Experimental manipulation of the timing of reproduction in the California vole

Summary The potential role of food and water in determining 1) the timing of the normal breeding season in the California vole, and 2) two frequently occurring exceptions to this normal pattern, are explored by using experimentally manipulated enclosed populations. Hypotheses derived from field observations were tested in the enclosed populations by the manipulation of green food, dry food, water, and vole density. The results of three experiments involving five populations are reported. In all populations densities were maintained at near‐normal levels (475 to 1750 per ha). In all three experiments, it was shown that breeding was possible in the summer if sufficient food and water were available, although perhaps with reduced litter sizes and rates of growth and reproductive maturation of young. In two cases, severe weight losses and high mortality were induced by insufficient water. When water was provided, survivors recovered weight and reproductive competency in between five weeks and eight months. In three populations, starvation induced weight losses and mortality. In these cases recovery of the voles took only one month or less. In two of these last, the inadequate food situation was produced during the normally wet season by outbreaks of a European slug. It is concluded that available moisture is the most important factor in triggering both the end and beginning of the normal breeding season. Of course, inadequately fed mice do not breed at any season. Summer (dry season) breeding can occur when food and water conditions are adequate due to locally favorable conditions or very low population densities. Prolonged delays in growth and reproduction following the return of favorable conditions are most likely the consequence of chronic and severe water deprivation.