The temperature profile of the cave interior reveals three clear divisions (Figure 5): (1) a shallow thermal zone from 0 to 15 m depth controlled by exterior air influx, (2) a heterothermal
medium zone between 15 and 50 m with a marked increase in temperature by 5[degrees]C at 35 m depth, and (3) the thermal deep zone with a progressive increase in temperature controlled by the geothermal gradient.
They also demonstrate that under field conditions, hibernation (the period of sequestration below ground) and the heterothermal period (the period between onset of first torpor and arousal from final torpor) are not necessarily equivalent; squirrels may spend several days euthermic but sequestered below ground before and especially after the heterothermal period (Michener, 1992; Barnes and Ritter, 1993).
For each individual, we document attributes of the heterothermal period shown to vary seasonally within hibernation, or between age cohorts (juveniles, adults) or sexes, in other free-ranging marmotines (Young, 1990; Michener, 1992; Zervanos and Salsbury, 2003; Buck et al., 2008).
Skin temperatures were plotted against time and the resulting profile inspected for the timing of onset and termination of the heterothermal period and patterns of skin-temperature variation during the heterothermal period.
The number of torpor bouts during the heterothermal period was determined for each ground squirrel and the duration of each torpor and arousal bout recorded.
Then, we calculated and plotted the mean monthly values per individual through the heterothermal period.
We compared male and female ground squirrels for duration and date of entry into and exit from the heterothermal period; number of torpor bouts during the heterothermal period; lowest skin temperature during hibernation; overall and monthly average torpor- and arousal-bout durations; monthly average minimum skin temperatures during torpor; and body mass at first capture (for squirrels first captured 22-27 Aug.).
Three animals last captured within 5 d of entering the heterothermal period weighed 132-144 g at last capture (Fig.
All squirrels with functional data loggers in spring displayed a characteristic skin-temperature profile during the heterothermal period: a series of 11-22 torpor bouts punctuated by brief arousal bouts (Fig.
3B) varied systematically and significantly through the heterothermal period (Kruskal-Wallis test statistic = 44.4 and 41.2, respectively; P < 0.001, PF = 6 for both variables): from shallow, short bouts in Oct.
Three males (collars 1, 6, and 13) differed from other squirrels in multiple attributes of the heterothermal period.