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Population processes in Peromyscus: an experimental approach Fairbairn, Daphne Janice


This study examined seasonal variability in demography, genetic composition, and behavior, in populations of deermice (Peromyscus maniculatus), Particular attention was paid to the possible role of aggression and dispersal in determining densities. Three types of populations were examined by means of biweekly live-trapping: undisturbed populations, a population removed continuously, and a population removed annually, at the onset of breeding. The two removal experiments sampled dispersing mice. The genetic data consisted of allelic frequencies at three blood protein loci, detected by starch gel electrophoresis: transferrin, an esterase, and glutamate oxalate transaminase. Behavior of field animals was examined in three laboratory tests. Females which began breeding in early spring suffered heavy mortality, and this resulted in a decline in female density. Females heterozygous at only one locus were selected for over this period. Males became aggressive and spaced themselves out as they began breeding, and light-weight, less aggressive males dispersed. This resulted in a decline in male density. While males were breeding, juveniles, particularly juvenile males, survived poorly, and few entered the populations. Light, non-breeding, subordinate males continued to disperse. As breeding stopped, males which continued to breed moved around, and juveniles survived well. The population increased to its maximum density in late fall. Over the winter, survival was good, dispersal low, and spacing behavior at its minimum. Dispersing males were lighter, showed less aggression in a neutral arena, and were more active than resident males. They differed genetically from residents, although there was no evidence of selection on males in control populations. Dispersing females were lighter, showed less exploratory behavior in an unfamiliar maze, and were more active than resident females. Dispersal was not selective in females. Populations settling in a depopulated area continued to differ genetically from control populations, but within two months resembled control populations demographically and behaviorally. The only difference was that a lower proportion of colonists bred, and this was compensated for by recruitment of juveniles from surrounding areas. The major hypotheses suggested by this study are: 1) Spacing out and dispersal of males in the spring is a response to mortality of early-breeding females, and thus it is this latter mortality which determines breeding densities. 2) Dispersal has two components: dispersal of subordinate mice in response to social pressure, and innate dispersal of more spontaneously active mice.

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