Elevated ambient temperature accelerates aspects of torpor phenology in an obligate hibernator.

Abstract

The thirteen-lined ground squirrel (Ictidomys tridecemlineatus) is assumed to be an obligate hibernator - commencing and terminating hibernation on a circannual rhythm, regardless of environmental conditions - but, until now, this assumption had never been fully tested. We housed three groups of captive-born ground squirrels from Aug. 2017 to Aug. 2018 under constant photoperiod (12hL:12h D) at 5, 16 or 25°C, and monitored hibernation using body temperature loggers. At 5 and 16°C all animals hibernated from autumn to spring with no differences in date of first/last torpor or duration of interbout euthermic periods (IBE), but torpor bout duration was 25% shorter at 16°C. One of 4 animals housed at 25°C did not hibernate. For the other three 25°C animals, the first torpor date did not differ from the other groups, but the last torpor bout (5 Feb.) occurred almost 8 weeks earlier. These animals aroused from torpor more frequently and IBE lasted significantly longer, so the total time spent torpid was less than 50% of the other groups. Unlike the 5 or 16°C animals, 25°C animals re-entered torpor in late spring 2018. Taken together these data suggest that this species is an obligate hibernator, but that high ambient temperatures can accelerate the endogenous circannual hibernation rhythm.