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The two male capuchin monkeys brace themselves against the floor. They each pull on a metal bar, slowly bringing a metal tray closer and closer. Seeing these two monkeys straining with all their might is an impressive sight; the tray is weighted down with as much as six or seven kilograms, yet pulling together they finally manage to lock the tray into place, in front of the cage. Phil quickly reaches into a dish on the tray and grasps some apple slices. A mesh partition between Phil and his partner, Drella, prevents Drella from reaching the apple slices and also prevents Drella from taking the apples by force. Will Phil share the apple with Drella?
If these were two humans instead of monkeys, the answer would be a simple "of course". One would expect Phil to feel gratitude towards Drella for helping him obtain the apples (one monkey can not pull the tray by himself). If Drella was not rewarded for his hard work, surely he would feel dislike or disgust for Phil and refuse to help him again. It is easy for us to assess the action a person would take in this situation. Most of us participate in many of these exchanges every day. We treat a friend to lunch because they paid last time. We buy an expensive birthday present in exchange for a similarly expensive present we received. The rules regulating these types of exchanges are obvious to us and we are sensitive enough to pick up on the slightest infringement of them. An invitation not reciprocated, a favor not returned. These transgressions can seriously strain a relationship. Our emotions serve to maintain this system, specifically the feelings of friendship, dislike, aggression, sympathy, trust and suspicion. How did such a system come about?
According to one scientist, Robert Trivers, this system of reciprocal altruism evolved in early hominids during the Pleistocene. To explain reciprocal altruism, one must first understand altruism. In his words, an altruistic act is any action that "benefits one unrelated individual while being apparently detrimental to the organism performing the behavior". Altruistic acts had long been a source of confusion for the evolutionary biologist. How could such behaviors, harmful to the performer and advantageous to the recipient, be selected for? For example, a fireman running into a burning building to save another individual. If individuals who had these altruistic tendencies were also more likely to die because of their actions, it seems as though the trait for these tendencies would quickly die out. The theory of kin selection, proposed by William Hamilton, helped explain the existence of altruistic behavior towards related individuals. By helping related individuals, individuals would increase their family members' chances of survival while decreasing their own. But because related individuals share genetic material, these altruistic acts would increase the survival rate of their own genes, if not the survival rate of the individual and therefore would be selected for in the long run.
By contrast, an individual who runs into a burning building to save the life of a stranger is not preserving future generations of their own offspring. But acts of altruism like this one could be selected for if other individuals reciprocated these acts. In a stable society of animals with a long life span, individuals who performed altruistic acts for each other would all increase their chances of survival. If individual A has a surplus of food and shares with individual B, who is too sick to hunt for food, individual B may recover and reciprocate when times get tough for individual A. In this way, individuals who were altruistic would have an evolutionary advantage as long as all individuals in a society participated in the even give and take of altruistic acts. This is why the emotions discussed above are so important; they serve to regulate behavior and encourage all individuals' participation. While we make think of complex emotions such as disgust and sympathy as uniquely human, our closest relatives, non-human primates, show an amazing degree of cognitive, and social complexity. Scientists have begun focusing on these animals to model the early hominids among which reciprocal altruism first evolved.
Both capuchin monkeys, the New World primates mentioned above, and chimpanzees, great apes native to Africa, are being studied in a variety of experiments. Both species are ideal candidates for study in this area of investigation. Both species and, indeed, almost all primates, live in small, stable social groups and have relatively long life spans. But chimpanzees and capuchins are unique in that they have relaxed dominance structures. In both societies, individuals do have identifiable ranks, with higher rank allowing an individual access to benefits such as more food and more opportunities for reproduction. However, these structures are not nearly as rigid as they are in other primate species. This relaxed social structure contributes to a rare occurence with in primates: both species commonly share food outside the mother-infant relationship. This is absolutely unheard of in other species such as baboons or macaques.
In a series of experiments conducted by Dr. Frans de Waal at the Yerkes Regional Primate Center of Emory University, two capuchin monkeys were separated by a mesh partition and one monkey was given some apple slices. Female capuchins shared reciprocally; that is they shared more with individuals who shared more with them. Experiments with chimpanzees showed that chimpanzees also share reciprocally. Chimpanzees also showed reciprocity across different behaviors. Not only was individual A more likely to share with individual B if B shared with her, individual A was more likely to share with individual B if B had groomed her the morning of the test session. The chimpanzees seem to have set up an economy where grooming, an affectionate and hygienic examination of one individual's fur by another individual, can be exchanged for food. Another factor that seems to increase the likelihood of sharing is cooperation to obtain the food.
As gruesome as it may sound, wild chimpanzees and capuchins are both occasional carnivores. Chimpanzees hunt colobus monkeys and capuchins hunt coati pups. Sharing may have evolved out of hunting behaviors because successful hunting often requires more than one hunter. But if the dominant male were to claim all the meat at the end of the hunt, other animals would have no incentive for participating in the hunt. In chimpanzees, individuals who participate in the hunt are more likely to receive pieces of the carcass than other males of any rank, who did not take part in the hunt but are merely interested in the food. In chimpanzees, an individual who cooperates with another individual to obtain food is rewarded for their participation.
The experiment described in the opening paragraph of this article is a approximation of this situation in captivity. The capuchins must work together and cooperate for one monkey to obtain food. Experimenters are interested to see how food is distributed. Control tests were conducted where only one individual has a bar in front of them to pull and the apple is in front of this individual. Rates of sharing in this condition, when only one individual worked to obtain food for himself, were compared to rates of sharing in the cooperative condition. Results from the experiment have been inconclusive so far. Capuchins do obtain a high rate of success on cooperative trials but sharing seems to be the same on cooperative and control conditions. In order for the capuchins to truly be cooperating they have to be taking their partner's actions into account and adjusting their own actions to complement the partners. If the capuchins were not truly cooperating they could just be acting similarly and both responding to a stimulus (for example, the presence of apple) by pulling on the bar.
Experimenters designed a twist on the cooperative test to determine if the capuchins truly understood cooperation, that is, if they were monitoring their partner's actions and responding accordingly. Experimenters replaced the mesh panel between partners with an opaque panel with a hole in it. The panel decreased opportunities for visual communication but sharing and visual communication were still hypothetically possible. In the opaque panel condition, success rates dramatically decreased. By examining the visual and vocal components of communication in both conditions, Dr. de Waal concluded that communication was necessary for success and that capuchins do understand cooperation.
These results are encouraging for Dr. de Waal and others who hope to examine the origins of reciprocal altruism and the social rules that form the basis of our society. Future studies that continue in this vein should reveal that there is alot to learn from our primate relatives. After all, as Dr. de Waal is fond of saying, "You don't have to be human to be humane".
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