Egg failure rates are increasing for many reptile species, threatening population stability. Yet the mechanisms of reptile egg failure have not been studied for most species, especially in the context of global climate change. One reason an egg might fail is because eggshell morphology could become mismatched with the environment as climate change continues and environmental conditions deviate from normal ranges, like during drought. Unfortunately, drought frequency and severity are increasing across large areas of the globe as climate change continues, potentially threatening eggs that are dependent on wet environments. We explored the effects of drought on reptile eggs by studying American alligator (Alligator mississippiensis) eggs exposed to drought conditions within incubators. Specifically, we determined the effects of low humidity conditions on alligator eggshell thickness, structure, porosity, and gas conductance rates, as well as embryo viability and hatchling growth. Our results are valuable because alligators are important ecologically, economically, and culturally, and they will be increasingly exposed to drought in the future. Our results are also applicable to some of the most endangered (and drought-exposed) reptile species, including all the other crocodilian species and many turtle species whose eggs tend to share traits with crocodilian eggs.
Results/Conclusions
We exposed alligator eggs to high ( >92% relative humidity, or RH), medium (86-90% RH), and low (80-84% RH) humidity conditions. Eggs in the high humidity treatment had a 73% hatch rate, while eggs in the medium and low humidity treatments had dramatically lower hatch rates of 24% and 9%, respectively. The main cause of egg failure in the medium and low treatments appears to have been water loss from the eggs, which resulted in desiccation and separation of the interior membrane of the eggshell and eventual asphyxiation of the embryo. These results suggest that even very small changes in humidity within an alligator’s nest can lead to a precipitous drop in embryo survival, likely affecting population dynamics over the long-term if drought conditions persist. Future research should focus on how drought may affect humidity levels in actual alligator nests and how eggs respond in more realistic incubation conditions. Furthermore, expanding this type of humidity and drought research to other potentially vulnerable reptile species will help determine how widespread the threat of drought might be for reptile eggs more broadly.
