How do fluctuating temperatures alter the cost of development?

Quantifying how variable temperature regimes affect energy expenditure during development is crucial for understanding how future thermal regimes may impact early life survival and population persistence. Developmental Cost Theory (DCT) suggests that there is an optimal temperature (T opt ) that minimises energy expenditure during development (the “cost of development”). Exposure to fluctuating temperatures around an average of T opt are anticipated to increase either development time or metabolic rate. As a result, embryos will rapidly deplete yolk reserves, and consequently hatch at a smaller size or with less residual yolk to support postnatal survival and growth. Here, we studied total embryonic energy expenditure (development time and rate of CO 2 production) and conversion of yolk into tissue in common wall lizards ( Podarcis muralis ) under three incubation treatments anticipated, based on DCT, to increase the cost of development: no variance (T opt constant, 24 °C), low variance (22 – 26 °C), and high variance (18 – 30 °C). As predicted, we found that increasing variance around T opt increased the cost of development, despite reducing time to hatching. As a consequence, embryos on average hatched with 59% lower residual yolk reserves under high variance versus constant incubation temperatures. Our results highlight how the relative temperature sensitivities of development time and metabolic rate determine the cost of development, which in turn may predict the ability of egg-laying ectotherms to persist in variable environments. We show that DCT can provide a mechanistic framework for understanding the widespread, but often seemingly idiosyncratic, effects of fluctuating incubation temperatures on hatchling tissue and residual yolk mass.