Temperament is a complex behavioral trait that describes characteristic patterns of response to environmental, particularly social, conditions and perturbations. Disparities in the tendency to approach or avoid novelty or readiness to engage in aggressive interactions have been documented in comparisons between species (Christian 1970), subspecies (Gonzalez et al. 1981), populations within species (Champoux et al. 1994), inbred lines of laboratory animals (Scott and Fuller 1965), domesticated versus wild populations (Price 1984), and individuals within a species (Benus et al. 1992). Differences in physiological stress response systems (Selye 1937) are commonly identified as an important proximate mechanism underlying these temperament differences (Huntingford and Turner 1987, Kagan et al. 1988). Social systems of animals are perceived as emerging from relationships between individuals (Hinde 1983). Individual interactions, in turn, are hypothesized to reflect individual behavioral strategies which maximize inclusive fitness (Silk 1987). Selection on a physiological system, which can dramatically affect the pattern and outcomes of individual interactions, could produce evolutionary change in social organization and social behavior. Many workers explicitly suggest that temperament differences among primate species are adaptive in many instances, yet admit that the specific ecological and social selection pressures to which the neuroendocrine system is responding are often unclear (Thierry 1985, Clarke et al. 1988, Richard et al. 1989). Species-level comparisons have not offered many testable comparative models, probably because of confounding effects such as large phylogentic distances or uncertain phylogeny, inadequate knowledge of ecological and social conditions in the wild, drift, and convergent evolution. In short, little progress has been made toward understanding the evolution of stress-response patterns in primates. In this chapter I suggest that comparisons of geographically and genetically separated primate populations or subspecies may be an alternative and more successful approach to addressing the evolution of stress responses and the disparate social behaviors that result. Population and geographic comparisons are likely to be profitable for three reasons: (1) comparisons are less likely to be confounded by phylogenetic disparities (Arnold 1992), (2) the factors imposing different selective regimes among localities can perhaps be more readily identified, (3) hypothesis testing may be facilitated because populations suitable for testing a model will be easier to identify than new species.
Commentary: Grounded procedures: A proximate mechanism for the psychology of cleansing and other physical actions
