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Summary 1.In reptiles, growth is subject to proximate environmental influences, such as food availability and temperature, that may be crucial during the early stages of postnatal development. Mediterranean regions, with their severe summer drought, offer an excellent opportunity to examine the effects of environmental variations in precipitation and productivity on the timing of reproduction and growth rates of lizards. 2. In this study, we compared the incubation time, size at hatching, growth rates and changes in juvenile body condition of two nearby populations of the lizard Psammodromus algirus separated by 600-700 m altitude in central Spain. We combined a reciprocal incubation experiment at 27 and 30 ° C with a reciprocal transplant experiment to distinguish between environmental and population-specific sources of geographical variation. 3. At both temperatures, eggs from the high-elevation site hatched sooner. Several important phenotypic traits of juveniles were primarily determined by the growing environment: the high-elevation site provided more food and allowed hatchlings to grow faster and reach larger size. 4. Environmental effects overrode familial ones, as shown by the larger growth rate of half-sibs released at the high-elevation site. However, both the size and the growth rate of juveniles were also influenced by their mother's site of origin, which means that population differences may reflect a genetic differentiation and/or different maternal effects. 5. Low precipitation and associated food scarcity were seemingly the main factors constraining the growth rates of juveniles at the lowland site.

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