The evolutionary economics of embryonic-sac fluids in squamate reptiles
Résumé
The parchment-shelled eggs of squamate reptiles take
up substantial water from the nest environment, enabling the conversion
of yolk into neonatal tissue and buffering the embryo against
the possibility of subsequent dry weather. During development, increasing
amounts of water are stored in the embryonic sacs (i.e.,
membranes around the embryo: amnion, allantois, and chorion).
The evolution of viviparity (prolonged uterine retention of developing
embryos) means that embryonic-sac fluid storage now imposes a
cost (increased maternal burdening), confers less benefit (because
the mother buffers fetal water balance), and introduces a potential
conflict among uterine siblings (for access to finite water supplies).
Our data on nine species of squamate reptiles and published information
on three species show that the embryonic-sac fluids comprise
around 33% of neonatal mass in viviparous species versus 94% in
full-term eggs of oviparous squamates. Data on parturition in 149 vipers
(Vipera aspis, a viviparous species) show that larger offspring
store more fluids in their fetal sacs and that an increase in litter size
is associated with a decrease in fluid-sac mass per offspring. Overall,
the evolutionary transition from oviparity to viviparity may have substantially
altered selective forces on offspring packaging and created
competition among offspring for access to water reserves during embryonic
development.