Lerista bougainvillii, a case study for the evolution of viviparity in reptiles

Many factors, both environmental and biotic, have been suggested to facilitate or hinder the evolution of viviparity (live-bearing) in reptiles. Viviparity has evolved recently within the Australian scincid lizard Lerista bougainvillii and the species includes oviparous, viviparous, and reproductively intermediate (with prolonged egg retention) populations; thus, it offers an exceptional opportunity to evaluate the validity of these hypotheses. We carried out such tests by (i) comparing environmental conditions over the geographic ranges occupied by oviparous, viviparous, and intermediate populations (to identify possible selective forces for the evolution of viviparity), and (ii) comparing morphological, reproductive and ecological traits of L. bougainvillii with those of other sympatric scincid species (to identify traits that may have predisposed this taxon to the evolution of viviparity). The areas occupied by viviparous L. bougainvillii are significantly colder than those occupied by both their intermediate and oviparous conspecifics, in accord with the 'cold-climate' hypothesis for reptilian viviparity. Rainfall is similar over the ranges of the three forms. Climatic unpredictability (as assessed by the magnitude of year-to-year thermal variation) is lower for viviparous animals, in contradiction to published speculations. Comparison with 31 sympatric scincid species showed that L. bougainvillii is not atypical for most of the traits we measured (e.g., body size, clutch size, thermal preferenda and tolerances). However, oviparous L. bougainvillii do display several traits that have been suggested to facilitate the evolution of viviparity. For example, pregnancy does not reduce locomotor ability of females; the lizards are semi-fossorial; even the oviparous females produce only a single clutch of eggs per year; and they ovulate relatively late in summer, so that the time available for incubation is limited.