Reconstructing an adaptationist scenario: what selective forces favor the evolution of viviparity in montane reptiles?

It is notoriously difficult to test hypotheses about the selective forces responsible for major phylogenetic transitions in life-history traits, but the evolution of viviparity (live bearing) in reptiles offers an ideal model system. Viviparity has arisen in many oviparous reptile lineages that have invaded colder climates. Thermal advantages (eggs retained within the mother's body will be warmer than those laid in the nest) are almost certainly important, but the actual selective pressures remain controversial. For example, the benefit to retention might involve faster development, protection against freezing, predation, or desiccation, or modification of hatchling phenotypes. I experimentally manipulated incubation regimes of a montane scincid lizard (Bassiana duperreyi, Scincidae) to test these ideas. Eggs maintained in cooler "nests" in the laboratory developed more slowly, were more likely to die before hatching, and produced inferior (small, slow) hatchlings. A 2-wk initial period of higher-temperature incubation (simulating uterine retention, an intermediate step toward viviparity) ameliorated these effects. In the field, I placed eggs in artificial nests at the upper elevational limit of natural nests and also extending a further 100 m higher on the mountain. The results mirrored those in the laboratory: retention at maternal body temperatures accelerated hatching, enhanced hatching success, and increased locomotor speeds of hatchlings. This selective advantage of uterine retention was greater at higher elevations and increased with longer retention. The causal process responsible was prolonged low-temperature incubation rather than freezing, desiccation, or predation, and both hatching success and hatchling viability were affected. Field experiments that directly re-create selective regimes may thus provide robust tests of adaptationist hypotheses.