activity patterns of Whiptail Lizards (Squamata: Teiidae: Aspidoscelis):
a proximate response to environmental conditions or an endogenous rhythm?
T. Winne1 and M. B. Keck2
River Ecology Laboratory, Drawer E Aiken SC 29802, USA
2Grayson County College, 6101 Grayson Dr, Denison TX 75020,
1. The hypothesis that high soil temperatures are required to induce
both initiation and cessation of daily activity in Whiptail Lizards (Aspidoscelis;
formerly Cnemidophorus), as well as the hypotheses that hunger
thresholds and high rates of evaporative water loss influence daily activity
patterns, were experimentally tested in A. inornata and A.
2. Although a critical soil temperature was required to elicit
the initiation of morning activity, high temperature was not a necessary
stimulus for the cessation of activity.
3. Access to prey did not influence the pattern of daily activity;
moreover, evaporative water loss did not appear to explain the cessation
of afternoon activity.
4. Reversing the photoperiod during our experiments led only to
a change in the time of initiation of daily activity (i.e. activity began
12 h later), not a significant change in the duration of daily activity.
5. These results provide strong evidence that circadian cycles
can play a critical role in not only the initiation but also the cessation
6. While the ultimate cause (i.e. selective advantage, if any)
of this unusual circadian rhythm may be related to extreme temperature,
limited water supplies or some other exogenous factor, clearly, the rhythm
persists in the absence of limiting environmental conditions.
Circadian rhythm, Cnemidophorus, environmental constraints, reptile,
T. and M. B. Keck. 2004. Daily activity patterns of Whiptail Lizards (Squamata:
Teiidae: Aspidoscelis): a proximate response to environmental
conditions or an endogenous rhythm? Functional Ecology 18:314-321.
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