A Taste for Blood
Humans have always feared animals that roam the dark. Anything that conducts its business after sundown must surely be malevolent in nature. Bats, in particular, have been held in very low esteem for centuries, especially in the western world. In the realms of human imagination, bats are often little more than creatures of evil, associated with the deepest night and devilish goings-on. They are linked more intimately with vampires than any other animal, and indeed many traditional fictional vampires could transform into bats to fly around. But do bats deserve such bad press?
The legend of the vampire, born many centuries ago in eastern Europe, originally described them as mindless undead creatures that were often bloated and with a ruddy complexion – markedly different from the pale, gaunt and often charismatic, sophisticated beings of modern vampire mythology. These early vampires had no connection with bats because blood-sucking bats do not exist in Europe. But when the Spanish Conquistadors returned from Central and South America in the 16th century and told macabre tales of giant bats that dropped down on you as you slept and sucked the very blood from your veins, they were incorporated into vampiric folklore. Clouded by their own ideas of what a vampire should look like, early naturalists jumped to all sorts of conclusions and assumed that any big, ugly-looking bat in the region must be a bloodsucker.
These early accounts were certainly sensationalist but not zoologically accurate. First of all, vampire bats don’t suck blood – they lap it up from an open wound like a cat drinking milk. Secondly, rather than being large, vicious creatures, vampire bats are about the size of a mouse. And finally, although over 100 species of bat were mistakenly described as vampires, we now know that there are, in fact, only three.
That’s right, three. Out of a grand total of more than 1,400 bat species worldwide. These three bats are the only mammals on the planet that exclusively feed on blood.
Of these three species, two – the white-winged and the hairy-legged – prefer the blood of birds and are adept at clambering through the branches to reach nests and their occupants. But the third species – the common vampire bat, and the one that this article will be focusing on – feeds mainly on mammalian blood. It is also the only one to feed on humans with any regularity.
The Bloodthirsty Bat?
As any fan of horror films can tell you, sucking blood relies on stealth. And there’s a reason why Dracula and his undead allies are so closely associated with these flying mammals – they’re experts at finding and consuming blood.
The common vampire bat usually sneaks up on its victim (preferably one that is asleep) from the ground. A bat on the ground may seem out of place, and it certainly moves somewhat awkwardly, but it is surprisingly fast and agile, capable of running at up to 2 metres per second. A vampire bat’s wild prey consists of deer, tapirs and a type of wild pig called a peccary, although today they feed mostly on livestock such as horses and cattle. Once a suitable victim has been found, the bat is able to use an arsenal of special adaptations to get its meal.
Thermoreceptors on its nose allow it to locate areas on its prey where blood flows close to the skin and is therefore easier to access. If there is fur on the skin of the host, it uses its canine and cheek teeth like a barber’s blades to shave away the hairs. Then it uses its razor-sharp incisor teeth to make a wound and starts to feed, lapping up the blood with rapid flicks of its tongue. Its saliva contains chemical components that make its job easier: anaesthetic that numb the pain receptors of the prey so that it doesn’t feel the bite and attempt to dislodge the bat; and an anticoagulant (appropriately named draculin) that maintains the flow of blood from the wound so that it doesn’t clot until the bat has completed its meal.
In just half an hour, a vampire bat can drink up to 50% of its own body weight in blood. This can be a serious problem for a flying animal because it risks becoming temporarily grounded if it gains too much weight. The bat deals with this by converting the nutrient-poor blood plasma to a stream of dilute urine within two minutes of starting a meal.
A vampire bat requires at least 20 grams of blood – equivalent to two tablespoons – every day to survive. Blood-hunting is a skill that improves over time, but even so, failure is a fact of life for even the best blood-seeker and every night up to 30% of vampire bats fly home with an empty stomach. Since blood is made up of water and protein and contains no fat, vampire bats have no opportunity to build up fat reserves. Three consecutive nights of feeding failure means starvation. If a female has a pup with her, she must not only provide it with milk but also regurgitated blood, so finding food is particularly important during this time.
Vampire bat colonies consist mainly of females and their pups, young males, and a small number of dominant adult males. The females in particular have strong bonds with other members of the colony. Any bat that has had an unsuccessful raid and is feeling hungry can return to the roost and (at the risk of sounding unforgivably anthropomorphic) lick the face of an ‘old friend’. The second bat will often respond by regurgitating a bit of congealed blood for the first, so keeping it going until the following night.
Vampire bats often share their blood meals with close relatives, but they’ll also donate food to unrelated individuals as well. When this behaviour is between relatives, it is very easy to explain because those individuals share a very high proportion of genes with one another. This action aids the propagation of genes in a similar way to a parent caring for its young, though to a lesser degree. Therefore, giving to blood relatives (pun intended) isn’t altruism – it’s just self-interest at the genetic level.
But why do unrelated individuals help one another out? Could this be an example of truly selfless behaviour? Or is there an advantage to an individual that acts in this way?
It is certainly to a bat’s benefit to give blood to a starving companion if she could be sure that when she herself had bad luck and had failed to feed, the recipient could be relied upon to behave in the same way. And that is exactly what happens. On another night, the situation may be reversed and the bat receiving blood on this occasion may repay the favour. Feeding non-kin probably expands a bat’s social network, making it more likely that it will itself be helped in times of need. In fact, it seems that the largest predictor of blood-sharing behaviour is not necessarily relatedness between bats, but whether they have been helped by that individual before.
The giving of blood between two unrelated animals in this way is a reciprocal form of altruism that benefits both parties. Cheats who solicit blood and take it but do not repay the debt when required are soon detected and not helped again, which could lead to their early demise. The altruistic behaviour is therefore a characteristic that can selected and encouraged by the processes of evolution.
Typecast as the bad boys of the mammalian world, bats are to warm-blooded creatures what sharks are to fish, or what snakes are to reptiles: widely reviled and demonised by an often ignorant media. Yes, they have a penchant for blood, and yes, their gruesome reputation is not entirely undeserved. But there is another, gentler side to these unloved creatures that few people would ever have imagined. Who could have guessed that these bats live in caring, sharing communities where individuals form strong bonds and help one another out? Bloodthirsty, perhaps, but also seemingly benevolent.
Unfortunately, just as the light was being shed on the true nature of these bats, the fashion for Romantic Gothic literature in the 19th century ensured that vampires penetrated the popular consciousness. And then, in 1897, bat fact and vampire fiction became entwined when an Irish novelist published a book that would seal these flying mammals’ reputation. Bram Stoker’s classic Dracula combined Europeans myths of vampires that come to haunt the living with stories of bloodsucking bats from South America. It’s an association that vampire bats – and indeed all bats – have struggled to shed.
Make sure to check out the other Halloween-themed articles that The Nature Nook has been posting this week. Find out how to care for a ghost mantis, discover the grisly habits of the burying beetle, and learn about the strange ‘vampire fish’ wriggling around in British rivers and streams!
And starting next Friday, The Nature Nook will be starting a new regular feature, where every fortnight we’ll be looking at a fantastic but freaky frog!