Active and hibernating ground squirrels, Citellus tridecemlineatus, were exposed to whole-body <tex-math>${}^{60}{\rm C}$</tex-math> gamma rays at dose rates of 190 to 175 rads per minute over a range of doses of 900 to 20,000 rads. The results showed that over the entire range of exposures those animals that were irradiated while hibernating (body temperatures circa 5°C) and then immediately aroused from torpor were more radioresistant than the similarly irradiated active animals (body temperatures circa 37°C). This conclusion applied to both mean survival times and percentage of survival. Summer animals, as compared with winter animals, both active and hibernating, were found to be less resistant. The fact that these experiments involved levels of radiation which spanned the three radiation syndromes-hematopoietic, gastrointestinal, and central nervous system-suggest that the protective effects afforded by hibernation are broadly operative.

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