Growth, Seasonality, and Lizard Life Histories: Age and Size at Maturity

Abstract

Temperature influences the activity seasons, reproductive phenology, survival rates, and growth rates of lizards. We present a model of lizard growth that predicts phenotypic patterns of age and size at reproductive maturity in different thermal environments (i.e. different activity seasons). The model predicts a threshold in length of activity season: above this threshold (long season), lizards can mature one year earlier, but at a smaller size, compared to populations with activity seasons below the threshold. This environmentally imposed pattern reflects the proximate consequences of temperature. together with simple rules about the timing of maturation. A key prediction of the model is that age and size at maturity can vary non-linearly with the length of the activity season, and with the timing and duration of egg laying and hatching. We tested these predictions with published data from field studies of the phrynosomatid lizard Sceloporus undulatus, which is geographically widespread and occupies a range of thermal environments. We estimated activity seasons for each population by modeling the links between climates. microclimates and lizard body temperatures using heat-transfer principles. Female age at maturity showed the predicted threshold in length of activity season. whereas female size at maturity did not show the predicted threshold, but instead was negatively correlated with length of activity season. Two prairie populations were exceptions to this pattern: females matured in one year despite their short activity seasons, and consequently matured at an unusually small size. Prairie populations may have evolved differences in growth response and reproductive timing. The thermal environment appears to be an important correlate of life history variation among populations of Sceloporus undulatus.