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Factors affecting health and disease are somewhat neglected in the study of the dynamics of mammalian populations. The present paper summarizes a number of studies on social stress in rats and mice. Social situations induce classical stress responses in terms of corticosterone and catecholamines. More important is the fact that there is a considerable individual differentiation in the magnitude of the responses in relation to the behavioural stategy in a social situation. These physiological differences render individuals differentially susceptible for stress pathologies such as cardiovascular diseases and immune deficiencies. Moreover, the behavioural strategy largely determines the position of an animal in the social structure and hence the frequency of behavioural and physiological activation. The data suggest that the individual behavioural and physiological reactivity determine in interaction with the social environtment the susceptibility for diseases. The relevance of these observations for the dynamics of a population is discussed.

This study tries to explain the discrepancy between the frequencies of t-alleles found in house mouse populations and those expected on the basis of a deterministic model, by hypothesizing a difference in trappability between + /+ and +/t genotypes. We found variation in t-genotypes to be of no influence on trappability of the individuals in ten experimental populations.