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J Comp Physiol A (2008) 194:701712 DOI 10.1007/s00359-008-0341-3
ORIGINAL PAPER
Interseasonal variation in the circadian rhythms of locomotor activity and temperature selection in sleepy lizards, Tiliqua rugosa
David J. Ellis Bruce T. Firth Ingrid Belan
Received: 30 October 2007 / Revised: 18 April 2008 / Accepted: 19 April 2008 / Published online: 29 July 2008 Springer-Verlag 2008
Abstract Few studies in non-mammalian vertebrates have examined how various eVectors of the circadian system interact. To determine if the daily locomotor and behavioural thermoregulatory rhythms of Tiliqua rugosa are both controlled by the circadian system in diVerent seasons, lizards were tested in laboratory thermal gradients in four seasons and in constant darkness. Circadian rhythmicity for both rhythms was present in each season, being most pronounced in spring and summer and least evident in autumn. Most lizards displayed a unimodal locomotor activity pattern across all seasons. However, some individuals presented a bimodal locomotor activity pattern in spring and summer. Seasonal variations in the phase relationships of both rhythms to the light:dark (LD) cycle were demonstrated. No seasonal diVerences in the free-running period lengths of either rhythm were detected, raising the possibility that a single circadian pacemaker drives both rhythms in this species. Our present results demonstrate that both rhythms are similarly controlled by the circadian system in each season. Although seasonal variations in the thermal preferences of reptiles both in the Weld and laboratory have previously been well documented, this study is the Wrst to demonstrate circadian rhythms of temperature selection in a reptile species in each season.
Keywords Reptiles Behavioural thermoregulation
Locomotor activity Circadian rhythms Seasonality
Introduction
The Australian sleepy lizard (Tiliqua rugosa, Hutchinson 1981) is a diurnal and largely herbivorous scincid lizard that displays daily rhythms of behavioural temperature selection and locomotor activity in the Weld during diVerent seasons (Firth and Belan 1998). In the Weld, the locomotor activity pattern of T. rugosa changes from unimodal during mild weather to bimodal during hot weather (Firth and Belan 1998). When held on continuously operating laboratory thermal gradients under artiWcial light:dark (LD) cycles that mimic the natural photoperiod in each season,T. rugosa display well deWned daily rhythms of behavioural thermoregulation characterised by the selection of higher overall day-time and lower overall night-time temperatures (Firth and Belan 1998). This...