If asked what makes us human, many might explain that our natural inclination towards compassion, generosity, and altruism towards our fellow humans makes us who we are. They might even define altruism as a strictly human trait, one that makes us human by virtue of being absent in the animal world. This is not an accurate assumption. There are a variety of animals that practice altruism, benefitting another at their own expense. Vivid examples of altruism have been found in one of the most feared and maligned of mammal species: Vampire bats.
Vampire bats are creepy. They have the face only a mother (or a bat enthusiast like myself) could love. They drink blood, sometimes even the blood of humans. They live in colonies that can number in the thousands and within those colonies, have fascinatingly complex social behaviors. It’s a good thing that they do, too. It can be a tough life being a vampire bat, and without a little help from their friends, many would die.
Bats, as flying animals, have to keep their weight down and can’t afford to keep much fat on their bodies. This is a problem. Bats have very high metabolisms, and yet blood is not a very energy-rich food source. This delicate combination means that vampire bats die after only 70 hours without food. A sick or injured bat that cannot fly out to feed for one night is immediately at high risk for death. Luckily for them, vampire bats have their own version of the soup kitchen.
Vampire bats are a chummy bunch. Within colonies, female vampire bats naturally form groups. In these groups, females often regurgitate blood to feed hungry companions. This provides a safety net for bats that cannot feed. Gerald Carter, a researcher at the Smithsonian Tropical Research Institute, decided to examine this behavior in an experimental setting. He worked with zoos to establish a population of several dozen vampire bats, some related and some not. For three years he watched them, carefully recording their behavior and unraveling the complexities of vampire bat societies.
To simulate a condition where a bat returned home hungry, he artificially induced fasting in individual bats for 24 hours. He then returned the bats to their groups and kept track of which of their companions fed them and which did not. He repeated this experiment hundreds of times and began to piece together some very intriguing results. Bats that had previously shared their food were given more when they went hungry themselves. Bats that were stingier with their offerings were sometimes ignored by those they had rebuffed when it was their turn to be hungry instead. The bats seemed to have some sort of concept of “fairness,” or at least a notion of equal exchange.
However, their behavior was even more complicated than that. Bats who may have willingly given blood to a fasting neighbor but did not have extra food to share gave even more once they were able to. It was apparent that bats remembered who had fed them, who they had fed, and who they wanted to feed, but were unable to. They did feed relatives, but social groupings dictated who they would feed more than relatedness. Though a bat might feed a stranger, they were most likely to feed a bat that had fed them previously.
Can we call the behavior of these bats reciprocal altruism? It’s a complicated question. Reciprocal altruism has traditionally been defined as an organism temporarily disadvantaging itself to benefit another, with the expectation that the other creature would return the favor in the future. While the bats did share food most among those that had shared with them, it was not an exact trade. On average, hungry bats were fed by three donors. Sometimes a donor would reject a begging bat, and other times a hungry bat would reject a specific donor. The behavior is too complex to dub it simple tit-for-tat. Gerald Carter suggests that we call vampire bat cooperation a “biological market” rather than a 1:1 relationship.
Brock Fenton, another scientist involved in the study, says that with the complexity of behavior, social interactions, and cooperation that vampire bats share, "you're almost looking at an elephant in a bat package." There is still a lot we don't know about vampire bat altruism, but it just goes to show, you shouldn't judge a book by its cover, no matter how creepy that cover might be!
- Kate Dzikiewicz, Paul Griswold Howes Fellow