to explain the absence of other sanguivorous bats: if wound feeding led to sanguivory as Fenton (1992) argued, then why – so far as we know* – haven’t other insectivorous bats also evolved wound-feeding and sanguivory? Fenton did draw attention to the fact that many Old World insectivorous bats lack the robust incisors of phyllostomids, and it is conceivable that this was an additional constraint to non-American bats [the adjacent image shows the predatory phyllostomid Chrotopterus auritus eating a smaller bat. It's not relevant to the text but sure is a cool photo. It's from Tropical Bats]
* I say ‘so far as we know’ as there are tales of a sanguivorous bat dubbed the ‘death bird’ from Ethiopia. The case was discussed by Shuker (2003) but is just as likely to result from superstitious fear of bats as from a hitherto undocumented species.
At long long last, that’s it for the time being on vampire bats, and ‘next’ I will get round to dealing with those promised alleged vampire pterosaurs. For previous posts of mine on bats please see We flightless primates, Chewed bones and bird-eating microbats and Greater noctules: specialist predators of migrating passerines. The latter is particularly relevant right now as a new paper on this subject has just appeared. Popa-Lisseanu et al. (2007) used stable isotopes to trace the diet of Greater noctules, and they provide strong substantial support for the remarkable idea that this bat is a specialist predator of nocturnally migrating passerines.
Refs – -
Fenton, M. B. (1992). Wounds and the origin of blood-feeding in bats Biological Journal of the Linnean Society, 47, 161-171 DOI: 10.1111/j.1095-8312.1992.tb00662.x
Ferrarezzi, H. & Gimenez, E. d. A. 1996. Systematic patterns and the evolution of feeding habits in Chiroptera (Archonta: Mammalia). Journal of Comparative Biology 1, 75-94.
Gillette, D. D. 1975. Evolution of feeding strategies in bats. Tebiwa 18, 39-48.
Popa-Lisseanu, A. G., Delgado-Huertas, A., Forero, M. G., Rodriguez, A., Arlettaz, R. & Ibanez, C. 2007. Bats’ conquest of a formidable foraging niche: the myriads of nocturnally migrating songbirds. PLoS ONE 2(2): e205.
Shuker, K. P. N. 2003. The Beasts That Hide From Man. Paraview Press, New York.
Wetterer, A. L., Rockman, M. V. & Simmons, N. B. 2000. Phylogeny of phyllostomid bats (Mammalia: Chiroptera) data from diverse morphological systems, sex chromosomes, and restriction sites. Bulletin of the American Museum of Natural History 248, 1-200.