Desmodontines: the amazing vampire bats

I am fascinated by bats, and I wish I knew more about them. And among the most fascinating of bats are one of the few groups that all people around the world have heard of: the vampires....

Various non-sanguivorous bats are regarded as vampires by various people in various parts of the world, but true vampires - the only truly sanguivorous bats and indeed only sanguivorous mammals - are unique to the American tropics. Though often classified in their own microbat family, Desmodontidae, it is universally agreed today that vampires are part of the American leaf-nosed bat family Phyllostomidae*. Although separate 'family' status for vampires was championed by some workers as recently as the 1980s (e.g., Corbet & Hill 1980), inclusion of vampires among phyllostomids isn't a new idea: this is what Paul Gervais (1816-1879) thought as early as 1854, as did John Edward Gray (1800-1875) and several other 19th century mammalogists. Among phyllostomids, recent phylogenetic studies have mostly agreed that desmodontines are basal members of the group, and in fact one of the first lineages to diverge (Wetterer et al. 2000, Jones et al. 2002).

* Not Phyllostomatidae, as it used to sometimes be written.

There are three living vampire species: the reasonably large and widespread Common vampire Desmodus rotundus, the small, poorly known and rare White-winged vampire Diaemus youngi (only discovered in 1893), and the Hairy-legged vampire Diphylla ecaudata, the only species that has ever been recorded as far north as the USA. The White-winged and Hairy-legged species are mostly bird predators, while the Common vampire is mostly a mammal predator [adjacent image of White-winged vampire from here].

Vampire bats are fascinating for a whole host of reasons, and not just for the sanguivory. For their size, vampires are phenomenally strong, and able to carry up to a third of their weight and fly after consuming their own body weight in blood. They are reportedly far harder to euthanize with the chest-squeeze method than is normal for bats. They are long-lived, reaching 19 years or so both in captivity and in the wild. They are particularly agile compared to other bats and among the most terrestrially capable of all bats. Their large, powerfully muscled thumbs enable them to spring up from the ground - they are the only bats capable of 'flight-initiating vertical jumps' (Schutt et al. 1997) - and they are also unique among bats in being able to gallop (Riskin & Hermandon 2005). Bats appear to have lost the ability to run early on in their evolution, meaning that vampires have evolved running quite independently from all other mammals.

Needless to say, vampires are anatomically specialized. They have a particularly short muzzle that lacks a nose leaf, a peculiarity given that most other phyllostomids possess one. Pit organs in the nose are sensitive to infrared radiation and help them to locate capillaries just under the skin. Their upper incisors and canines are large and projecting, and both Common and White-winged vampires only have one pair of upper incisors (the Hairy-legged vampire has two pairs) [pic of teeth from here]. These teeth lack enamel and are shear-like with sharp cutting edges; they appear to be kept sharp by constant contact with the smaller lower incisors and canines. Vampires use their upper jaw teeth to shave away a small area of fur or feathers, and to then make a small, shallow incision. Using the long, pointed tongue they lap at the wound, the special grooves on the side helping capillary action (the tongue's upper surface remains free of blood) and the enzyme desmokinase preventing clotting. Desmokinase actually doesn't work on all sorts of blood: it does the job for cattle, horse and people, but not for the blood of sheep and dogs.

The lower incisors are widely spaced (to allow passage of the tongue during feeding) and bilobed, apparently to help the bat grip to the skin of a prey animal. The Hairy-legged vampire has a peculiar fan-shaped, multi-lobed lower incisor which recalls the similar incisors of the colugos. I don't know if its function has been determined (in colugos it's supposed to be a grooming tool). Molars are absent in the Common vampire, giving this bat the lowest tooth count (20) of any bat. The White-winged vampire has large glands at the corners of its mouth that can emit jets of a nauseating fluid (!).

