ALLMAN, Phil*; ROOSENBURG, Willem; BOWDEN, Rachel; Ohio University; Ohio University; Illinois State University: Egg size and yolk hormone levels in the diamondback terrapin across a latitudinal gradient

Life history theory predicts that the number and size of offspring produced in one reproductive event should optimize maternal fitness for a given population. This optimization has been used to explain population differences in egg size in several ectothermic species, but a mechanistic understanding of how such differences are regulated remains unclear. The recently described physiological constraint hypothesis predicts that maternal hormones that influence vitellogenesis may provide a mechanism for the regulation of egg size. We tested this hypothesis by collecting eggs from three populations of diamondback terrapins (Malaclemys terrapin) that exhibit significant differences in egg size. The amount of maternally derived testosterone and progesterone in the egg yolks was determined using a competitive-binding steroid radioimmunoassay. Testosterone and progesterone are known to inhibit vitellogenesis so we predicted that smaller eggs would contain larger mass-specific amounts of these steroid hormones. Although we detected significant differences in testosterone (F_{2, 33}=35.4,P=0.02) and progesterone (F_{2, 32}=7.1,P=0.01) among the populations, the within population trends did not match our prediction. The population with the smallest eggs did have the highest testosterone values, but regression analysis indicates a positive relationship between testosterone and egg size within each group. Additionally, within a population the largest eggs did not have the lowest mass-specific hormone levels. These results suggest that the maternally derived yolk hormones may be constrained at the population level in this species and that egg size is regulated through additional processes. Further investigation is needed to explain the observed patterns.