Data from: Viviparity does not affect the numbers and sizes of reptile offspring

Viviparity (live-bearing) has independently evolved from oviparity
(egg-laying) in more than 100 lineages of squamates (lizards and snakes).
We might expect consequent shifts in selective forces to affect per-brood
reproductive investment (RI = total mass of offspring relative to maternal
mass) and in the way in which that output is partitioned (number versus
size of offspring per brood). Based on the assumption that newly-born
offspring are heavier than eggs, we predicted that live-bearing must
entail either increased reproductive investment or a reduction in
offspring size and/or fecundity. However, our phylogenetically-controlled
analysis of data on 1,259 squamate species revealed no significant
differences in mean offspring size, clutch size or RI between oviparous
and viviparous squamates. We attribute this paradoxical result to (1)
strong selection on optimal offspring sizes, unaffected by parity mode,
(2) the lack of a larval stage in amniotes, favouring large eggs even in
the ancestral oviparous mode, and (3) the ability of viviparous females to
decrease the mass of uterine embryos by reducing extra-embryonic water
stores. Our analysis shows that squamate eggs (when laid) weigh about the
same as the hatchlings that emerge from them (despite a many-fold increase
in embryo mass during incubation). Most of the egg mass is due to
components (such as water stores and the eggshell) not required for
oviductal incubation.  That repackaging enables live-born
offspring to be accommodated within the mother’s body without increasing
total litter mass. The consequent stasis in reproductive burden during the
evolutionary transition from oviparity to viviparity may have facilitated
frequent shifts in parity modes.