A, MitoTracker staining shows accumulation of subsarcolemmal and cytoplasmic mitochondria in quadriceps muscle of hibernating squirrels. Scale bar, 10 μm. B, Western blot and density analyses of mitochondrial fractions shows significantly increased protein levels of ATP Synthase, Mitofusin-1 (Mfn-1), Mitofusin-2 (Mfn-2), and Fis-1, suggesting enhanced mitochondrial biogenesis during periods of hibernation. C, Relative increase of mitochondrial transcription factor A protein during hibernation. (**P<0.01, ***P<0.001)