Evidence for Circannual Rhythms
Although a considerable amount of evidence suggesting endogenous annual clocks in plants and animals was accumulated during the first half of this century, their existence was not demonstrated before the early 1960’s. The most compelling evidence then was provided by the work of Pengelley and Fisher (1957, 1963) with golden-mantled ground spuirrels (Spermophilus lateralis). Figure 2.1 shows the seasonal changes in body weight and food consumption, as well as the occurrence of hibernation, in two ground squirrels maintained for about 2 years under a constant LD 12:12 and constant temperature conditions. Both animals continued to hibernate about once a year, and each period of hibernation was preceded by a dramatic increase in body weight and food consumption. The general pattern of these seasonal functions was rather similar to that in free-living squirrels with an obvious difference that can be seen particularly clearly in the squirrel maintained in 0 °C. In the first year of the experiment this animal entered hibernation in late October, but in the second the onset of hibernation had shifted forward to mid-August and in the third to early April. In other words, the period measured between onsets of hibernation was shorter than 12 months.
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