Temperature cycles induce a bimodal activity pattern in ruin lizards: Masking or clock-controlled event? A seasonal problem

The daily locomotor activity pattern of Ruin lizards in the field is mainly unimodal, except for summer months when soil temperatures exceed 40 degree sC to 42 degreesC around midday. In such a situation, lizards reduce their locomotor activity around midday to avoid overheating, and thus their daily activity pattern becomes bimodal. The bimodal pattern expressed in the fie ld is usually retained in the free-running rhythm under constant temperatur e and DID for a couple of weeks, after which the bimodal pattern changes in to a unimodal pattern. In the present study, the authors examined whether 2 4-h temperature cycles (TCs) would change lizard activity from a unimodal t o a bimodal pattern. Administration of TCs to unimodal lizards free-running in DID is able to entrain locomotor rhythms and to induce a bimodal patter n both in summer and autumn-winter. There are, however, striking seasonal d ifferences in the effectiveness with which TCs achieve bimodality: (a) Numb ers of lizards rendered bimodal are significantly higher in summer than in autumn-winter; (b) TCs require less time to achieve bimodality in summer th an autumn-winter; (c) bimodality is retained as an aftereffect in the poste ntrainment free-run in summer, but not in autumn-winter; (d) TCs change act ivity duration in summer, but not in autumn-winter. All this demonstrates t he existence of seasonal changes in responsiveness of the circadian oscilla tors controlling activity to the external factors inducing bimodality. Osci llators' responsiveness is high in summer, when bimodality is the survival strategy of Ruin lizards to avoid overheating around midday in open fields, and low in autumn-winter, when bimodality has no recognizable adaptive sig nificance.