Variation in nesting patterns affecting nest temperatures in two populations of painted turtles (Chrysemys picta) with temperature-dependent sex determination

Abstract

Mechanisms maintaining sex ratios in populations with temperature-dependent sex determination (TSD) remain elusive. Although geographic variation in embryonic sex determination (i.e., pivotal temperature) has been widely investigated in reptiles exhibiting TSD, no previous studies have directly addressed geographic variation in maternal behavior affecting nest thermal conditions. I evaluated patterns of nest-site selection and its effects on thermal and hydric nest conditions for a population of painted turtles (Chrysemys picta bellii) exhibiting TSD in New Mexico. These results are compared to data collected from a well-studied, conspecific population experiencing relatively cooler climatic conditions in Illinois. Since canopy vegetation cover reduces nest temperatures in Illinois, I expected females in New Mexico to nest under high amounts of canopy vegetation cover. However, females from New Mexico placed nests under significantly less canopy vegetation cover, but closer to standing water, than did females from Illinois. Experimental nests in New Mexico demonstrated that increased canopy vegetation cover and soil moisture reduced nest temperatures. By nesting close to standing water rather than under canopy vegetation cover, females in New Mexico nested in habitats more closely associated with maximizing moisture around nests rather than reducing nest temperatures through shading. Mean July nest temperatures were similar for both populations. Since nest hydric conditions affect hatching success and hatchling size in C. picta, nesting patterns in New Mexico may primarily reflect selection for microhabitats affecting offspring survivorship or size.