“The evolutionary economics of embryonic-sac fluids in squamate reptiles”

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Xavier Bonnet, Guy Naulleau, and Richard Shine

Transition from oviparity to viviparity limited embryonic-sac fluids and created competition among offspring for water

How much water should a pregnant snake provide her babies?

A female aspic viper (Vipera aspis) with her six offspring a few minutes after parturition. Litter size varies between 1 and 15 in this species with a mean value of 6. Litter mass (37 g on average) represents 55% (max 111%) of the post-maternal mass (70 g on average). Fluids stored in embryonic sacs are vital for development, at parturition they correspond to an additional burden of 15 g for the mother; they represent 21% of the post-maternal mass and thus occupy an important intra-uterine volume. Siblings compete for finite amounts of water and pose strong hydro-mineral challenge to their mother.
(Credit: Xavier Bonnet)

Although most species of reptiles reproduce by laying eggs, many groups—including vipers—have made the evolutionary leap to retaining those developing eggs inside the mother’s body instead of laying them in a nest. Thus, a mother viper gives birth to fully developed young. A team of French and Australian scientists has examined this evolutionary transition in detail, using an approach from economics to understand how the evolutionary switch from eggs to babies changes patterns of investment into the offspring. In particular, a reproducing mother in a live-bearing species can’t afford to put too much water into an “egg” inside her body—both because water is often hard to obtain, and because it takes up a lot of space. And so, a female reptile that produces live offspring instead of eggs is forced to be miserly with water for her babies.

A neonate Vipera aspis halfway in the process of birth. The small snake is expelled by the mother while still inside the embryonic sac. The fluids contained in the sac weigh approximately 2.5 g on average (range: 0.3–7.7 g) and represent 33% of the neonate mass.
(Credit: Xavier Bonnet)

But she can’t reduce the water supply too far. An aquatic environment is essential for any embryo to develop. Water is needed for the yolk to be converted into the offspring’s tissues, and provides cushioning against shocks. The water is kept inside special membranes in the “eggs” of all species, even humans. In humans, when the amniotic sac ruptures (water breaks), approximately 1 liter of fluids is lost. What is the total amount of fluids lost at birth in species with multiple offspring? This was measured in France in more than 140 aspic vipers (a viviparous snake) that produced >1000 babies. Embryonic-sac fluids averaged 21% of maternal body mass, thus represent a heavy burden and challenge the mother’s hydric balance. Moreover, siblings compete for access to finite water supplies: In large litters, each baby has less water. These results reveal for the first time strong conflicts for water between the mother and her offspring, and among siblings. Read the Article