Use of field‐portable ultrasonography reveals differences in developmental phenology and maternal egg provisioning in two sympatric viviparous snakes

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.

Temperature‐induced multi‐species cohort effects in sympatric snakes

“…25 days among years, shortening or extending the time available for post‐natal growth accordingly. Unfortunately, accurate estimates of the timing of ovulation and parturition are difficult to obtain and are likely to vary among individuals in the field (but see Sparkman et al, 2018).…”

“…25 days among years, shortening or extending the time available for post-natal growth accordingly. Unfortunately, accurate estimates of the timing of ovulation and parturition are difficult to obtain and are likely to vary among individuals in the field (but seeSparkman et al, 2018).Cohort effects at my study site were of sufficient magnitude to accelerate attainment of reproductive maturity in at least some individuals during warm years. For example, in 2010, the end-of-season SVL of S. dekayi neonates (185.5 mm) exceeded the minimum SVL of reproductively mature males (175 mm based on presence of sperm in cloacal smears, personal observation) and at least some neonatal males exceeded 175 mm in 2010, 2014, 2016, and 2018.…”

Dehydration during egg production alters egg composition and yolk immune function

“…Previous studies examining trade-offs between maternal and embryonic hydration during water deprivation concluded that maternal hydration is compromised in favor of embryonic hydration (Dupoué et al, 2015, Sparkman et al, 2018. However, these studies relied on egg…”

The thermal ecology and physiology of reptiles and amphibians: A user's guide

“…For a number of reasons, the best approach will depend on the circumstances, and may differ between amphibians and reptiles. For instance, assessing morphological features of embryos in transparent amphibian eggs can sometimes be done visually without impeding development, whereas in reptiles, assessment usually involves sacrificing embryos (but see Sparkman et al, 2018 on reptilian ultrasound), such that different methodologies may be favored in each group.…”