1. One of the primary axes of life-history variation involves the proportion of embryonic development for which the offspring is retained within its parent's body; understanding trade-offs associated with prolonging that period thus is a critical challenge for evolutionary ecology. 2. Prior to oviposition, most oviparous squamate reptiles retain developing eggs in utero for about one-third of embryogenesis; the strong conservatism in this trait is a major puzzle in reptilian reproduction. To clarify fitness consequences of this prolonged uterine retention, we need to experimentally modify the trait and examine the effects of our manipulation. 3. We used transdermal application of corticosterone to induce gravid scincid lizards (Bassiana duperreyi) to lay their eggs 'prematurely', with relatively undeveloped embryos. Corticosterone application induced females to oviposit sooner (mean of 5.41 ± 0.51 days post-treatment) at earlier embryonic developmental stage (27 ± 0.21) than did controls (13.2 ± 1.22 days; embryonic stage 30.4 ± 0.16). 4. Corticosterone levels in the egg yolk were unaffected by maternal treatment, so effects of earlier oviposition should not be confounded by endocrine disruption of embryogenesis. Nonetheless, early oviposition reduced hatchling fitness. Hatching success was lower, incubation periods post-laying were increased, and neonates from eggs laid at earlier embryonic stages were smaller and slower. 5. These results suggest that retention of developing eggs in utero by oviparous squamates enhances maternal fitness, and does so via modifications to offspring phenotypes rather than (for example) due to accelerated developmental rates of eggs in utero compared to in the nest. 6. More generally, our data support optimality models that interpret interspecific variation in the duration of maternal-offspring contact in terms of the selective forces that result from earlier vs. later termination of that maternal investment.
- uterine retention