Figure 1. Body temperature (T_b) changes over the annual cycle of the 13-lined ground squirrel.

The T_b trace during hibernation was obtained with a telemeter implanted in a squirrel housed in an environmental chamber maintained at 3–4°C. T_b lines in spring and summer are hand-drawn for illustration. Winter torpor bouts last from a few days to 2–3 wk, and interbout arousals (IBA) to normothermia typically last <24 h. ET, early torpor; LT, late torpor.