Vampires are small, with the biggest individuals reaching 90 mm in head and body length, 350 mm in wingspan, and at most 50 g in weight. Their wing and leg bones have peculiar longitudinal grooves that help anchor their hypertrophied limb muscles. They have a T-shaped stomach that has a large secondary pouch, and during feeding it swells considerably: a bat drinks between 0.5 and 1.4 times its own weight in blood. Even while still feeding (a feeding bout lasts for between 10 and 60 minutes) the bat starts to urinate in order to get rid of the excess water and nitrogen contained within blood (blood is about 90% water). Accordingly their kidneys are highly specialized. Vampires are tailless and have a strongly reduced uropatagium (the membrane between the legs). The calcar - an ankle bone unique to bats that normally helps support the trailing margin of the uropatagium - is either absent or reduced to a tiny spur.

Because blood is pretty much pure protein, vampires have no fat reserves and hence are highly sensitive to cold. They also have a fast metabolism and will starve after three days without food. Common vampires unable to get a meal for themselves are now known, remarkably, to practice reciprocal altruism: they have a 'buddy system' where hungry bats beg blood from their roost-mates (usually, but not always, a related individual), and they apparently help those that have helped them in the past (Wilkinson 1984). Regurgitated blood is also fed to juveniles pretty much as soon as they're born, and then periodically during the pup's first year (yes, baby bats are called pups). The single pup is not only proportionally enormous (representing 20% of the mother's weight at birth), it is also cared for for a much longer time than any other bat and is only weaned when about 9-10 months old. Juveniles take a year or two before they can feed properly on their own, and juveniles will use the same wound sites as their mothers and presumably follow her as she hunts and feeds (Wilkinson 2001) [adjacent image from here].

Vampires are highly sensitive to dehydration - in fact they dehydrate by way of evaporation faster than any other mammal - and they roost in areas where humidity is at or approaching 100%. These physiological factors are debilitating if vampires aren't able to keep warm and suitably well hydrated.

It's perhaps worth noting by the way that myths about humanoid vampires do not come from the bats: if anything it's the other way round, with European myths and ideas about humanoid vampires being transferred to the American bats in the 1750s. At this time people thought that various bats from the Old World and elsewhere were blood-feeders, and in fact the view that the false vampires, ghost bats and yellow-winged bats of tropical Africa, Asia and Australasia were blood-feeders persisted until well into the 19th century. While people knew that certain bats were in the habit of attacking livestock and drinking blood, they seem to have assumed that it was one of the large, scary looking species that was responsible. Some explorers and naturalists had noted as early as the 1760s that the true vampires were the small desmodontines, but this was widely overlooked until much later. The history of the discovery of vampire bats and their sanguivorous habits is a fascinating and lengthy story, best reviewed in David Brown's excellent Vampiro: the Vampire Bat in Fact and Fantasy (Brown 1994).

And one more thing on vampire lore and legend: apparently (and ironically), bats were never incorporated into Old World vampire mythology until Bram Stocker did so in his 1897 book (Altringham 2003).

Lots more to come. Have you ever wondered how a behaviour as remarkable as sanguivory may have evolved? (though, for inspiration, you might like to check the previous articles on blood-feeding in birds here and here).

Refs - -

Altringham, J. D. 2003. British Bats. HarperCollins, London.

Brown, D. E. 1994. Vampiro: the Vampire Bat in Fact and Fantasy. High-Lonesome Books (Silver City, New Mexico).

Corbet, G. B. & Hill, J. E. 1980. A World List of Mammalian Species. British Museum (Natural History), London.

Jones, K. E., Purvis, A., MacLarnon, A., Bininda-Emonds, O. R. P. & Simmons, N. B. 2002. A phylogenetic supertree of the bats (Mammalia: Chiroptera). Biological Reviews 77, 223-259.

Schutt, W. A., Altenbach, J. S., Chang, Y. H., Cullinane, D. M., Hermanson, J. W., Muradali, F. & Bertram, J. E. A. 1997. The dynamics of flight-initiated jumps in the common vampire bat Desmodus rotundus. The Journal of Experimental Biology 200, 3003-3012.

Riskin, D. K. & Hermanson, J. W. 2005. Independent evolution of running in vampire bats. Nature 434, 292.

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.

Wilkinson, G. S. 1984. Reciprocal food sharing in the vampire bat. Nature 308, 181-184.

Wilkinson, J. 2001. Bat blood donors: feeding and sharing in vampire bat colonies. In MacDonald, D. (ed) The New Encyclopedia of Mammals. Oxford University Press (Oxford), pp. 766-767.