The embryonic stage is the most vulnerable part of a reptile’s life. Although cocooned inside a leathery shell, young lizards are defenceless against threats such as hungry predators and, as a new study in Conservation Physiology reveals, extremely high temperatures.
Eastern fence lizards (Sceloporus undulates) are a widespread species in the US, and are found all the way from the warmth of mid-Florida to the cooler regions of New Jersey. Females lay 3-16 eggs in early summer, which they carefully bury in shallow soils to protect them from drying out and overheating.
Yet in recent years, there has been a surge in the number of brutal heat waves blasting across the continental US. Such severe temperatures, even if only for a day, can place a massive amount of stress on animals that already live in exceptionally hot environments. With this in mind, a study from researchers at Arizona State University sought to determine the impact of extreme temperatures on the embryos of eastern fence lizards.
Using infrared sensors, they monitored the heart rate of the embryos under rising temperatures. When the eggs reached 47°C (~116°F), their hearts could no longer beat any faster and the embryos suffered cardiac arrest. Surprisingly, this lethal temperature was the same for lizards regardless of whether they originated from the northern or southern extent of their range. The authors therefore suggest that this is the highest possible temperature that these embryos could ever hope to endure, no matter where they come from. Troublingly, soils in some natural nests are already known to reach these temperatures. And if the frequency and severity of heat waves continues to rise, as predicted under current climate change scenarios, such events could severely reduce the ability of local populations to successfully reproduce.
With many record high temperatures again set in 2013, findings such as these only begin to hint at some of the very real biological impacts of global warming. This is especially worrying for endangered reptile species such as the blunt-nosed leopard lizard (Gambelia sila) and the Coachella Valley fringe-toed lizard (Uma inornata). If these species have a similar critical threshold for extreme temperatures during their embryonic phase, then escalating heat waves could spell disaster for their already tiny populations.
Reference: Michael J. Angilletta Jr, Maximilian H. Zelic, Gregory J. Adrian, Alex M. Hurliman, & Colton D. Smith (2013). Heat tolerance during embryonic development has not diverged among populations of a widespread species (Sceloporus undulatus) Conservation Physiology, 1 (1) DOI: 10.1093/conphys/cot018