[Autoregulation in mammalian populations and stress: an old theme revisited]

Abstract

We consider the current state of four hypotheses explaining the auto-regulation of population numbers in mammals: 1. Density -- and frequency-dependent selection of genetically determined stereotypes of behavior (Chity, 1960, 1967). 2. Physiological effects of stress conditioned by redundant density (Christian, 1950, 1963; Christian, LeMunyan, 1958). 3. Maternal effects on offspring (Ward, 1984; Lee, MacDonald, 1985). 4. Kin hypothesis based on assumption of temporal variation in relatedness of neighbors (Charnov, Finerty, 1980). All four hypotheses consider the density of individuals or their signals (visual, acoustic, or chemical) to be the main factor of regulation, and are based on an assumption of density-dependent variation in spacing behavior. The local density is considered to be derived from the ability of individuals to disperce. This ability appears to be determined by internal (including genetic), as well as external limiting factors. The above hypotheses are not quite alternative. In fact, all four hypotheses imply stress (directly or indirectly) as a regulatory mechanism within the population, although its role is usually associated with J. Christian's hypothesis. Besides the direct density-dependent, physical exclusion of competitors, social behavior may activate and control neuroendocrine regulatory mechanisms which lead to a reduction in breeding intensity and maturation, and increased mortality due to functional disorders and immunity syppression. To date we cannot reject any of these hypotheses. We believe that progress in understanding the role of physiological stress in the regulation of population density in mammals will depend on attention to the following factors: 1. Abundance (availability) of feeding resources, predators and parasites are real factors affecting levels of physiological stress in wild mammals. Population density depends on these factors, and their role in the stress dynamic in the population can be decisive. 2. Social conflicts (density-dependent behavior) are usually considered to be the main factor of physiological stress and a keystone of autoregulation mediated by stress. We still pay little attention to the role of stress hormones and neurotransmitters in the regulation of animal behavior. Both hormones and neurotransmitters can greatly affect individual behavior, including interactions between individuals in the population. 3. A common viewpoint is that stress negatively influences ontogenesis, reproduction, and survival. At the same time, in nature we often observe a positive relationship between stress and reproduction, or stress and survival, and we must pay more attention to this phenomenon. 4. In spite of a well developed theory about the role of stress in the regulation of reproduction and survival in mammalian populations, we still do not pay much to species-specific characteristics of the neuroendocrine mechanism of stress and its effects on population demography.