Research

We study the adaptive design of cooperative relationships. How do cooperative animals choose, maintain, and regulate their social relationships?

Theory: The evolution of cooperation is a central topic in biology. Cooperative traits pose an evolutionary puzzle: how do cooperative traits emerge and persist when non-cooperative traits can better exploit the public good of cooperation? Social evolution theory has largely solved this puzzle with three key ideas. First, inclusive fitness theory explains why altruism can evolve among kin. Second, reciprocity theory explains how cooperation between two repeatedly interacting individuals can be enforced by reward and punishment. Third, biological market theory explains how cooperation can be favored by partner choice and the threat of partner switching, and how asymmetries in cooperation can result from supply and demand. Together, these ideas lay the foundation for understanding why even costly cooperative traits can evolve and persist.

The problem: The mechanisms that enforce cooperation have been demonstrated in systems that are easy to manipulate in the lab (like microbes, plants, ants), but in more complex vertebrate societies (including humans), the mechanisms stabilizing cooperation are more controversial. In part, this is because a gap exists between simple evolutionary models of cooperation and the complex psychology of helping decisions in animals. For animals that live in individualized societies, much cooperation takes place in the context of a stable social bond that seems analogous to a human friendship. We don’t know how these relationships develop, why they develop, how they differ across species, or to what extent they they require complex cooperative traits.

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Food sharing relationships in captive vampire bats. Most sharing occurs among females (red) rather than males (blue). The size of a node shows the number of roost-mates fed by that bat. Arrow width (grey) shows the average rate of food sharing from one bat to another.

Our goal is to use experiment and observation to test predictions (from inclusive fitness theory, reciprocity theory, and biological market theory) about the design of cooperative relationships. We study the cooperative relationships of vampire bats (pictured left on my shoulder) 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 2-3 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.

bat on shoulder
me and a vampire bat

With Rachel Page at Smithsonian Tropical Research Institute, we are looking at how vampire bats that are strangers can eventually develop cooperative relationships.

With Alex Ophir and Angela Freeman at Cornell University, we plan to look at the neuroendocrine basis of cooperation in bats.

With Simon Ripperger, we are tracking foraging movements and looking for social foraging in free-ranging vampire bats using newly developed tracking devices that can be placed on both bats and their “prey”.

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