Egg-laying reptiles in cold climates: determinants and consequences of nest temperatures in montane lizards

Studies on reptilian life-history evolution have emphasized the role of cold climates as a selective force for the evolution of viviparity, but have tended to neglect the many cold-climate reptiles that retain oviparity. Many of these species avoid low incubation temperatures by selecting warm nest-sites, and the evolution of viviparity (by uterine retention and maternal thermoregulation) is an extension of this strategy. However, an alternative pathway exists: reptiles may adapt to low-temperature incubation rather than avoid it. The scincid lizard Nannoscincus maccoyi from high-elevation areas of south-eastern Australia follows this alternative strategy. Field studies show that Nannoscincus selects cooler oviposition sites than does the sympatric heliothermic skink Bassiana duperreyi, owing to differences in the type of cover object (log vs. rock), the size of cover object and the depth to which eggs are buried. These interspecific differences in natural incubation regimes are reflected in the responses of laboratory-incubated eggs to incubation temperature. Bassiana eggs tolerate higher temperatures than do Nannoscincus eggs, but do not develop as rapidly at low temperatures. Incubation at lower temperatures produces a larger, faster hatchling in Nannoscincus, whereas the reverse is true in Bassiana. Thus, Nannoscincus and Bassiana follow different pathways to overcome the difficulties of reproducing in cold climates. Only the Bassiana pathway is likely to be compatible with the evolution of viviparity.