Thermal ecology of melanistic and non-melanistic species of cordylid lizards in the Western Cape of South Africa

Susana Clusella Trullas

doi:10.17918/00000629

Melanistic (uniformly black) species of cordylid lizards in the Western Cape of South Africa are generally distributed on cooler mountain tops and in peninsular areas, whereas lighter-colored congeners are found in warmer inland areas. The aim of this thesis was to study the thermal biology of closely-related melanistic and non-melanistic cordylid species and test the hypothesis that higher skin absorptivity of melanistic species provides a thermal advantage under low temperature and limited solar radiation compared to lighter species. I repeatedly determined preferred body temperature (Tse1) in a laboratory thermal gradient for Cordylus niger (melanistic sp.), Cordylus oelofseni (melanistic sp.), Cordylus polyzonus (melanistic population) and Cordylus cordylus (non-melanistic sp.). Mean Tsel (30.8-34.0°C) was fairly consistent although, within and among-day repeatability values of Tse1 were low indicating that individual differences in Tsel were not always consistent over time. I modeled the effects of skin color difference between C. niger (5.3% reflectance) and C. cordylus (15.2%) using a steady-state energy balance equation. This model indicated that at low wind speed and under various conditions of solar radiation and air temperature, melanistic lizards reach higher equilibrium temperatures than non-melanistic ones. The greatest differences (~2-3°C) were predicted under highest incident radiation levels. Body temperatures of live lizards measured under controlled and natural conditions were in close agreement with the model predictions. The combination of operative and body temperatures in the field with Tse1 for C. niger, C. oelofseni and C. cordylus indicated that all species were active thermoregulators in summer and winter. All species thermoregulated at a higher level in winter, although high thermal constraints of the environment greatly reduced accuracy and effectiveness. Despite its small magnitude, the effect of melanism appeared advantageous in cold conditions rather than disadvantageous in warm conditions, especially for species such as C. oelofseni (montane) in which Te frequently fell below critical thermal minimum in winter. Finally, this study found little support for predictions of the thermal melanism hypothesis (i.e. correlations between several traits of thermal physiology and skin reflectance of squamates) across a wide range of species from heterogeneous habitats. However, further work is needed to determine at which scales these relationships may exist.

Thermal ecology of melanistic and non-melanistic species of cordylid lizards in the Western Cape of South Africa

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Thermal ecology of melanistic and non-melanistic species of cordylid lizards in the Western Cape of South Africa

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