Why do the eggs of lizards (Bassiana duperreyi: Scincidae) hatch sooner if incubated at fluctuating rather than constant temperatures?

Reptilian eggs hatch sooner if incubated at higher temperatures, but why should increased diel thermal fluctuation (independent of mean temperature) also modify the duration of incubation? In the montane scincid lizard Bassiana duperreyi, eggs incubated at 22 ± 7 °C hatched > 10% sooner than did sibling eggs kept at a constant 22 °C (55 versus 62 days). We non-invasively measured embryonic heart rates, which are highly correlated with rates of embryonic oxygen consumption, as well as with rates of embryogenesis, to test three explanations (exponential relationships between temperature and developmental rate; thermal acclimation of developmental rate; and facultative shifts in the degree of embryogenesis completed before hatching) aiming to explain why fluctuating-temperature incubation reduces the incubation period. Heart rates did not acclimate to incubation temperature regimes. Hatchling body sizes did not differ among the two treatments. We found an exponential relationship between temperature and heart rate, which means that embryonic cardiac output was increased more by a diel increase in temperature than it was decreased by a diel fall in temperature, leading to 7% more daily heartbeats at 22 ± 7 °C than at 22 °C. This nonlinear thermal sensitivity of developmental rate is the reason why diel thermal fluctuations accelerate embryogenesis, and thus hasten hatching, in B. duperreyi. The generality of this relationship implies that, in cool-climate squamate reptiles, mothers might be able to hasten the hatching dates (and thus viability) of their offspring by selecting nest sites that provide high diel thermal variation as well as high mean temperatures.