Elastic vipers: reproducing snakes adjust their size and internal volume to accommodate their developing offspring

In many taxa, natural selection favours the ability of a female to accommodate a clutch or litter that is very large relative to her own body, placing a selective premium on traits that increase available abdominal space relative to litter volume. Flexible changes during pregnancy might help to mitigate these constraints. Using ultrasound scanning of captive snakes and snakes captured in the field, we explored such traits in reproducing female vipers (Vipera aspis). First, the anteriormost embryos moved forwards as they swelled during pregnancy, taking up space previously occupied by maternal viscera. Second, the oviductal membrane-bound packages containing embryos changed shape to fit flexibly into thicker vs. thinner parts of the mother's body. Third, intervertebral spaces increased during pregnancy, allowing the mother's body to lengthen. Maternal size elongation during pregnancy was reversed shortly after parturition. The decrease in size was closely related to the degree of abdominal displacement and reproductive output and was also evident in field-collected females of three European snakes. In summary, our data suggest that elasticity of the mother's body and flexibility in packaging of the offspring can mitigate abdominal constraints on maximal litter volume and space competition among siblings.