Altruism is the performance of a behavior that will benefit the recipient at a cost to the donor, such as risking one’s life to save another. Numerous examples exist of animals exhibiting altruistic behavior. For instance, adult crows may act as “nannies” for other crows, instead of increasing their fitness by producing their own offspring. Another example is a ground squirrel that will warn others of the presence of a predator, even though making such a call may draw the attention of the predator to itself. In studying social insects, American biologist Edward O. Wilson (1929–) found that in many species of social insects, workers forego reproduction entirely (they are sterile) in order to help raise their sisters. This behavior can be explained in two possible ways. The donor is performing the act either in hope that the recipient will someday return the favor (reciprocal altruism) or because the recipient is a family member. In the game of evolution, winners are those who leave the greatest number of copies of their genes in the subsequent generations, so this “kin selection” form of altruism may not be so altruistic, after all.