Charity Among Vampires

Common Name: The Vampire Bat

A.K.A.: Subfamily Desmodontinae

Vital Stats:

• Subfamily contains three species; the common vampire bat (Desmodus rotundus), the hairy-legged vampire bat (Diphylla ecaudata), and the white-winged vampire bat (Diaemus youngi)
• All three feed only on blood, a phenomenon known as hematophagy
• The common vampire bat feeds primarily on mammals, while the other two species prefer avian blood
• Can live up to 20 years in captivity

Found: Throughout Mexico, Central America, and all but the most southern reaches of South America

It Does What?!

Several years ago while on a botanical expedition in the rainforests of South America, I woke one morning to find that one of the other team members, still fast asleep in his hammock, had – apparently – been stabbed in the shoulder during the night. A surprising amount of blood had run down his arm, and yet he snored peacefully away. What the hell had happened to this guy, and was he the world’s deepest sleeper, or what?

Nope. Turns out he had just unwittingly provided a good meal for Desmodus rotundus… the common vampire bat.

As horrifying as it may seem to have flying vermin drinking your blood whilst you sleep, it’s really not as bad as pop culture would have us believe. The bats are more scavenger than predator. To begin with, they prefer stealth and guile to any kind of open attack. Sleeping animals are best, and victims are never approached from the air, Caped Crusader-style. Instead, the bat will land nearby and walk on all-fours over to its prey. From there, it uses heat sensors in its nose (similar to some snakes) to detect where blood vessels pass close to the surface of the skin. In cows, another favourite blood donor of Desmodus, bites are usually just above the hooves or around the ears.

Also contrary to popular belief, the bites are never violent; they’re more like a tiny nick from a very sharp razor- painless, but they tend to bleed a lot. In this case, they’re bleeding a lot because the bat’s saliva contains anticoagulents, preventing the blood from clotting. The bat will lap at the cut with its tongue (no blood-sucking here), transferring saliva into the wound, which will sometimes continue to bleed for hours afterward.

An entire feeding session takes the bat only about 20 minutes, during which time it can consume up to half its own weight in blood. How is this possible? Vampire bats have an amazingly efficient excretory system; the plasma (liquid) portion of the blood is immediately absorbed and passed through the kidneys. Within minutes of beginning to feed, the vampire starts to pee at the same time, and continues to do so until its meal has been reduced to a manageable volume. (Did they leave this part out of the Twilight movies?)

Creepy as these little beasts may seem, they have a surprisingly enlightened social structure. Vampire bats have been cited by animal behaviourists as one of the few examples of reciprocal altruism (“tit for tat”) in nature. You see, the vampire lifestyle is a bit precarious- a bat will die if it fails to feed for two successive nights. As a lifesaving measure, a bat in such dire straits will actually beg another bat for food. The other bat will then regurgitate some of its meal – just enough to make do – into its hungry neighbour’s mouth. Impressively, the bats even keep score. A hungry animal will turn preferentially to a bat it has helped out in the past, and cheaters are recognised and allowed to starve.

Far from being mindless, aggressive little monsters, vampire bats are altruistic, intelligent creatures. How intelligent? Researchers who housed a vampire bat with a hen observed the bat to mimic the behaviour of a chick so effectively that the hen settled down on top of the bat as she would to keep a baby warm. The bat then nicked her on the stomach and drank her blood while she tried to mother it.

Now that’s just creepy.

[Fun Fact: Vampire bats listen to the rhythm of an animal’s breathing to determine whether or not it’s asleep. They prefer to return to a victim they’ve had previous success with, and evidence suggests that they can identify individual humans by their breathing noises in the same way that we recognise individuals by their voices.]

[Also: The common vampire bat can jump up to three feet off the ground to reach large prey.]

Says Who?

• Groger & Wiegrebe (2006) BMC Biology 4:18
• Lee et al. (2012) PloS ONE 7(8): e42466
• Schutt (2008) Natural History, November Issue, pg.22