Observations on Primate Aggression

From time to time in SMT310 we cover the progress of the "Human Genome Initiative", that is, the project the goal of which is to sequence the human genome, to compare the sequences with other species and to continue to ask the question "Just what is it that makes us human," in the context of the sequences we find, their placement in the genome and their interactions with each other. Often our assigned articles comment on the great sequence similarity of genes of dissimilar species. But how about similar species? DNA sequence analysis shows that humans are so similar to the two chimpanzee species that humans might be thought of as "the third chimpanzee," according to Jared Diamond. On the average, the human genome shows that humans and chimpanzees share at least 98.5% of their DNA sequences.

The following extract comes from "African Genesis" by Robert Ardrey and the research notes of C.R. Carpenter. Both Ardrey and Carpenter are careful not to draw conclusions or to reach generalizations which might be considered to be too anthropocentric. Still, lay people not hide-bound by the scholarly strictures of anthropology might find it amusing that some of the creatures described may remind them of people they know.

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For an example of how such an [exceedingly aggressive] male may affect the workings of an animal society--in this case, a nonpredatory one--we must turn to the studies of C. R. Carpenter, and once more inspect the record of the rhesus monkeys transported from India to Santiago Island. Among all the generalizations which one may tentatively put forward concerning tendencies in animal conduct, none rests more firmly on universal observation than Carpenter's own conclusion, that faced by territorial invasion the home team almost always wins. Warfare can be, and must be, continuous along territorial boundaries. Hostility for one's neighbour must be unremitting, and when one meets him at the fence line one must give every evidence of intended rape, pillage, and bloodiest invasion. But when the shouting is all over and perhaps a skull cracked here and there, it is the rule that both sides retire where they came from.

We cannot say that conquest never occurs in the world of animals. Our studies are too incomplete and have been so far too hit-or-miss to state that permanent conquest is a type of behaviour confined solely to that species currently most eminent in world affairs. A family of gibbons, we know, will conduct a grape raid into a neighbouring arboreal pasture; and a hawk, if he can get away with it, will sneak an occasional titbit from the next hawk's mouse field. But such disorderly conduct in the world of animals seems with rare exceptions directed towards an immediate objective with rapid retirement in mind from the beginning. The fundamental rights of territory are honoured even when they are being violated. Despite our incomplete knowledge of animal affairs, one would be sorely tempted to state flatly that territorial conquest is a mode of animal behaviour so sporadic as to be of no evolutionary significance, were it not for an exception so carefully observed and so carefully analyzed as to causes, that one can only conclude that conquest as man knows it must occur in the animal world when conditions properly combine.

The rhesus monkeys transported from India, we will recall, were about three hundred and fifty in number; went to pieces morally on the voyage; and, resettled on the little Puerto Rican island, within about a year divided their thirty-six acres into territories, established their societies and regained their self respect. In the light of what happened later one must keep in mind that these territories were new and perhaps lacked to a degree the authority over behaviour that older territories might exert. If this was so, however, it was not reflected in rhesus behaviour. Each group quickly established its proper hostility for neighbouring groups, and in customary isolation quickly welded its social life into an amiable, xenophobic whole.

One of the objects of Carpenter's study was dominance. He had developed in the course of other primate studies certain criteria for dominant behaviour: how often one male would be the leader in a move towards a new feeding place, and how often another; which made the first move towards food in the morning, and which towards rest at night; which took the lead in territorial disputes, or voiced the first call in an emergency. Such criteria compiled in the dossier of each male in a single society gave in sum the individual's rank order in the hierarchy. And it told more. It gave one an index of relative dominance that could be applied to a whole species. Primates are not like jackdaws. There is no rigorous rank order in which every individual must always assume the same status in relation to every other individual. In primate societies there is simply a tendency for one male rather than another to take leadership in situations. Among gorillas that tendency is at its peak, so that one male rules and is never disputed. Among baboons the tendency is strong; a few males in the troop will make almost all the decisions, and the dominant rank of one male in relation to another will be quite distinct. The howler, for all his violent vocabulary, asserts the least rank in relation to his fellows. His is the closest to a co-operative, live-and-let-live, equalitarian society to be found in the primate world.

