The Social Regulation of Population Density and Age-Sex Distribution in the Toque Monkey

Abstract

[The relationships between behavioral, ecologicaland demographic data are examined for a wild population of nearly 450 toque monkeys, Macaca sinica, of Sri Lanka, based on 3½ years of continuous, plus 3 years of intermittent study. Under relatively stable ecological conditions troops fluctuated in size within limits, and the net growth of the population was zero (Ro = 1). Decrease in food supply caused a net decrease in population size. An abundant food supply stimulated population growth. A lifetable indicated that mortality differed markedly according to age and sex, and that this pattern was not attributable to predation or disease alone. Behavioral analysis revealed that 81.5% of all threats occurred during foraging. Threatened individuals were prevented from feeding, and their foods were usurped directly in 36% of all threat interactions. The observed frequency of threats between age-sex classes during foraging differed significantly from two progressively stronger null hypotheses. These hypotheses took into account, respectively, the age-sex distribution of the society, and the frequencies of nearest neighbor associations between animals of different age and sex. The agonistic discrimination between animals of different age and sex was mirrored by a similar affiliative discrimination. Some conclusions of this analysis were that adults dominated the juveniles which in turn dominated the infants, and among juveniles and infants males dominated their female peers. Adults likewise discriminated against the young females. The behavioral relationships during foraging influenced the foraging efficiency of animals, which was measured by the amount of time spent in foraging, feeding rates, and the quality of foods that were consumed. By these measures adult males fared best, then adult females, then juvenile males, and juvenile females fared worst. I proposed that as a consequence of these behaviorally imposed feeding efficiencies, young animals died at greater rates than adults, and infant and juvenile females died at greater rates than their male peers; behaviorally mediated access to other resources is likely to influence mortality in a similar manner. During the mating season, threatening and wounding increased markedly among adult and subadult males that competed for mates and that migrated. The peak of migration rates in males occurred during the subadult phase and coincided with the peak of mortality in males. This suggested that the rigors of migration (low priority of access to rcsources and frequent wounding) underlie the observed mortality. Mortality in adult males was (1) less than in subadult males which migrated more frenuently, and (2) greater than in adult females which do not migrate. Competition between troops, in part, determines the amount of food available per troop, and thereby contributes to setting the upper limit to troop size. The molding of the age-sex structure is then determined by social processess within a troop. I hypothesized that the effects of social behavior (mainly aggressive and affiliative behaviors) and possibly its nature, change in accordance with the age and sex distribution of the society, its density, and available resources. Social behavior determines, through socially imposed mortality, the age and sex distribution and density of the society (and hence the population) in such a way as to maximize the reproductive success of some of its members, and results in bringing the society (and population) towards an equilibrium state (Ro = 1) with the resources and non-socially imposed mortality., The relationships between behavioral, ecologicaland demographic data are examined for a wild population of nearly 450 toque monkeys, Macaca sinica, of Sri Lanka, based on 3½ years of continuous, plus 3 years of intermittent study. Under relatively stable ecological conditions troops fluctuated in size within limits, and the net growth of the population was zero (Ro = 1). Decrease in food supply caused a net decrease in population size. An abundant food supply stimulated population growth. A lifetable indicated that mortality differed markedly according to age and sex, and that this pattern was not attributable to predation or disease alone. Behavioral analysis revealed that 81.5% of all threats occurred during foraging. Threatened individuals were prevented from feeding, and their foods were usurped directly in 36% of all threat interactions. The observed frequency of threats between age-sex classes during foraging differed significantly from two progressively stronger null hypotheses. These hypotheses took into account, respectively, the age-sex distribution of the society, and the frequencies of nearest neighbor associations between animals of different age and sex. The agonistic discrimination between animals of different age and sex was mirrored by a similar affiliative discrimination. Some conclusions of this analysis were that adults dominated the juveniles which in turn do