Thermal sensitivity of lizard embryos indicates a mismatch between oxygen supply and demand at near-lethal temperatures

Abstract

Aspects of global change (e.g. urbanization, climate change) result in novel, stressful thermal environments that threaten biodiversity. Though much research quantifies the thermal sensitivity of adult organisms, effects of global change on developing offspring (e.g. embryos) are also important. Oviparous, non-avian reptiles have received considerable attention because eggs are left to develop under prevailing environmental conditions, making them vulnerable to increases in ambient temperature. Though many studies assess embryo thermal tolerance and physiology in response to long-term (i.e. chronic), constant incubation temperatures, fewer assess responses to acute exposures which are more ecologically relevant for many species. We subjected eggs of the brown anole lizard (Anolis sagrei) to heat shocks, thermal ramps, and extreme diurnal fluctuations to determine the lethal temperature of embryos, measure the thermal sensitivity of embryo heart rate and metabolism, and quantify the effects of sub-lethal but stressful temperatures on embryo development and hatchling phenotypes and survival. Most embryos died at heat shocks of 45 or 46 {degrees}C, which is [~]12 {degrees}C warmer than the highest constant temperatures suitable for development. Heart rate and O2 consumption increased with temperature; however, as embryos approached the lethal temperature, heart rate and CO2 production continued rising while O2 consumption plateaued. These data indicate a mismatch between oxygen supply and demand at high temperatures. Exposure to extreme, diurnal temperature fluctuations depressed embryo developmental rates and heart rates, and resulted in hatchlings with smaller body size, reduced growth rates, and lower survival in the laboratory. Thus, even brief exposure to extreme temperatures can have important effects on embryo development, and our study highlights the role of both immediate and cumulative effects of high temperatures on egg survival. Such effects must be considered to predict how populations will respond to global change.