BOWDEN, RM*; HARMS, HK; JANZEN, FJ; Iowa State University; Iowa State University; Iowa State University: Age-Related Variation in Egg Size and Yolk Steroid Hormones in the Painted Turtle

Egg size is thought to be constrained by pelvic aperture morphology in turtles. Given this potential constraint, females should optimize egg size such that they produce the largest egg possible while maintaining a balance between investment in offspring and investment in self. We examined how female body size and relative age impact egg mass (used as a proxy for egg size) in the painted turtle, Chrysemys picta. Regardless of female size, we found that younger females produce smaller, less massive eggs than do older females. This finding suggests that egg size may not be constrained only by pelvic aperture morphology in younger females. We then investigated age-related and seasonal variation in yolk hormones to explore potential physiological mechanisms for limiting egg size in younger females. We found that yolk testosterone concentrations were significantly higher in eggs from younger females, but that testosterone levels did not significantly vary between first and second clutches within a nesting season. Yolk estradiol levels were significantly higher in second clutches, and did not vary with female age. We also found that less massive eggs contain higher concentrations of testosterone, and previous studies have shown a link between elevated plasma testosterone levels and decreased vitellogenic activity. Elevated testosterone levels in younger females may thus constrain egg size physiologically, beyond the well-known physical constraints imposed by pelvic aperture morphology, potentially playing an important but previously unrecognized role in the evolution of egg size.