Adaptive Blood Cell Variability in the Annual Life Cycle of the Ground Squirrel Urocitellus undulatus

The complete blood count (CBC) dynamics was investigated inlong-tailed ground squirrels Urocitellusundulatus (n = 100)during different periods of their annual activity. While blood cell (WBC),platelet (PLT), and red blood cell (RBC) counts were determinedin summer (normothermia, June–July), fall (preparatory period, October),winter torpor (hypothermia, December–February) and winter active(euthermia, interbout arousals, December–February) periods. In fall,PLT and WBC counts increased by ~40% compared to summer. Under wintertorpor, thrombocytosis and leukocytosis were replaced by extremethrombocytopenia (↓ ~90%), as well as leukopenia in all WBC subpopulations (↓ ~80%).During brief interbout arousals, PLT and WBC counts reached thesummer values except for a slightly increased MPV (mean plateletvolume). As fall approached, there was an insignificant erythrocytosis(↑ ~10%), which together with changes in other indices could havesuggested a mild fall-time hypoxia. During winter torpid and euthermicperiods, RBC counts were almost indistinguishable from the summercontrol values, however, atypical RBC forms occurred both in thefall preparatory period and during hibernation itself. The resultsare discussed in the context of blood cell adaptation to harsh conditionsof hibernation in ground squirrels, and can be useful in studyingthe adaptive abilities of homoeotherms, including humans.