Abstract
A thorough understanding of the life cycles underlying the demography of wild species is limited by the difficulty of observing hidden life‐history traits, such as embryonic development. Major aspects of embryonic development, such as the rate and timing of development, and maternal–fetal interactions can be critical features of early‐life fitness and may impact population trends via effects on individual survival. While information on development in wild snakes and lizards is particularly limited, the repeated evolution of viviparity and diversity of reproductive mode in this clade make it a valuable subject of study. We used field‐portable ultrasonography to investigate embryonic development in two sympatric garter snake species, Thamnophis sirtalis and Thamnophis elegans in the Sierra Nevada mountains of California. This approach allowed us to examine previously hidden reproductive traits including the timing and annual variation in development and differences in parental investment in young. Both species are viviparous, occupy similar ecological niches, and experience the same annual environmental conditions. We found that T. sirtalis embryos were more developmentally advanced than T. elegans embryos during June of three consecutive years. We also found that eggs increased in volume more substantially across developmental stages in T. elegans than in T. sirtalis, indicating differences in maternal provisioning of embryos via placental transfer of water. These findings shed light on interspecific differences in parental investment and timing of development within the same environmental context and demonstrate the value of field ultrasonography for pursuing questions relating to the evolution of reproductive modes, and the ecology of development.
Highlights
The ability to accurately assess population-level outcomes of demographic changes requires measuring responses across the full life cycle of demographic events that make up a species’ life history
T. elegans eggs showed a 68% increase in volume between early and late developmental stages, whereas T. sirtalis showed a 27% increase
Recent climate change has been associated with changes in reproductive phenology in a variety of species, we found no significant difference in distribution of females with embryos at different developmental stages across years
Summary
The ability to accurately assess population-level outcomes of demographic changes requires measuring responses across the full life cycle of demographic events that make up a species’ life history. Other research on V. aspis has shown that there is a trade-off between maternal and fetal hydration, such that when mothers are water-deprived, they become dehydrated, but their eggs derive water via placental transfer to the same degree as eggs in nondehydrated mothers (Dupoué et al, 2015) These findings call for further study of maternal water provisioning in other viviparous species, so that we can begin to understand how fluctuations in water availability might affect developing embryos in the wild. Differences in phenology and provisioning could indicate interspecific differences in genes or gene by environment interactions underlying developmental phenotypes In pursuing these questions, we provide basic information regarding dynamics of development in this system, and demonstrate the feasibility and value of a method for determining otherwise hidden patterns of developmental life history in the wild within and across species and environments
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