Three species of the approximately 1300 species of bat known to exist throughout the world have evolved to feed solely on blood. These members of the Desmodontinae family are commonly known as vampire bats. They are the common vampire bat (Desmodus rotundus), the hairy-legged vampire bat (Diphylla ecaudata), and the white-winged vampire bat (Diaemus youngi).
All three species are found in the Americas, sharing a similar range that extends from Mexico, through Central America, south as far as Argentina and Uruguay. Other species of bat also feed on blood and are classed as vampire bats, but rather than subsisting exclusively on blood they also feed on insects, fruit, fish, meat, nectar and pollen.
The reason why so few of the world’s 1300 bat species have evolved to feed on blood, and why feeding on blood in general throughout the animal kingdom –a practice known as hematophagy- is so uncommon, is very simple: feeding on blood involves a number of major challenges. In addition to escaping detection while feeding upon a host, a blood-consuming creature must also overcome the hazards to the kidneys and bladder caused by the consumption of large volumes of fluids, and the potentially negative impact on the organism resulting from excessive protein and iron content.
Vampire bats are so specialized that they are able to drink between a third and half their bodyweight in blood during a single feeding session, while processing that massive intake of fluids and excreting the excess as urine, before abandoning the host.
Since they diverged from fruit and insect eating bats some 26 million years ago, vampire bats have evolved to harvest blood efficiently from hosts, in what is an essentially parasitic relationship. They have large front teeth, designed for cutting into flesh, and their saliva contains an anticoagulant substance. The common vampire bat even possesses thermoreceptors in its snout, enabling it to locate through infrared radiation areas of the body where capillaries flow close to the surface of the skin. In addition, vampire bats employ their acute hearing to detect the regular breathing of a sleeping victim.
As a means of approaching their targets as stealthily as possible, vampire bats have also evolved the ability to walk and run upon their powerful legs, assisted by their wings. It is in this way that they are able to approach sleeping prey and begin to feed, without alerting their victim.
Vampire bats live in colonies that can number several hundred individuals. Typically, they will seek out locations for their colonies that are enveloped in perpetual darkness. In the rainforest, such sites may include caves or hollow trees.
Contrasting with their fearsome reputation, one of the most interesting characteristics of vampire bats is their formation of strong social bonds within the roosting colony. In addition to engaging in social grooming, vampire bats will also share food. Because a vampire bat must feed at least every forty-eight hours, and an individual is not guaranteed to find a food source during every foray it makes beyond the roost, successful bats will donate food to other members of the colony by regurgitating some of the blood they have harvested from a host.