Experimental manipulation of steroid concentrations in circulation and in egg yolks of turtles.

Steroid hormones in egg yolks are increasingly recognized as an important component of maternal and offspring fitness in oviparous vertebrates. Yet, except for in birds, the mechanism by which females allocate these resources is poorly understood. We manipulated systemic levels of hormones in reproductively mature female red-eared slider turtles (Trachemys scripta elegans) with silastic implants to test the hypothesis that hormones are allocated to developing follicles as a quantitative function of circulating levels in the females. Turtles exhibited similar amounts (<1 ng/ml) of circulating steroids (dihydrotestosterone, estradiol-17 beta, or testosterone) in early September immediately prior to experimental manipulation. After treatment with silastic implants, circulating levels of steroids increased markedly. By the following April after hibernation, circulating levels of dihydrotestosterone had returned to preimplantation levels, but circulating levels of estradiol-17 beta and testosterone in estradiol-17 beta- and testosterone-treated turtles, respectively, remained substantially elevated through April. Focusing on testosterone, we detected nearly six-fold higher concentrations in yolk from mature follicles from testosterone-treated turtles than in yolk from mature follicles from control turtles. Our results provide support for the hypothesis that concentrations of steroids in egg yolks of turtles reflect circulating concentrations of steroids during follicular development rather than the hypothesis that females selectively allocate specific amounts of steroid hormones to each egg separately. Our findings also highlight an unambiguous physiological mechanism by which nongenetic maternal effects in oviparous species can directly influence the nutritional milieu experienced by developing embryos.