Research

Friendships are essential for humans to survive and thrive. Some nonhuman animals also appear to form individualized friendship-like social bonds. How does this process happen? When a new individual joins a group, it initially encounters strangers. It might prefer to interact with some strangers over others based on their phenotypic traits or behaviors. With repeated interactions, those strangers become familiar. Relationships form and differentiate. Our focal individual begins to form preferences for some of these relationships over others. Those preferences might be reciprocated. Social bonds form which yield mutual benefits. Specific pairs prefer to associate, groom each other, forage together, and maybe even share their food. Our focal individual begins to have a “stake” in the survival and wellbeing of some of its partners, and it increasingly invests in helping those partners. Those investments might be reciprocated, creating cooperative relationships.

My lab hopes to build data-driven models of how this process of social integration unfolds and how it works. What traits and behaviors underlie these cooperative relationships? 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?

Our goal is to use experiment and observation to test predictions from a variety of theoretical frameworks about the forces or strategies that create and maintain cooperative relationships. We try to understand how individual traits and actions lead to changes in relationships and ultimately the structure of social networks.

Why do organisms evolve traits that benefit others? The evolution of cooperative traits 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). Cooperative relationships emerge from traits that are probably evolutionarily stable due to multiple factors that work together and interact. What are the relative importance of these factors and how do they 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.

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.

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