Circannual rhythms in European ground squirrels during nine years of entrainment

Body mass and hibernation/activity cycles have been measured in laboratory born European ground squirrels (Spermophilus citellus) during their entire life spans (up to 9.2 years). Effects of entrainment were tested applying f orced square-wave cycles of environmental temperature, with alternating hea t (30 degreesC) and cold phases (7 degreesC or 15 degreesC), and with 1.0-, 0.67-, or 0.5-year cycle periods. When exposed to 7 degreesC during the co ld phases, the animals were able to synchronize to external cycles as short as 6 months. When 15 degreesC was applied instead, some of the animals wer e unable to respond accurately even to 0.67-year cycles. Two types of "unlo cking" of the phase-locked biological rhythms have been identified in which either body mass or hibernation/activity cycling failed to conform to the exogenous rhythm. During post-entrainment, ground squirrels reverted to the ir inherent circannual rhythms, typical of free-running conditions.