. Advances in the study of mammalian behavior. Mammals. 704 Spec. Publ. Amer. Soc. Mamm. 7. m' j'j'a 's' o'n'd' j'f'm'a'm'j' j'a's' o'n'd' j'f'm'a'm'j' j'a's' o'n' d'j'f'm' a'm'j' j' 1972 1973 1974 1975 Fig. 1. Demography of a cycling vole population, Microtus pennsylvanicus, in southeastern Massachusetts. Shading identifies winter months. Summer breeding periods are denoted by vertical lines. (From Tamarin, 19776; copyright 1977, The Ecological Society of America.) ual voles, either through direct contacts or by indirect cues to density such as pheromonal signals (Bronson, this volume; Muller
. Advances in the study of mammalian behavior. Mammals. 704 Spec. Publ. Amer. Soc. Mamm. 7. m' j'j'a 's' o'n'd' j'f'm'a'm'j' j'a's' o'n'd' j'f'm'a'm'j' j'a's' o'n' d'j'f'm' a'm'j' j' 1972 1973 1974 1975 Fig. 1. Demography of a cycling vole population, Microtus pennsylvanicus, in southeastern Massachusetts. Shading identifies winter months. Summer breeding periods are denoted by vertical lines. (From Tamarin, 19776; copyright 1977, The Ecological Society of America.) ual voles, either through direct contacts or by indirect cues to density such as pheromonal signals (Bronson, this volume; Muller- Schwarze, this volume; Richmond and Stehn, 1976; Vandenbergh, this volume). However, a density increase does not necessarily mean that mice encounter each other more often. Pearson (1960) showed that as density increased, there was not a simple increase in runway usage; rather, mice built more runways. In fact, the relative number of active runways has been used as an indicator of density (Lidicker and Anderson, 1962). However, we will assume that increased den- sity does lead to more interactions, or at least that mice are aware of increased density as it occurs. According to Christian (1980), a higher rate of interaction results in the general adaptation syndrome (Selye, 1946), which causes an eventual exhaustion of the adrenal-pituitary system and leads to increased mortality, decreased reproduction, and a population decline. According to Chitty (1967; 1977), increased interaction at higher densities leads to selection for aggressive voles capable of functioning under higher density. Thus, as density increases there are more and more aggressive individuals. Eventually, however, a decline occurs because reproduction is depressed among highly aggressive individuals, and they suffer increased mortality. Both hypotheses suggest that evolutionary mechanisms affect population. Please note that these images are extracted from scanned page images that may have been digitally enhan
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