The rhesus monkey falls somewhere between the baboon and the howler. In a normal society containing half a dozen males, Number One will take the lead or win the argument on perhaps four or five times as many occasions as Number Last. And Carpenter found the ratio of dominance in his transported rhesus society on Santiago Island to be of such a moderate sort, differing little from that of untransported rhesus societies which he had studied in India and Siam. But while he was studying his various groups and making his calculations of dominance, an astonishing event took place.

Group I embarked on conquest. Group I seemed no different from any other troop on the island. It was average in size and contained the normal distribution of males, females, juveniles, and infants. Its territory was of the same order of magnitude, and the food supply which as I have mentioned was distributed daily by a caretaker--was equally available to all. No reason for systematic aggression could at first be discovered. But conquest nevertheless occurred. Daily Group I infringed on its neighbours--and got away with it. Daily, regularly, Group I made its feeding excursions on to the territories of not just one but five neighbouring societies. Group I was opposed, as it had to be opposed, by the injured societies. There was no weakness in the opposition. But Group I by some mysterious power broke that most fundamental of animal laws, that the home team wins. In this case the home teams, all of them lost; and Group I had its way on opposition territory. The mystery, however, became quickly solved.

GROUP I contained a male of almost unbelievable dominance. He was Number One, of course, and his factor of dominance as compared to Number Last was about fifty. While a normal maximum in the rhesus would be about five, Number One had as great an advantage as that even over Number Two. That all powerful natural accident, conception, had placed in the genes of a remarkable monkey such resources of strength, of energy, of courage, and of assurance that he had become a giant of dominance. And his very presence in a society amazingly enough communicated to all members of the society the resources of his nature. Group I was pervaded by its leader's character, and despite all laws of territorial behaviour acquired the capacity, as a society, to dominate its neighbours.

Carpenter removed the master monkey from the master society. The troop immediately fell back to its own territory. Not once during the exile of its leader did the society commit a single act of trespass. Then Carpenter restored the monkey to his fellows. Without hesitation Group I returned to its field of conquest. I find it difficult to review Dr. Carpenter's careful study without recalling certain enigmas of the lion and of man. Nowhere in all the unexplored jungle of animal behaviour does the observer catch more fleeting glimpses of profound, suggested truths than along the shadowed paths of animal dominance. So little do we truly know that observations are frequently little but hints. A movement is reflected in a still pool; but when we look up, nothing is there. A face peers at us from the depths of a vine; but when we stir the leaves, the face is gone. We may say with certainty that the instinct for hierarchy benefits many an animal society.

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Here is a description of the colony by Carpenter:

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The field work for this report was done on Santiago Island, a small Cay of about 37 acres which lies five-eighths of a mile off the Southeast coast of Puerto Rico. The nearest point on the mainland is La Playa de Humacao and it is from there that boats leave to cross the narrow channel to the Island. The topography of Santiago Island is irregular. Its surface is covered by a cocoanut palm grove, scrub trees, grass and weeds, rocks and new plantings of shade, fruit and ornamental trees. The Island offers a situation which, in many respects, is uniquely favorable for a breeding colony of Rhesus monkeys. Here according to original plans, field and laboratory research were to be closely integrated.

The original stock of the Santiago Colony was collected by the author in India during September, 1938. Three months later, 409 monkeys were released on Santiago Island. On March 1, 1940 the Rhesus Colony had about 350 monkeys in various stages of development ranging from infancy to senility. There were 24 adult males and four sub-adult males who lived in heterosexual groupings, while 12 subadult males lived in unisexual male groupings, thus making a total of 40 males in the Colony. There were approximately 150 adult females, making a ratio of more than 6 adult females to each adult male. In addition there were 3 sub-adult females, 48 infants born in India during July and August, 1938,90 infants born in the Colony between May 19 and December 31, 1939 and 20-odd infants born since January 1, 1940.

Besides these Rhesus monkeys there was a male, a female and an infant of the pig-tail type (Macaca nemestrinus) constituting a small family. The animals organized themselves into 6 heterosexual groups ranging in size from 3 in the case of the pig-tail group to 147 for the largest Rhesus group. The smallest heterosexual Rhesus group had 13 individuals. The average Rhesus group contained approximately 70 animals. This report is based on observations of 40 different Rhesus females of the Colony who passed through 45 periods of sexual receptivity between February 29 and April 27,1940. When this study began the Rhesus Colony had just been released from isolation cages, in which they had been held for six weeks for tuberculosis testing.