Sunny side up: lethally high, not low, nest temperatures may prevent oviparous reptiles from reproducing at high elevations

Oviparous (egg-laying) lizards and snakes generally inhabit warmer climates than do related viviparous (live-bearing) taxa. This pattern is widely attributed to the failure of oviparous reproduction in cold climates, but the thermal regimes of potential nest-sites above and below the elevational cut-off for oviparous reproduction have never been quantified. We studied oviparous (Bassiana duperreyi) and viviparous (Eulamprus heatwolei) scincid lizards at such a site in the Brindabella Range of south-eastern Australia. Miniature data-loggers monitored temperatures of nest-sites and lizards in midsummer, partway through the incubation period of eggs in natural nests. Our results contradict the simplistic notion that mean nest temperatures determine this elevational limit for oviparity. Instead, potential nest-sites with average temperatures suitable for embryogenesis in Bassiana are available well above the threshold elevation. However, thermal minima decrease consistently with elevation and thus the maximum temperature needed for any given mean incubation temperature increases rapidly with elevation. Potential nest-sites above the elevational threshold can only attain mean temperatures high enough to sustain embryogenesis by having lethally high thermal maxima. Such nest-sites are available close to the soil surface, but cannot support development. In contrast, behavioural thermoregulation allows viviparous lizards to maintain high mean body temperatures concurrently with relatively low maximum temperatures, regardless of elevation. Paradoxically, oviparous reptiles may be restricted to low elevations not because nests that provide appropriate mean incubation temperatures are unavailable further up the mountain, but because eggs laid in such shallow nests would overheat.