Research

Some nonhuman animals form individualized friendship-like social bonds. Bonded individuals might prefer to associate, groom each other, forage together, or share their food. We study the adaptive behaviors that underlie these cooperative social relationships in vampire bats. Do individuals choose more helpful social partners? How do they enforce returns on their cooperative investments? What are the consequences of these decisions on how relationships develop or on the structure of societies as a whole?

The big picture: The evolution of cooperative traits (why do organisms evolve traits that benefit others?) was a longstanding puzzle that has been essentially solved by more than four decades of integrating theory, observational field studies, and controlled experiments. Different theoretical approaches (e.g. inclusive fitness theory, interacting phenotypes, multilevel selection, reciprocity, and biological markets) each emphasizes different factors shaping the evolution of social behavior (e.g. relatedness, scale of competition, assortment by phenotype, repeated interactions, and partner choice). In the real world, cooperation is probably often evolutionarily stable due to multiple factors that work together and interact.

Why vampire bats? The relative roles of these factors for stabilizing cooperation are best understood in more simple systems that are easy to manipulate in the lab, like microbes or symbioses based on resource exchange (e.g. when plants and fungi trade carbon for phosphorus). By contrast, in species where individuals form complex long-term cooperative relationships (like primates), the relative roles of these factors remains unclear, because these relationships involve many different kinds of interactions over time. Social bonds are almost by definition hard to manipulate. We study the cooperative relationships of vampire bats because they form long-term bonds and performs natural, frequent, and costly helping behaviors that can be monitored, measured, and manipulated over long time-periods.

Our goal is to use experiment and observation to test predictions (from inclusive fitness theory, reciprocity theory, and biological market theory) about the forces or strategies that create and maintain cooperative relationships. We try to understand how individual traits and actions leads to changes in relationships and the structure of social networks.

Vampires are blood-feeders on a tight energy budget—they can starve to death after just 3 nights of unsuccessful hunting, but they can also live for more than two decades. Females regurgitate food to their offspring but also related and unrelated adults. These helping decisions appear to be based on past social experience, because reciprocal donation rates are more important than genetic kinship for predicting food-sharing rates. Nonkin bonds appear to act as form of “social bet-hedging” as relying exclusively on one or a few kin donors is too risky. We can experimentally manipulate cooperative behavior by preventing sharing in specific pairs or by administering intranasal oxytocin or lethargy-inducing LPS. By manipulating the social experiences individuals have, we hope to understand how individual traits lead to different relationship outcomes, and how relationship dynamics influence the broader social network structure.

Ongoing projects

• With Rachel Page at Smithsonian Tropical Research Institute, we are looking at how vampire bats that are strangers can eventually develop cooperative relationships.
• We are tracking foraging movements and looking for social foraging in free-ranging vampire bats using newly developed encounter-tracking devices that can be placed on both bats and their “prey”.
• With Alex Ophir and Angela Freeman at Cornell University, we plan to look at the neuroendocrine basis of cooperation in bats.
• With Liz Hobson and Ian Hamilton, we are trying to understand how social networks emerge from the traits of individuals and relationships.

To see examples of our most recent work, see Publications.