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Power among monkeys

Social disputes and the capacity to track through the environment in search of food, molded the brain of primates



Like a boss: the emperor tamarin waits for its subordinates to find food and then steals it from them

Known locally as the bigodeiro (Emperor Tamarin), the tamarin at the side likes to dominate. At the moment of eating he remains at a distance and leaves the other tamarins  in the group to look for food in the crown of the trees. When he sees that they have found something, he immediately lets out sharp screams like a jeer and expels his companions from nearby, making it clear who it is that gives the orders around there. This Italian mafia like behavior is not just valid among this species of monkeys. When he goes out looking for food with smaller species, the capo also imposes his superiority upon the others… with the scream. But the capacity to recognize the role that each animal performs in its group is not the only thing to rule the life of these two species of capuchin monkeys. After daily accompanying two groups of emperor tamarins for four months and two of the saddle-backed tamarin in a stretch of the Amazonian rainforest in plain urban area of the city of Rio Branco, the primatologist Júlio César Bicca-Marques, from the Catholic Pontifical University of Rio Grande do Sul (PUC-RS), verified that just as important as knowing who is the boss is the capacity to use signs available within the environment to find food. Associated, these abilities help to mold the intelligence of these monkeys and of other primates – the group of animals that includes human beings, although these results cannot be transposed directly to our species, which is subject to more intriguing social relationships and capable of altering the very environment.

This conclusion was born from two independent ideas about the development of the brain and intelligence of primates launched during the 1970’s. When observing African monkeys, the anthropologist Sue Taylor Parker concluded in 1977 that the capacity to deal with environmental or ecological information, such as finding the route back home or discovering a tree with fruit, had been essential for the survival of primates. Thus, over thousands of years nature had favored the survival of those with the greater ability to make use of this type of information. According to this rationale, the greater and greater need for dealing with environmental information would have brought about a more and more  voluminous brain – those of the tamarin, some 35 million years distant from human beings from the evolutionary point of view, weigh approximately 30 grams, while ours, approximately 40 times greater, have an average weight of 1,350 grams.

Machiavellian behavior
Not everyone had agreed. In 1976 the British psychologist Nicholas Humphrey had suggested that the factor that would have guided brain evolution in primates would have been social order. Nature would have benefited those with the ability to relate with other members of the group – and even manipulate them with the objective of keeping the group cohesive. In the opinion of Humphrey, this ability would be related to the capacity to deal with another information category, known as social or Machiavellian, in reference to the Florentine thinker Nicolas Machiavelli, who in 1513 described in his work The Prince the political and social behavior used by rulers to preserve their power. It is this information category that the offspring of the emperor tamarin, or even an adult saddle-backed tamarin, uses when it abandons a recently discovered hog plum or inga and allows the dominant male to feast itself alone. Respecting the particularities of each species, this is a decision similar to that of someone who allows an armed robber to steal his car without letting out any reaction because he knows that there is a greater chance of not being injured and that he can get another car later.

Humphrey argued that primates need to be “calculating beings”:  they had  be capable of evaluating the consequences of their own behavior, of the behavior of others and of balancing between losses and gains, decisions being made based on information that is not always reliable. Supposing that this was the situation most frequently found in nature, this ability or intelligence would have been the main force to model transformations because it had passed through the primates’ brain since the evolution of this animal group, around 50 million years ago.

For almost three decades the supporters of one or other hypothesis collected evidence without coming to a consensus of opinion. Now, in this series of experiments with the Amazonian tamarins, the Bicca-Marques arrived at a conciliatory conclusion. It is impossible to determine the supremacy of one form of intelligence over another: both are essential for the survival of the tamarins. “A consequence of life in a group”, says  Bicca-Marques, “is that the primates must decide about where to search for food taking into consideration the probability of finding food in a determined location, environmental information, allied to the possibility of having access to food or of sharing it with other group members, a piece of social information”.

Bicca-Marques began to suspect that these factors didn?t act in isolation upon the development of the brain during observations of how these monkeys behave at lunch time. In 1993 he resigned from his post at the Ministry of the Environment in Brasilia, and installed himself at the Federal University of Acre (Ufac) in order to study these tamarins that he had known only from books. Parallel to this, he looked for the North American anthropologist Paul Garber, from the University of Illinois in Urbana, a specialist in the behavior of these little primates, who helped to plan the experiments that would allow him to control the access of the monkeys to food.

On a 3-hectare area of the Ufac’s Zoo-Botanic Park, Bicca-Marques set up food stations in which it was possible to control the conditions in which emperor tamarins (Saguinus imperator) and the saddle-backed tamarins (Saguinus fuscicollis) found food – each station was formed by eight trays set in a circle of 10 meters in diameter. At 15 steps from each station an observation tower was mounted similar to a house upon piles, from whose interior it was possible to see the monkeys without being noticed. From the 22nd of September 1997 until the 29th of January 1998, Bicca-Marques and three biology students got up every day at 3:30 a.m. and went into the forest to the towers where they would spend, often under almost 40 degrees of heat, from nine to ten hours seated accompanying the monkeys’ meals. During almost 4,000 hours of monitoring, the monkeys visited the stations some 1,294 times. In the majority of cases, five or six tamarins  of the same species – S. imperator or S. fuscicollis – appeared for lunch.

During the 120 days of the experiment the primatologist?s team simultaneously prepared within the four stations tests in which the tamarins had to learn that the bananas were always in the same trays – whilst the others exhibited plastic bananas – or that a yellow cube or a colored wooden pole had indicated the position of food. The monkeys came out well in the first test, but only some members of the emperor and saddle-backed groups discovered that the yellow cube and the wooden pole indicated the tray with the banana. The fact that some monkeys did not make use of these signals to find food does not mean  that they would not be capable of making the association. When these results were analyzed, taking into consideration the species – S. imperator or S. fuscicollis – and not each individual group member, it was concluded that both the emperor and saddle-backed monkeys know how to deal with environmental information in order to find food.

Gains and loses
But it was the behavior of these tamarins – when they would arrive to feed in groups of a single species or in mixed groups – that revealed: it is truly not possible to separate the influence of environmental intelligence upon the brain?s development from the influence of social intelligence. Always when one of the two bands of emperor tamarin appeared to eat unaccompanied, the group’s strongest male – called dominant or alpha, a type of capo – waited for his subalterns to locate the bananas before manifesting himself and taking care of what he considered to be his. This is something similar to what happened in the mixed groups. Only among the saddle-backed tamarin was the level of collaboration greater: frequently all of them had exerted themselves in search of the bananas in the trays. This apparently unjust cooperation, the proto-cooperation, in truth benefited both sides. The emperors won, as they saved energy while their subordinates searched for food in the lower parts of the forest, and the saddle-backed won, who waited their turn to eat the fruit found by the emperors in the crown of the trees or captures the insects that escaped from them and fled close to the ground. As well as this, both benefited through the watched against predators carried out by their companions.

Another peculiarity of the acquaintanceship between these two species is that the weight of each type of information appears to vary from one moment to the next. “These small primates deal with both forms of information alternately throughout the day”, says t Bicca-Marques, who described his discoveries in a series of articles, the most recent published in the American Journal of Primatology, in the International Journal of Primatology and in the Journal of Comparative Psychology. When they learn that a certain tray always contains a piece of banana, the subordinates use the environmental information to find the food. For the dominant tamarin it is the social information that is valid when they use their hierarchy position to take the food found by the others, although they also know how to use environmental signals.

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