The effect of moisture during development on phenotypes of egg-laying reptiles: a systematic review and meta-analysis

The embryonic environment is critical for the development of many ectothermic vertebrates, which makes them highly vulnerable to environmental change. Changes in temperature and moisture, in particular, are known to influence embryo survival and offspring phenotypes. Although most papers concerning phenotypic development of terrestrial ectotherms focus on the role of temperature on eggs and embryos, the comparatively small number of studies on the effects of substrate moisture are well suited for quantitative analysis aimed at guiding future research. To accomplish this goal, we compiled data from 37 studies on 28 different reptile species and used a meta-analytic approach to quantify the effect of substrate moisture on several offspring outcomes: hatching success (survival), incubation duration, hatchling mass and length, and sex ratio. We found that substrate moisture had a small effect across most traits but significantly affected body size (i.e. length and mass), with wetter conditions producing longer and heavier hatchlings. Temperature also moderated the effect of moisture on hatching success, with higher temperatures resulting in lower success. Additionally, the effect of moisture on hatching success and hatchling mass was enhanced by larger differences in moisture concentration between treatments, yet the effect was small. Lastly, substrate moisture affected sex ratio in turtles, but not in other squamates. Overall, these analyses provide a foundation for further research investigating the effects of moisture on oviparous reptile development. Increasing the diversity of environmental variables for which we understand their impact on animal phenotype will be beneficial in an era with wide-ranging global change.