Phenotypic variation in smooth softshell turtles (Apalone mutica) from eggs incubated in constant versus fluctuating temperatures.

Temperatures experienced during embryonic development elicit well-documented phenotypic variation in embryonic and neonatal animals. Most research, however, has only considered the effects of constant temperatures, even though developmental temperatures in natural settings fluctuate considerably on a daily and seasonal basis. A laboratory study of 15 clutches of smooth softshell turtles (Apalone mutica) was conducted to explicitly examine the influence of thermal variance on phenotypic variation. Holding mean temperature constant and eliminating substrate moisture effects permitted a clear assessment of the impact of thermal variance on hatching success, incubation length, hatchling body size, swimming speed, and righting time. Incubation length and swimming speed varied significantly among temperature treatments. Both traits tended to increase with increasing thermal variance during embryonic development. Clutch significantly affected all traits examined, except righting time, even after accounting for the effects of initial egg mass. These results highlight the importance of accounting for the impact of both thermal mean and variance on phenotypic variation. The findings also strengthen the increasing recognition of maternal clutch effects as critical factors influencing phenotypic variation in neonatal animals.