Population differences in embryonic developmental timing in the lizard Diploderma swinhonis.

Geographic variation in reproductive timing can influence how embryonic development is partitioned between maternal egg retention and external incubation. We investigated population differences in embryonic developmental timing in the lizard Diploderma swinhonis by comparing eggs from Orchid Island and mainland Taiwan. Gravid females from mainland Taiwan and Orchid Island were collected and maintained under standardized laboratory conditions. Following oviposition, eggs within each clutch were sequentially dissected at 10-day intervals to minimize maternal effects and avoid pseudoreplication. Embryonic developmental stages were determined using a standardized developmental staging table, and developmental trajectories were compared between populations. Embryos from Orchid Island were deposited at significantly more advanced developmental stages, indicating that females retain eggs in the oviduct for a longer period prior to oviposition. In contrast, embryos from mainland Taiwan were laid at earlier developmental stages but subsequently exhibited faster post-oviposition developmental rates. Developmental synchrony also differed between populations, with embryos on Orchid Island showing lower synchrony and more gradual stage transitions among individuals. Despite these differences in developmental onset, rate, and synchrony, total incubation duration did not differ between populations, suggesting that embryonic development is partitioned differently between maternal retention and external incubation environments. Standardized maternal body size showed no detectable effect on developmental rate. These patterns may be associated with geographic differences in ecological conditions, including the presence of egg-eating snake predators on Orchid Island but not on mainland Taiwan. Such variation is consistent with facultative egg retention and highlights how reproductive timing can vary among populations within a species.