Five populations of marmots were studied in Colorado from 1962 through 1972. Males are classified by age as yearlings, two-year olds, and adults. Socially, marmots are colonial, peripheral, or transient. Each population was divided into four age-sex classes, male yearlings, female yearlings, male adults, female adults, for analysis of social behaviour. Social behaviour was amicable or agonistic. Both types of behaviour occurred between male yearlings and each of the other age-sex classes. Play-fighting characterized social behaviour among male yearlings. Agonistic behaviour characterized social interactions between male yearlings and male adults. Male yearlings avoided male adults and usually dispersed from the locality. Adult male aggression enhanced but was not necessary for dispersal of yearling males. Agonistic behaviour between males and the four age-sex classes was not related to population density.
Territorial males are conspicuous and advertise their presence by tail flagging. Many colonial males patrol their territory. Adult male social behaviour is predominantly agonistic. Mean rates of amicable and of agonistic behaviour between adult males and adult females were greater for years of male turnover than for years of male returns.
Territoriality is characterized by exclusive use of the area by the colonial male, vigorous defence against transient or peripheral males, and by a relatively fixed area. A decrease in the number of males in an area causes an increase in the size of territories. Additional males in an area causes a decrease in the size of territories. The mean size of 24 typical territories is 0.67 ha. Reduction in the number of territories in an area was associated with increased agonistic behaviour among adult males. Fights were rare and occurred when the territorial system was unstable.
The major functions of the territorial system are enhanced reproduction in a limited environment, enhanced outbreeding and enhanced fitness of colonial males.