1. Viviparity (live-bearing) has independently evolved from oviparity (egg-laying) in more than 100 lineages of squamates (lizards and snakes).
2. 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 vs. 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 RI or a reduction in offspring size and/or fecundity.
3. 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.
4. We attribute this paradoxical result to (1) strong selection on 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.
5. 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.
6. The consequent stasis in reproductive burden during the evolutionary transition from oviparity to viviparity may have facilitated frequent shifts in parity modes.