Variations in environmental conditions during embryonic development can have a profound influence on the trajectory of on organism's life. For example, if a mother breathes in polluted air, then the toxins in the air can alter gene expression in the developing baby and change a phenotype – an observable trait associated with a gene. Thus, genes can produce vastly different traits if they are affected by environmental triggers. Biologists call this phenomenon ‘phenotypic plasticity’ and it has particular ecological relevance for many ectothermic (cold-blooded) animals, such as lizards. Unlike humans, whose babies are well protected from environmental fluctuations inside their mothers, lizards lay eggs that are very susceptible to outside changes in climate.
Climate change is a man-made effect that has intensified extreme-weather events globally, including making temperatures soar. Given the potential risk rapidly increasing temperatures will have on the survival of heat-sensitive animals, Alex Gunderson and colleagues from Tulane University, USA, and Auburn University, USA, wanted to know if exposing embryonic lizards to high temperatures would make them more susceptible to hot temperatures or prime their physiology to become heat-tolerant.
The team selected a common lizard, the Cuban brown anole (Anolis sagrei), and subjected their eggs to three temperatures that represent the natural range that anoles experience in nature: a cool (27°C), warm (33°C) or hot (40°C) temperature. The team observed an immediate effect of temperature on the eggs; only 40–57% of eggs exposed to the warm and hot temperatures survived, in contrast to the cool eggs, which had a 90% survival rate. Considering that climate change is expected to push temperatures in some regions to their upper limits, this finding is concerning, as it shows that an enormous proportion of eggs that are persistently exposed to higher natural temperatures might succumb to heat-stress.
Of the eggs that hatched, the team raised the lizard youngsters at 25°C (room temperature) to adulthood, before testing their heat tolerance. Then, knowing that lizards eventually hit a critical temperature at which their muscles fail, the scientists slowly warmed the lizards from 36°C and flipped them on to their backs. Every time a lizard successfully turned over, indicating that their muscles were still functioning, the team raised the lizard's body temperature by 1°C and tested its righting ability again. If the lizards had benefited from their warm incubation, then they might be able to continue righting themselves at temperatures where the cooler lizards failed. However, the results were both surprising and alarming. Surprising, because all the lizards, regardless of whether they were raised in cool, warm or hot conditions, lost the ability to right themselves when temperatures were higher than 39°C. In other words, the temperature that the lizards experienced while they were embryos had no effect on their heat-tolerance as adults, which is alarming, because of the implications for reptile populations worldwide.
Given that scientists expect climate change to elevate global temperatures in the years to come, reptiles might not benefit from phenotypic plasticity as embryos if exposed to extreme temperatures. Considering that a number of reptilian species live and reproduce in areas with very high daily temperatures, an increase of only a few degrees could have severe ecological consequences for their populations.