Analysis of Two Freely-Growing Enclosed Populations of the California Vole

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Abstract
Two populations of California voles (Microtus californicus) were established in enclosures, and supplied with adequate food and water. Density effects on reproduction, social behavior, molt, ectoparasite load, obesity, and sex ratio were examined. Voles were live-trapped regularly, and all were removed and killed at the end of the experiments, which lasted 10 to 12 months. Both populations grew rapidly after an initial adjustment period during which the founder adults were reduced to one pair. One population grew slightly more rapidly than the other, and reached 2.4 voles/m2. Sex ratios stabilized as numbers increased, favoring a different sex in each population. Wounding was similar in intensity to that found in field populations, except that in the denser population it increased in both sexes toward the end of the experiment. Although all members of each population constituted a single social group or clan, 37% of the older adults showed nonrandom movement patterns. Moreover, there was a clear attempt at social subdivision in both populations although the boundaries were imperfectly maintained. A mite infestation acted in an anti-regulating fashion. Molting was much more frequent and less strongly seasonal than in the field. Females showed a positive correlation between molting and reproduction rather than the expected negative one. While all voles over 25 g contained some kidney fat, obesity was not correlated with reproduction or age, except that the youngest cohort and reproductively active males were slightly less fat. Litter sizes were high and unaffected by density. However, a large percentage of both males and females failed to mature sexually, and this tendency was age-related. Neonatal mortality was also high. Overall population growth, while much lower than the theoretical maximum, was similar to that of other enclosed populations, thus emphasizing the importance of dispersal in normal demography. Other suggested features of this vole's social system are small group size, a tendency toward monogamy, strong presaturation dispersal, and only weak social inhibitions of reproduction.