Incubation temperature in the wild influences hatchling phenotype of two freshwater turtle species

Background: The nest environment influences phenotypic traits of hatchling turtles. Female turtles select nest sites that promote hatchling survival, and alter nesting behaviour in response to changing environments. Differences in phenotype generated by incubation environment could provide variation in traits that natural selection can act upon. The relationship between incubation temperature in the laboratory and post-hatching phenotype is well documented, but whether incubation in nature generates biologically meaningful levels of phenotypic variation is less well studied. Questions: (1) What are the effects of canopy cover, laying date, and nest depth on incubation temperature? (2) What are the relationships between incubation temperature, egg mass, and hatchling phenotype? (3) What are the sex-specific effects of incubation temperature on phenotypic variation in two turtles with temperature-dependent sex determination? Organisms: Painted turtle (Chrysemys picta) and snapping turtle (Chelydra serpentina) nests and hatchlings from Algonquin Park, Ontario, Canada. Methods: In 2010 and 2011, we measured canopy cover at nests and hourly temperatures within nests throughout incubation. Post-parturition, we measured egg mass of each clutch. After emergence, we measured hatchling righting response (time taken to flip from carapace to plastron), carapace length, and mass. Conclusions: Canopy cover and oviposition date did not affect nest temperature, but nest depth influenced daily temperature variance in snapping turtle nests. However, limited variation in environmental characteristics suggests that a female's ability to select microhabitats that adaptively affect offspring survivorship or phenotype is limited. Female painted turtles with heavier eggs selected nest sites that were warmer. Nest incubation temperature was related to multiple hatchling characteristics. Painted turtle hatchling carapace length was positively related to mean incubation temperature, but snapping turtle hatchling size was not related to incubation temperature. Painted turtle hatchling righting response was not related to incubation temperature, but snapping turtle hatchlings from warmer nests righted themselves more quickly and hatchlings from nests with greater temperature variance righted more slowly. Our predicted nest sex ratios suggested that warmer nests with heavier eggs would produce female hatchlings. Also, in both species, carapace length was greater for hatchlings from nests predicted to produce females than from nests predicted to produce males. These findings from a natural setting help to inform future research on the adaptive significance of temperature-dependent sex determination.20 page(s