Blood cell dynamics during hibernation in the European Ground Squirrel

Hibernation is a unique natural model to study large and specific modulation in numbers of leukocytes and thrombocytes, with potential relevance for medical application. Hibernating animals cycle through cold (torpor) and warm (arousal) phases. Previous research demonstrated clearance of leukocytes and thrombocytes from the circulation during torpor, but did not provide information regarding the timing during torpor or the subtype of leukocytes affected. To study the influence of torpor-bout duration on clearance of circulating cells, we measured blood cell dynamics in the European Ground Squirrel. Numbers of leukocytes and thrombocytes decreased within 24h of torpor by 90% and remained unchanged during the remainder of the torpor-bout. Differential counts demonstrated that granulocytes, lymphocytes and monocytes are all affected by torpor. Although a decreased production might explain the reduced number of thrombocytes, granulocytes and monocytes, this cannot explain the observed lymphopenia since lymphocytes have a much lower turnover rate than thrombocytes, granulocytes and monocytes. In conclusion, although underlying biochemical signaling pathways need to be unraveled, our data show that the leukocyte count drops dramatically after entrance into torpor and that euthermic cell counts are restored within 1.5h after onset of arousal, even before body temperature is fully normalized.