Thursday, December 4, 2008

Twinkle, Twinkle, Vampire Bat

As a young boy I can remember going through a brief phase where I was fascinated with bats, especially vampire bats; but what did I really know about these animals? I'm pretty sure I heard that they don't really bite people, but is that true? Has anyone ever died after being attacked by a vampire bat? A few months back I decided to do some internet research to find out the real facts.

Vampire Bats

There are three species of vampire bats: the common vampire bat (desmodus rotundus), the hairy-legged vampire bat (diphylla ecaudata), and the rare white-winged vampire bat (diaemus youngi). They are all New World leaf-nosed bats (Phyllostomidae family) and together they make up the subfamily Desmondontinae. These three species more or less share the same habitat, stretching from southern Texas/Mexico to South America and including some Caribbean islands such as Trinidad, where they can be found primarily in forests but also in other humid to arid parts of the tropics/subtropics.

The major characteristic which distinguishes Desmondontinae from other leaf-nosed bats is their diet: they all feed on the blood of warm-blooded vertebrates. Although the species are different enough to be placed in three separate genera (each consisting of just a single species), scientists believe that they probably share a common ancestor and thus that hematophagy ("bloodsucking") is a trait which evolved once among bats. Whereas the hairy-legged and the white-winged vampire feed primarily on birds, the common vampire bat likes to feed on mammals. They prefer to feed on livestock such as cattle, goats, pigs, and horses due to their abundance.


Like other bats, the vampire is nocturnal and thus its victims are normally asleep during its feeding time. It is also very agile and, unlike other bats, the common vampire can walk, run and hop on its strong hind legs. Normally, the common vampire bat first lands nearby its victim and then walks up to it and climbs up to a suitable feeding spot. The bat usually chooses a part of the body which is difficult for the animal to reach such as the shoulder so as to protect itself from retaliation if its prey should awaken during feeding, and if need be it can jump to avoid attack.

Vampire bats do not actually "suck" blood. First they pierce the animal's hide with their sharp front teeth making a 3mm incision which is painless and usually doesn't awaken the victim. Then they lap up the blood aided by the groove in their tongue and lower lip and by a powerful anticoagulant in their saliva (which scientists have dubbed "draculin").

On average the common vampire bat weighs something like 50g and in a single 20-minute feeding it can ingest around 50% its body weight (20g or 1 fluid ounce). When its belly is full like this it is actually too heavy to get off the ground, so as soon as it begins to feed the bat's body starts digesting the blood and within 2 minutes it starts to excrete most of the plasma in the form of highly-concentrated urine. Even after this process, the bat may still have taken on 20-30% of its body weight and in order to fly away it needs to gain extra lift by jumping: crouching and springing itself into the air with its strong hind legs.

Vampire bats have been known to return to the same victim (such as tethered livestock) night after night, in which case they just need to break the scab in order to get that sweet blood flowing once again.

Attacks on humans

Human attacks by common vampire bats are rare because (a) they usually won't go after humans when there is livestock around, and (b) someone has to be sleeping outside or in a home without windows (or with open windows) in order for the bat to gain access to them. When vampire bats do feed on humans they don't go for the jugular but usually prefer a less conspicuous feeding spot such as the earlobe or big toe given that if they wake you up mealtime is over.

Even if you were attacked by a vampire bat you wouldn't die of blood loss although birds and smaller/sickly animals who are preyed on by multiple bats may be weakened or even killed. Indeed, if you've ever had blood work done you've probably lost more blood than a vampire bat would remove during a feeding.

There is, however, a much more serious threat facing victims, which is that vampire bats are often carriers of diseases such as rabies. Damn, did you know that if it's untreated the rabies virus is almost always fatal and that symptoms include fever, paralysis, paranoia, hallucinations and brain damage!? Thus, people who do not receive treatment (such as farmers in rural areas of South America who may have limited access to medical services) do sometimes die from rabies after being bitten by vampire bats. In addition to occassional human fatalities, rabies outbreaks in livestock spread by vampire bats represent a real problem for farmers and it is estimated that they cause millions of dollars in losses for the Latin American cattle industry every year.


So, basically, you don't really need to be afraid of vampire bats, but you do need to be afraid of rabies! Seriously, if you're ever bitten by a bat or for that matter by a racoon, an unfamiliar dog, or any other mammal you'd better head straight for the emergency room to get vaccinated.

Photo of common vampire bats roosting taken at Cinncinnati Zoo by Philip Meyers of U Mich, Museum of zoology, closeup of common vampire bat by Michael & Patricia Fogden/Corbis.

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