A classic example of how friends helped each other is the hunter-gatherer theory. When men killed an animal that had too much food for his family, and another man did not hunt as well that time, they would share it and later reciprocate the favor. If efforts were not reciprocated, the hunter would no longer share with him. Many examples of cooperation are found in nature. Vampire Bats share blood with friends that were not successful in a hunt by regurgitating extra blood from their stomachs.
Baboons seek help from friends in an effort to gain sexual partners, and higher status in the group.
Studies have found that when an ape helps another gain sexual favors, the female ape mates with the friend who received the help. Apes who help another are much more likely to receive help in the future. A study has shown that chimpanzees not only reciprocate their shared endeavors, but they also recruit the aid of others in an attempt to keep control of group status and mating dominance. The concept of the social contract theory has many components. Reciprocal altruism benefits organisms, however, the benefit is not always reciprocated immediately.