Environmental temperature is a key driver of variation in physiological developmental rates in reptiles. Cooler temperatures extend development time and increase the amount of energy required to achieve hatching success, which can pose fitness consequences later in life. Yet, in locally-adapted populations, genetic variation often opposes environmental variation across ecological gradients, known as countergradient variation (CnGV). It is therefore not only the presence, but the absence of phenotypic variation that can reveal insights into the mechanisms underlying local adaptation across environmental gradients. While evidence for genetic variation opposing environmental variation in physiological rates has been summarised in other taxa, the generality of CnGV variation in reptiles is yet unknown. Here I present a summary of studies measuring developmental time and metabolic rates in locally-adapted populations across thermal clines for 15 species of reptiles across 8 families. CnGV in developmental time is found to be common, while no clear pattern emerges for the thermal sensitivity of metabolic rates across locally-adapted populations. CnGV in developmental time may be an adaptive response in order to decrease the costly development in cool climates, however empirical work is needed to disentangle plastic from genetic responses, and to uncover potentially general mechanisms of local thermal adaptation in reptiles.
Countergradient Variation in Reptiles: Thermal Sensitivity of Developmental and Metabolic Rates Across Locally Adapted Populations
