C.R. Carpenter, a student of Yerkes (see Gould's paper) set up a colony of rhesus monkeys. This was on the Caribbean island of Cayo Santiago. The animals were to supply the needs of medical research, and at the same time afford students of animal behaviour (ethologists) an opportunity to study rhesus monkeys in the wild. In December 1938 450 rhesus monkeys and 14 gibbons were released on the island. The animals had been caught in Indo-China, Thailand and Malaya. After 18 months the rhesus monkeys, after much in-fighting, had settled into six groups. The gibbons could not peacefully co-exist with the rhesus monkeys, and therefore had to be recaptured and removed from the island.
The colony was disturbed during the second world war. It was difficult to get supplies out to the island. Four hundred and ninety animals were shipped to the mainland. These were used for research on disease. The colony and island went into decline.
In 1956 Stuart Altmann reintroduced the regular census of the rhesus monkeys, thus allowing a thorough longitudinal study of social organisation to be conducted.
Until the 1970's the primary purpose of Cayo was to supply animals for medical research. This meant that animals were randomly selected for export haphazardly. No regard was given to the animal's position within the social structure of it's troup. This obviously interfered with the study of social organisation. Fortunately this policy was changed, allowing four troops to remain undisturbed.
The advantages of studying this island colony of rhesus monkeys are that, unlike field studies, all animals are generally clearly visible. In field studies many unknown animals contribute to the social organisation of the troops. The main disadvantage of using a small island to house a colony of rhesus monkeys, is that food has to be provided for the animals; this precludes the possibility of studying their feeding ecology.
At the time of writing, there were 610 rhesus monkeys on Cayo. Apart from six solitary males, the rhesus monkeys live in six social groups. The sizes of these groups vary from 53 to 139 animals. The age, sex and maternal geneology of every animal are known. The animals are trapped only once a year to tattoo the youngsters born that year and to take blood samples for the continuing genetic work.
Different behaviours can be observed at different times of the year. The year can be split into two main seasons: the mating season (July to December) and the birth season (January to June).
The activities of the animals vary according to the time of day. The animals look for food and eat during the cool of the morning. During the heat of the day, the animals rest and groom each other. Towards evening they play and eat again, finally retreating to the trees to sleep. Grooming is not only for sanitary reasons, removing parasites and matted hair, it is also for social reasons. Grooming cements the bonds between group members. Grooming tells us about the hierarchical structure of the troop. Lower status animals groom higher status animals. Higher status animals are groomed more frequently and for longer periods compared to lower status animals.
Each social group consists of a number of adult males and two to four matrilines. A matriline consists of an adult female, her adult daughters and all their juvenile offspring. Males leave the troop upon reaching maturity (about three to four years), whereas females stay within the group. The adult males in a troop have originally come from another troop. They might join another troop at some time, but they will never return to their natal troop (i.e. the troop where they were born).
The hierarchy allows dominant animals to have access to desirable objects, such as grooming and food. The hierarchy serves to keep aggression to a minimum, with a well-developed code system for signalling dominance or submission. Overt fighting rarely ends in death (although wounds received can go septic and cause premature death). The dominant animals win their social status by fighting rivals, but afterwards aggression is kept to a minimum. The monkeys get to know what the likely outcome would be without engaging in battle; this avoids time wasting, allowing the animals to get on with other tasks.
Adult males generally are of a higher status than juvenile males and all females. Each matriline will have a status, established by fighting amongst the females when the troop was formed. All member's of the dominant matriline will be of a higher status than members of other matrilines. Within a matriline, the most dominant female after her mother, is the youngest mature female Young males benefit from their mother's rank, but when they leave for another troop they start at the bottom and slowly work their way up the hierarchy.
Figure 29.1 (p495) illustrates the splitting of troops that occurs when the troops become too large.
This paper concentrates on the laboratory and raises fundamental questions about methodology. The paper also raises ethical issues.
Because Behaviourists argued that as all higher organisms evolved out of lower organisms, they considered that it was valid to study lower animals. They feel that lower animal behaviour is merely quantitatively different from that of higher animals. Behaviourists therefore studied animals like rats and pigeons in the laboratory, and emphasised the importance of learning. Ethologists feel that Darwinian theory has been misinterpreted, and therefore study animals in the wild, with an emphasis on the importance of genetically determined behaviour. Ethologists are zoologists and are concerned with viewing behaviour as a biological entity. They are also happy to study animal behaviour for its own sake, without necessarily comparing the behaviour to that of humans. They consider two critical questions when interpreting animal behaviour: (i) what is the behaviour for (function)? (ii) How has it evolved within the species (phylogeny) (Hinde, 1982)? Behaviour is ultimately seen as important for the survival of the species. Behaviour that does not help the species survive would become extinct, like the animals who practised the behaviour. Living in groups is seen to be important for the survival of the species, according to Clutton-Brook (1974) for three reasons: (i) defence and avoidance of predators; (ii) finding and handling food; (iii) reproduction. On Cayo Santiago, there are no predators, and food is provided; because of this only the reproduction aspect of the social group can be rigorously studied.