Abstract Seasonal differences in the locomotor activity pattern of Podarcis sicula held in constant conditions in the laboratory were found to be associated with systematic differences in both the freerunning period of locomotor rhythms (τ) and the circadian activity time (α). In order to establish whether the pineal played a role in the control of seasonal changes in circadian parameters, we examined the effects of pinealectomy in constant conditions on the locomotor behavior of lizards displaying the bimodal activity pattern typical of summer. In most lizards pinealectomy lengthened τ, shortened α and abolished the bimodal activity pattern. Pinealectomy induced a sudden transition from the typical locomotor behavior of summer, characterized by a marked bimodal pattern, short τ and long α, to the typical locomotor behavior of autumn, characterized by a unimodal pattern, a long τ and short α. These results demonstrate that the pineal plays a central role in the seasonal reorganization of the circadian system that occurs in P. sicula.
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