Thermal benefits of melanism in cordylid lizards: a theoretical and field test

Authors

  • Susana Clusella-Trullas,

    Corresponding author
    1. Department of Bioscience and Biotechnology, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104 USA
    2. Ecophysiology Laboratory, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa
    •  Current address: Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa. E-mail: sct333@sun.ac.za

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  • Johannes Hvan Wyk,

    1. Ecophysiology Laboratory, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa
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  • James R. Spotila

    1. Department of Bioscience and Biotechnology, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104 USA
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  • Corresponding Editor: A. M. Bronikowski.

Abstract

The hypothesis that low skin reflectance (melanism) provides an advantage for thermoregulation under cold conditions has received mixed support in ectothermic vertebrates. We selected a model system, three allopatric closely related species of cordylid lizards that differ in skin reflectance, to test this hypothesis. Cordylus niger and Cordylus oelofseni are melanistic and inhabit peninsular and montane areas, respectively, whereas Cordylus cordylus is more widespread and inhabits low inland areas. By combining theoretical, experimental, and field data on these species, we demonstrate that the difference in body temperature (Tb) between melanistic and non-melanistic lizards under ecologically relevant climate variation ranged from 0° to 2°C. Despite its small magnitude, however, the faster heating rate and higher Tb of melanistic species relative to non-melanistic species conferred an advantage under cold conditions. Comparison of habitat thermal quality (de) and thermal accuracy (db) across species indicated that, in winter, melanism conferred the greatest advantage during small windows of thermal opportunity. This finding was most pronounced for C. oelofseni, which is most constrained by cold temperatures in its habitat. By contrast, due to their rock-dwelling habits, melanistic and non-melanistic species benefited from rock refugia in summer, giving similar levels of thermoregulatory effectiveness across species, regardless of skin reflectance. This study therefore demonstrates that skin reflectance variation across cordylids has significant effects on their thermal balance. Furthermore, studies investigating the role of varying skin reflectance in field populations and species should incorporate fine and broad temporal scales (daily, monthly, and seasonal), environmental variability, and cost–benefit trade-offs of thermoregulation.

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