Daily activity patterns of Whiptail Lizards (Squamata: Teiidae: Aspidoscelis): a proximate response to environmental conditions or an endogenous rhythm?
C. T. WINNE1 and M. B. KECK2
1Savannah River Ecology Laboratory, Drawer E Aiken
SC 29802, USA, and
2Grayson County College, 6101 Grayson Dr, Denison TX 75020,
USA
Summary
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.
gularis.
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 playa
critical role in not only the initiation but also the cessation of activity.
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.
Key-words: Circadian rhythm, Cnemidophorus, environmental
constraints, reptile, temperature
SREL Reprint #2843
Winne, C. 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.
