Seasonal variation in the natality, mortality, and nutrition of the pine vole in two orchard types

Pine voles (Microtus pinetorum) were collected for one year in a maintained and an abandoned orchard. Higher densities of voles were indicated in the maintained orchard. Estimates of mortality were lower and reproduction was higher in the maintained orchard. Net reproductive rates indicated a stable or increasing population in the maintained orchard and a decreasing population in the abandoned orchard. Digestible energies and consumption indexes were higher in the maintained orchard.

A drastic decline in reproduction occurred in the abandoned orchard in November while reproduction remained high in the maintained orchard. The largest increase in theoretical energy requirements for thermoregulation occurred from September to November. At this time the digestible energy in the abandoned orchard was much lower than in the maintained orchard, there was a larger reduction in ground cover, a lower consumption index, and a sharp decline in female fat content in the abandoned orchard. Although cause and effect cannot be shown it seems likely that the lower energy intake, the higher energy demands for thermoregulation and for pregnancy and. lactation would cause energy requirements to be greater than energy intake. It is postulated that such an energy restriction would lower circulating levels of LH which would result in a decline in reproduction in the abandoned orchard. It is also postulated that energy intake and photoperiod have an interactive effect on reproduction in the pine vole and that energy intake in the maintained orchard in the November to January period was not sufficient to override the effect of decreasing daylength and to keep a high proportion of females in a reproductive state.