ampire bats, killer whales, and human beings have something in common other than our occasionally bloodthirsty reputations. All of us share.

Take the vampire bat, for instance. Bats burn energy quickly, so they starve if they have to go two or three days without food. On any particular night they have a good chance of failing to find a meal. Unsuccessful for one or two nights, a hungry vampire bat will stroke the full belly of a well-fed comrade until it regurgitates a share of its meal. So the hungry bat lives to fly another day.

"This goes beyond what you’d normally expect," says Bruce Winterhalder, professor of anthropology and chair of the Curriculum in Ecology. "You wouldn’t be surprised if a mother would share with her offspring, or maybe close relatives would share, because there’s a genetic or evolutionary interest in that. But what is surprising is that there appear to be sharing friendships that develop in these bats irrespective of their biological relatedness."

So the impulse toward generosity is neither uniquely human nor a matter of ethics alone, Winterhalder says. "Anthropologists for decades have observed that hunter-gatherers routinely share food. Various models of what it means to be human have been built around the idea that sharing was a matter of generosity central to the ethic of these people — and that’s true. But behavioral ecologists have now developed five or six different explanations for why people would share food. And those explanations have come out of evolutionary theory."

or people as well as for bats, sharing reduces risk and helps ensure survival of the group. But another motive for sharing has to do with what Winterhalder calls scrounging, or tolerated theft. "If you live in a small group and you happen to be the one sitting there with a big chunk of something to eat, it’s very difficult to say no to someone who comes up and asks for part of it," he says. "Because it’s probably going to be more trouble for you to deny them a share of that chunk than it is to give up some of it. This can be worked out mathematically, so that you can analyze the costs and benefits, the marginal returns to each party, of this kind of tolerated theft."

Mathematical models for doing such analysis have become key tools in the young field of human behavioral ecology, which emerged in the mid 1970s as a synthesis of biology and anthropology. As a student at Cornell University, Winterhalder studied with Steven Emlen, whose research on the formation of families in birds reminded him of humans. But while others were mapping these patterns in summary fashion — as trends in populations of, say, people or birds — Winterhalder wanted to understand how the trends arose from the choices of many individuals.

Early in his career, Winterhalder conducted fieldwork with hunter-gatherers — the Cree in northern Canada. Later, he moved into the Andes to study agricultural production — a serendipitous sidetrack.

"Having spent eight or ten years trying to understand the Cree as hunter-gatherers, I wanted to gain similar experience with agriculturalists from the same perspective — how individuals make decisions about subsistence. The idea was that if I could have some experience in both kinds of systems it would help me with what I think is one of the grand problems of anthropology — what evolutionary circumstances brought about the change from hunter-gatherer populations to agricultural populations through the process of domesticating plants. After hundreds if not thousands of papers, we still don’t understand that process well. We know an awful lot about it, but we don’t understand it. So I think of it as one of the great issues of human prehistory."