To prevent starvation, vampire bats will share their blood meals with an underfed friend. New research uncovers the steps leading up to this remarkable prosocial behaviour and the importance of mutual grooming.
Vampire bats share their blood meals, but they don’t do this with just any ol’ bat. These flying mammals are known to form tight social bonds and build relationships that last for years. At the risk of anthropomorphizing, they actually seem to have friends. New research published today in Current Biology shows how these relationships emerge in vampire bats, and how social grooming leads to blood sharing among non-related paired individuals.
“To my knowledge, nobody rigorously studied how two animals go from being ‘strangers’ to having a natural cooperative relationship, where one partner would even help the other at a cost to themselves,” behavioural ecologist Gerald Carter, the lead author of the new study and an assistant professor at Ohio State University, wrote in an email to Gizmodo.
Vampire bats are obligate blood-eating mammals, meaning they subsist entirely on blood meals, which they gather by stealthily latching onto cattle, goats, horses, and the occasional human. Their vampiric existence puts them in perpetual risk of starvation, however, as they need to feed at least once every three days or they’ll likely die. So while out on their nightly patrols, vampire bats tend to either hit the jackpot or go home hungry. Three consecutive nights of failure means almost certain death.
That is, unless they’ve got a buddy who’s willing to be a blood donor.
As the new research points out, vampire bats have evolved an effective cooperative strategy in which individuals pair off and create tight social bonds among themselves. When times get tough, one member of the pair can rescue their friend from starvation by regurgitating the blood they sucked earlier, in a manner similar to birds feeding their nestlings.
For the new study, Carter and his colleagues wanted to learn more about this process, including how vampire bats are able to forge these tight relationships and then take it upon themselves to share such a valuable resource.
Now, most scientists who study cooperation tend to observe relationships that already exist (e.g. primates who groom each other) or relationships they themselves fostered in experimental settings (e.g. monkeys helping cohorts by pulling a lever that delivers food), according to Carter.
“What we did was introduce unfamiliar vampire bats in captivity and then let them form a cooperative relationship as they would in nature,” he told Gizmodo. “You can simulate a bat’s natural social life in captivity because they are small and live in tight spaces like hollow trees. To induce helping, we just fasted a bat, which gave each stranger the opportunity to help it. We did not have to train them, and we got way more observations than you could from a purely observational field study. And most importantly, we could see them go from strangers to food-sharing donors.”
For the experiment, the scientists gathered vampire bats from two distinct sites in Panama: 19 bats from Tolé and eight bats from Las Pavas. The bats were either isolated as pairs (each from a different site) or in small groups (i.e. one Las Pavas bat and three Tolé bats). All groups were tested by withholding food from some individuals and then observing their resulting behaviours. In total, the experiment took 15 months, involving 638 fasting trials.
“In each case, we repeatedly fasted all the bats in the group, but one at a time, so that everyone had the opportunity to help and to be helped, and we looked at how both grooming and food-sharing rates changed over time,” said Carter.
As the experiment progressed, several bats—especially those in the pairs—suddenly instigated grooming behaviours, in which they licked each other, similar to cats. The practical reason for this is to remove parasites, but it serves a social function as well, allowing the bats to get used to each other and form a bond.
Over time, the researchers watched as these grooming sessions got more vigorous and frequent over time. These grooming sessions often led to blood sharing with underfed cohorts. The bats switched between being a donor and a recipient (which happened multiple times), and when a bat helped their buddy for the first time, this bold action made it more likely that the buddy would return the favour for their first time.
Interestingly, the same social networks were maintained even after the captive bats were released back into the wild, according to Carter.
The new study affirms previously proposed theories about the emergence of cooperative behaviours in animals, humans included. The initial “investment” of grooming is a way to “test the waters,” according to the research. Reciprocated grooming signals that the investment worked and that the investment should be escalated—what the researchers called “raising the stakes.” This intense, frequent grooming serves as stage setting for the blood sharing when times get tough.
This would seem to violate the “selfish gene” hypothesis—a gene-centered approach to understanding the emergence of traits. Ultimately, the only thing a gene “cares” about is self-replication, according to this perspective of evolution. But under this view, it’s often a challenge to understand how altruistic behaviours emerge, given that the focus shifts to the survival and perpetuation of another individual’s genes—especially unrelated individuals. But as Carter explained to Gizmodo, his team’s observations work rather nicely within the context of the selfish gene hypothesis and how evolution gives rise to cooperation.
“Food donations appear to promote reciprocal donations back to the original donor. That’s one hypothesis for which we have increasing evidence,” said Carter. “Every bat has a different food donor network, and vampire bats that feed more non-kin [unrelated bats] also cope better with the loss of one of the main donors in their donor network. So bats that are more generous might do better in the long run. Also, most food sharing occurs with close kin, like extended parental care, and these food donations also help copies of your genes in others,” he said.
These findings could be applied to other species that cooperate, such as cleaner fish (they eat dead skin and parasites from larger fish with impunity) and primates, who, like vampire bats, also engage in grooming behaviours that lead to coalitions. It could even apply to humans, as Carter explained.
“‘Raising the stakes’ or ‘testing the waters’ could also be important in behaviours like courtship and mating,” he said. “In humans, before marriage there is dating, and before dating, there is that period where you just get to know each other…[but] it’s obvious that you want to escalate gradually. So there are many other social situations where animals might want to ‘test the waters,’” said Carter.
Looking ahead, Carter and his colleagues would like to study how sickness, particularly illnesses that cause lethargy, in vampire bats might affect cooperative relationships. Blood sharing in vampire bats, it would appear, doesn’t seem to be affected by this immune challenge, which makes bats listless, but the scientists would like to learn more.
“When you are sick you are less social, but you maintain the most important kinds of social relationships,” said Carter. “We are living that reality right now,” in reference to the ongoing covid-19 pandemic and the need for social distancing.
Indeed. Never thought I’d say this, but perhaps we should be more like vampire bats as we struggle through this awful moment in human history.