Reputation: Blind and bloodthirsty flappy things. The friends of vampires. Pure evil. They scare me and I don't want to know.
Reality: Your fear is irrational. You will love vampire bats and their amazing sanguinivorous lifestyle, especially their caring, sharing blood regurgitation scheme.
Vampire bats have taught me many things. One of them is the word "sanguinivore", a creature whose diet consists solely of vertebrate blood. There are plenty of sanguinivorous invertebrates (think mosquitos and leeches), but only three mammals that have gone in for sanguinivory: the common vampire bat, the white-winged vampire bat and the hairy-legged vampire bat.
Given their unusual diet, it is understandable that vampire bats should have been named after the shrouded, undead figure of European folklore. But the more I learn about vampire bats, the more I realise that this association is defamatory.
The indulgent, almost luxuriant pace of the mythological vampire suggests that a sanguinivorous existence is a relaxed affair. Vampire bats, if they could talk, might beg to differ. Whilst blood is high in protein, it is poor in carbohydrate, so they must feed a lot and often. So vampire bats have evolved an impressive range of tricks for getting drinks.
Compared to other bats, the common vampire bat (and probably the other vampire bats too) is much more sensitive to low-frequency sounds. This talent might help them to locate their prey. For instance, common vampire bats can recognise individual humans by their breathing sounds.
Having located a target, a bat will often make its approach from the ground, using its hind-limbs and wings to bound along with remarkable agility and speed. One bat, put through its strides on a treadmill, reached speeds of more than 1 metre per second.
Once on the animal, a vampire bat will show off its phlebotomy skills, using heat-sensitive cells in its nose to locate a suitable blood vessel. The bat then prepares the site, clipping away the hair or feathers, swabbing the skin with its tongue and then tapping into the bloodstream with its razor-sharp incisors. A complex cocktail of salivary proteins, one of which is called draculin, keeps the blood vessels open and prevents the blood from coagulating.
It will typically lap at a carefully crafted incision site for up to an hour. During this time, it may more than double in weight, thanks to its highly distensible stomach. "They swell up like a mosquito," says Gerald Wilkinson of the University of Maryland in College Park.
The extra baggage makes it a little tricky to get airborne after a meal. But the vampire bat's springy forelimbs can propel it vertically – like a Harrier Jump Jet – at a speed of over 2 m/s. This is also useful if the bat's ruse is rumbled and it needs to make a swift get-away.
In spite of these nifty adaptations to sanguinivory, bats have no guarantee of finding food. By catching bats returning to their roost sites before dawn, Wilkinson estimated that almost one in ten adults, and one in three juveniles, come home hungry. "It's very obvious whether or not they've fed," he says.
This is not a trivial problem. If a bat goes without food for three consecutive nights, it will lose about a quarter of its body weight. It will also die.
So rather wonderfully, vampire bats have set up a blood-sharing scheme that reduces the chances of starving to death. Each night, well-fed individuals regurgitate packages of congealed blood for hungry roostmates. "It's very much like a cat lapping from a bowl of milk, only it's lapping at the mouth of the other bat," says Wilkinson.
In the late 1970s and early 1980s, Wilkinson and an assistant spent hundreds of hours staring up into the dark innards of hollow trees in Costa Rica, watching for one bat regurgitating blood for another.
In many cases, he saw mothers spewing up blood for their offspring. But often the donor and recipient were not obviously related. They just seemed to spend a lot of time hanging out, developing their bond by grooming each other. I asked Wilkinson if it would be fair to describe them as friends. "Friends would be another term," he says.
Recently, Wilkinson revisited the food-sharing behaviour of vampire bats and pinned down the context in which it occurs. He and a colleague recorded almost 1000 food-sharing bouts in a captive colony.
There was no sign that one bat was coercing another to donate blood. "If anything it's the opposite," says Wilkinson. "They are eager to give."
Some of the sharing bouts occurred between family members. More often, though, regurgitation was between friends. Sicking up blood isn't exactly cute, but it does show community spirit. It's all a far cry from the bats' nefarious reputation.
The common vampire bat usually feeds on mammals: mostly domestic species like cattle and horses, but also wild animals such as capybara, peccaries and tapir. But they do sometimes feed on humans and it's pretty common to sleep through it. When Wilkinson went on a field trip to Costa Rica in the 1970s, one of the other students got bitten. "His foot was sticking out from under a mosquito net, a bat came and fed on him," he says.
I ask Wilkinson if he ever got bitten during his years with these fascinating creatures. "I got occasionally nipped," he says. "When you catch them they are very agile. They squirm around and if you're not careful you can be bitten very easily." But, he says, they tame amazingly quickly. "I am quite fond of them."
Tweetable truths about vampire bats
Vampire bats can survive at least 15 years in the wild
If a vampire bat does not feed, it will be dead within three days
Vampire bats can run on all fours, reaching speeds of more than 1 metre per second
Vampire bats can take off vertically, like a Harrier Jump Jet