Heart rate, metabolic rate, and developmental time: local adaptations in thermal plasticity along the latitudinal thermal gradient

Embryonic development in ectotherms is strongly shaped by temperature, yet the multivariate nature of metabolism complicates attempts to link local thermal environments to developmental physiology and energetic costs. In this study, we examined embryonic heart rate, metabolic rate (V˙CO2), and developmental time in two populations of the prairie lizard, Sceloporus consobrinus, from a cool, northern versus a warm, southern environment. Eggs from each population were incubated at 25 and 31°C to test for countergradient variation in developmental physiology and assess how populations differentially use these physiological pathways in relation to developmental cost and time. To that end we evaluated predictions of developmental cost theory, which posits that the energetic cost of development is a product of metabolism and developmental time, with natural selection acting on each to minimize developmental costs, although both phenotypes respond to temperature differently. Developmental time decreased with increasing temperature in both populations, but embryos from the cool environment developed faster overall, consistent with countergradient variation. Across populations and temperatures, heart rate and metabolic rate were dissociated: at the warmer incubation temperature, cool-population embryos exhibited higher heart rates but lower metabolic rates relative to the warm population, achieving faster development at lower estimated energetic cost, whereas reliance on increased metabolic rate in the warm population produced slower development and higher costs. These results show that populations can evolve distinct combinations of physiological traits to meet similar developmental challenges and underscore the complex nature of metabolism as a multivariate trait when predicting ectotherm responses to thermal variation.