Vampire bat reciprocityLogic would suggest that people behave selfishly when resources are uncertain. If I were a hunter/gatherer—or an economist—and didn't know when the next bison would appear, I might be less inclined to share my pile of dead rabbits with my neighbors. (Or, I suppose being a vegetarian I'd be stingy with my portobella mushrooms or something.)
However, large-scale mathematical simulations indicate that communal sharing is the advantageous strategy: Small groups that share are resistant to attacks by selfish competitors and can themselves conquer groups of selfish actors . But the simulations control for too many factors (e.g., they assume all actors have equal strength), so it's satisfying to see this effect in the real world, as well.
Expecially in bats. "Field studies of the much-maligned vampire bat have revealed that they are highly social animals. They spend a great deal of time grooming each other. They have special friends with whom most of this grooming is done. When one of them has a bad night and fails to find an animal from which it can suck blood—licking is really what they do—its friends will regurgitate part of their own dinner for it. Days later, it will repay that debt when its friend has a bad day" .
 Kameda, T., Takezawa, M., & Hastie, R. (2003). "The logic of social sharing: An evolutionary game analysis of adaptive norm development." Personality and Social Psychology Review, 7, 2-19.
 Dunbar, R. (1997) Grooming, Gossip, and the Evolution of Language. Cambridge, Massachusetts: Harvard University Press.