Disturbances in the annual weight and hibernation cycles of thirteen?lined ground squirrels kept in constant conditions and the effects of temperature changes

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Abstract
Thirty‐two thirteen‐lined ground squirrels, Citellus tridecemlineatus, were kept in environmental rooms for 1–3 years. One group was kept at 11.5 ± 1.75°C, another at 4± 3°C. There was a 12 hr photoperiod in both rooms. Nearly all the animals hibernated and lost weight normally in their first winter in captivity. Subsequently the weight cycle disappeared or was greatly reduced in 50 % of the animals in the 11.5°C room. In some animals weight remained essentially steady for more than 18 months. In the 4°C room weight cycles were weak or absent in 75% of the animals after their first winter in captivity. In both rooms hibernation was more frequent in those animals that showed persistent weight cycles. Experimental increases in the ambient temperatures were associated with marked weight gains in most animals and decreases of temperatures, down to levels close to the original ones used, were associated with well developed hibernation. The results show that being fat facilitates hibernation and that temperature can be relatively important in controlling the weight cycles of thirteen‐lined ground squirrels. Within this species there appears to be a continuum in the degree on internal‐external control in the programming of their cycles.