Thermal tolerance in the urban heat island: thermal sensitivity varies ontogenetically and differs between embryos of two sympatric ectotherms

Most studies of thermal tolerance use adults, but early-life stages (e.g. embryos) are often more sensitive to thermal agitation. Studies that examine effects on embryos rarely assess the potential for thermal tolerance to change with ontogeny or how effects differ among sympatric species, and often utilize unrealistic temperature treatments. We used thermal fluctuations from nests within the urban-heat island to determine how thermal tolerance of embryos changes across development and differs among two sympatric lizard species ( and ). We applied fluctuations that varied in frequency and magnitude at different times during development and measured effects on embryo physiology and survival, and hatchling morphology, growth and survival. Thermal tolerance differed between the species by ∼2°C: embryos of , a lizard that prefers warmer, open-canopy microhabitats, were more robust to thermal stress than embryos of , which prefers cooler, closed-canopy microhabitats. Moreover, thermal tolerance changed through development; however, the nature of this change differed between the species. For , thermal tolerance was greatest mid-development. For , the relationship was not statistically clear. The greatest effects of thermal stress were on embryo and hatchling survival and embryo physiology. Hatchling morphology and growth were less affected. Inter-specific responses and the timing of stochastic thermal events with respect to development have important effects on embryo mortality. Thus, research that integrates ecologically meaningful thermal treatments, considers multiple life-history stages and examines interspecific responses will be critical to make robust predictions of the impacts of global change on wildlife.