Vampire bats drink blood. A lot of it, in fact — in order to survive, they need to consume roughly 50 to 100% of their body weight in blood each night, in part because they urinate out half of their meal within an hour after drinking it. After sixty hours without drinking blood, the vampire bat wastes away, losing as much as 25% of its body weight, and is unable to maintain its temperature and, in the end, its own life.
So, vampire bats share.
A 1990 study demonstrates that vampire bats are altruistic, something uncommon among non-human mammals. When community members are unable to fend for themselves (due to say, injury) those who can, help. When the successful foragers return to the roost, an unable bat, per the study, “solicits food from a potential donor, first by grooming around the stomach area and then licking the donor’s face. The donor bat then responds by regurgitating blood if receptive.” The practice is limited typically to females and in practice, only those bats with fewer than 24 hours to live receive these odd transfusions. The regurgitated blood allows the otherwise-doomed animal to live for as much as 18 hours — enough time to make it to the next night’s hunt. Further, bats only assited those bats which they had a previous relationship, and tend to prefer assisting those with whom they are related.
Is it pure altruism? Perhaps — but perhaps not. One other finding of the study is that bats were more likely to receive future regurgitative transfusions from those bats which they themselves had previously assisted. So perhaps vampire bats practice the Golden Rule.
Bonus fact: In the late 1700s, a Frenchman by the name of Tarrare was afflicted with the strangest of diseases — an insatiable appetite, despite showing no weight gain from his exorbitant eating. Tarrare would eat, literally, everything he could — even things typical people would never consider “food.” Per Wikipedia, doctors tried to treat him for his affliction, but Tarrare’s hunger won out: “[H]e would sneak out of the hospital to scavenge for offal in gutters, rubbish heaps and outside butchers’ shops, and attempted to drink the blood of other patients in the hospital and to eat the corpses in the hospital morgue.”
From the Archives: Bat Man: How Daniel Kish — a blind person — “sees” the same way bats do.
Related: The cutest vampire bat you will ever see. Blood not included, nor, thankfully, required. (Or advised.)