Will Groom for Food: Social Bonds in Vampire Bats

Featured Article: Gerald G. Carter, Damien R. Farine, Rachel J. Crisp, Julia K. Vrtilek, Simon P. Ripperger, Rachel A. Page. Development of New Food-Sharing Relationships in Vampire Bats. Current Biology, 2020. https://doi.org/10.1016/j.cub.2020.01.055

Friendship: it’s natural

In nature, sometimes animals form relationships that resemble what we call “friendships.” Like humans, animals must be weary of who they befriend and identify the risks of forming friendships. They must assess whether their new “friend” is exploiting them for their resources, such as food or shelter. One theory, called the “raising the stakes” model, suggests that to reduce the risk of forming social bonds, animals start with small, cooperative actions, like huddling together for warmth, and gradually increase investments, like grooming one another. 

However, there is little empirical evidence to support this theory. Though there are documented social bonds in nature, this evidence is limited to interactions where there is usually a power asymmetry, such as the cleaner-client fish mutualism, or a specific behavior within an already established relationship, such as social grooming in primates. The “raising the stakes” model suggests that forming relationships may involve animals testing the waters by gradually increasing the cost of cooperative behaviors. To determine if there is a pattern of cost-behaviors in animals, random strangers with no prior relationships must be introduced, and the emergence of natural cooperative behaviors that vary in “cost” (low, medium, high) should be measured. 

Following this methodology, a group of researchers led by Dr. Gerald G. Carter from the Ohio State University tracked the development of social grooming (low cost behavior) and regurgitated food-sharing (high cost behavior) among captive vampire bats (Desmodus rotundus) over 15 months. 

Vampire bats: a social experiment

Researchers collected female vampire bats from two different sites in Panama. They introduced the bats in isolated pairs (one bat from each location) and in quartets (three bats from one location, one bat from the other). Then, they merged the bats into one large mixed group. For both the controlled introductions (pairs and quartets) and the mixed group, bats were fasted to try to encourage food-sharing behavior, which is when fasted bats lick the mouths of another bat in hopes of being fed through regurgitation. Vampire bats eat only blood and can die of starvation if not fed frequently. The act of food-sharing is more like a life-saving measure than a simple meal-share. When these behaviors occur, it suggests that the two bats have close social ties. 

You scratch my back, I’ll feed you

Bats were more likely to share food after developing new social grooming relationships in pairs. Even then, new food-sharing relationships were rare, likely because of the high energetic cost. Within pairs, the more that a fasted bat groomed a fed bat, the more likely it was to get fed. In the cases when food-sharing between bats never occurred, grooming rates were low, which reinforces the “raising the stakes” theory. What’s even more interesting is that the more grooming that occurred in pairs, the more likely reciprocal food sharing would develop – both bats grooming each other and sharing food. 

When in mixed groups, the bats were more likely to groom and share food with bats from their original cohort. This suggests that the more familiar bats are with one another, the more likely they are to share their food. 

Friends forever

This display of behavioral hierarchy is a first glimpse at the mechanisms behind relationship forming in social animals. Trust-building behaviors like grooming can prove to a stranger that “I’ve got your back”.  When it comes to vampire bats, proving your worth could be the difference between life and death.