Blood Viscosity of Two Mammalian Hibernators: Spermophilus tridecemlineatus and Tamias striatus

Blood viscosity of the hibernators Spermophilus tridecemlineatus and Tamias striatus is similar to that characteristic of nonhibernators when measured in vitro under euthermic conditions (temperature = 37 C and shear rates greater than 100 s⁻¹). However, under conditions occurring in parts of the microcirculation during torpor (temperature = 23 C or lower and shear rates less than 100 s⁻¹), blood viscosity is substantially lower in the hibernators. This is largely because the viscosity of blood from the hibernators has a reduced shear-rate dependence resulting from a property of the plasma that minimizes erythrocyte aggregation when blood is flowing slowly. In addition, the erythrocytes from the hibernators are more deformable at low temperature than those from nonhibernators. These characteristics are present during both the hibernating and nonhibernating seasons and appear to be important in facilitating blood flow to tissues in torpid animals.