Synchronization of circannual cycles: a cold spring delays the cycles of thirteen-lined ground squirrels

Summary Ground squirrels, show circannual cycles with periods normally less than a year when kept under laboratory conditions. The way in which environmental factors synchronize these cycles with the geophysical year under natural conditions is not known. We tested the possibility that cold temperatures can cause long-term phase delays in circannual cycles of thirteen-lined ground squirrels.Two groups of animals were kept in the cold (4 °C) for either 8.5 or 13 months, after which they were returned to the warm (21 °C) and kept there until they had completed at least one additional cycle. A third group was kept in the warm for the entire experiment. Most of the males in the cold room groups became arrested in the spring phase of their cycles while they were in cold. When returned to the warm, these males resumed cycling. When animals showed prolonged spring phases, their cycles were phase-delayed and continued to reflect this delay even after they were returned to the warm. Both body weight cycles and molt cycles were delayed. In contrast to the males in the cold room, none of the females in the cold room groups and none of the warm room animals of either sex showed this response.Our results demonstrate that cold temperatures can phase-delay both body weight and molt cycles and that the spring phase is a critical stage in this effect. We suggest that spring temperatures are largely responsible for the seasonal synchronization of circannual cycles in ground squirrels and are, therefore, a possible Zeitgeber for these cycles.