| Summary: | Faculty of Science
School of Animal ,Plant and Enviromental Sciences
9707660j
McConnachie@ukzn.ac.za === Characteristics comprising an energy budget of Pseudocordylus melanotus melanotus were investigated in terms of limiting factors for the lizard’s distribution, and included measures of thermal biology, metabolism and digestive physiology. Pseudocordylus m. melanotus is rupicolous, is limited to the temperate parts of southern Africa and exhibits adaptations to these environments. The distribution of the lizard was modelled using two different climate envelope modelling techniques, which are discussed in terms of the energy budget and factors limiting the distribution of the lizard.
Pseudocordylus m. melanotus exhibits wide thermal tolerances. It can withstand body temperatures (Tb) below freezing, but freezing of body water, which occurs at ca. -5 ºC, is lethal. The lower critical minimum temperature was a surprisingly high measure for an apparently cold-adapted lizard. In the laboratory, the selected body temperature (Tsel) was ca. 30 ºC. In the wild, lizards thermoregulate by shuttling between hot and cold microclimates, modifying body postures and regulating activity times, and select Tbs of ca. 29 ºC in summer and ca. 26 ºC in winter.
Energy expenditure was measured over a range of temperatures. MR increases significantly with increasing temperature. Body mass and metabolic rate (MR) were significantly related at 30 ºC only. The lack of significance at other temperatures can be ascribed to the small range of body mass over which measures were made.
Energy gain in P. m. melanotus was quantified in terms of prey capture rates and by measuring aspects of digestive physiology. Prey capture attempts were 35 % successful and occurred ca. every 2 h 30 min in the field. Lizards consumed a variety of arthropods, but mainly ate beetles. Digestive rate and appetite increased significantly with increasing temperature. Apparent digestive efficiency (ca. 94 %) and apparent assimilation efficiency (ca. 87.2 %) were not affected by temperature.
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Two energy budgets were calculated; one based mainly on laboratory measures and the other based on the thermal profile of lizard body temperature measured in the field. The laboratory energy budget was approximately half that of the field energy budget. The field energy budget, however, provided a more realistic view of energy expenditure since it covered almost the entire thermal range experienced by lizards. Between 592.53 kJ (field energy budget) and 940.06 kJ (laboratory energy budget) per year is required for a standard 30 g P. m. melanotus to remain in energy balance. This means that lizards need to consume between 73.55 and 116.69 g of mealworms per year, which equates to ca. 740 to 1200 average mealworms. Any energy gained over and above these requirements can be allocated to growth, reproduction and storage.
In terms of the lizard’s distribution, it will be excluded from areas where the available thermal environment and prey abundance do not allow the lizard to maintain a positive energy balance. The lizard is apparently at the cold limit of its range, so predicted climate change in southern Africa is unlikely to have a significant effect on the distribution of the lizard. Suitable crevices are essential to the lizard’s survival, particularly during periods of extreme cold when lizards may experience Tbs where they are incapacitated, or effectively ‘comatose’, while in retreats.
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