Circannual variations of lizard circadian activity rhythms in constant darkness

The daily pattern of locomotor activity of Podarcis sicula in the field cha nges from unimodal in spring to bimodal in summer, becoming unimodal again in autumn. Short-term experiments in which P. sicula collected in different months were tested under constant conditions immediately after capture sho wed that the activity pattern typical of each season is retained in the liz ard circadian locomotor rhythm. In constant conditions, the bimodal pattern is associated with a short free-running period (tau) of the circadian loco motor rhythm and a long circadian activity (alpha), while the unimodal patt ern is associated with a long tau and short alpha. To test whether seasonal changes in circadian locomotor rhythms are driven by a circannual clock, w e recorded locomotor activity of lizards over 12-15 months in constant temp erature and darkness. The present results demonstrate, for the first time i n a vertebrate, the existence of circannual changes in constant darkness of both t and a. In most lizards, the longest tau along its circannual cycle is associated with a short alpha, and the shortest tau in the same cycle wi th a long a, so that the pattern of mutual association between tau and alph a is the same as in short-term experiments. Most lizards, however, stayed u nimodal all the time. This shows that changes in activity pattern from unim odal to bimodal (and vice versa) are induced by seasonal changes in environ mental factors, instead of being incorporated into a circannual rhythm. Cir cannual changes in tau and alpha of locomotor rhythms may adaptively predis pose the circadian system of P. sicula to a change in activity pattern as s oon as seasonal changes in the environment demand it.