The developmental effect of sex ratio on a sexually dimorphic scale count in the garter snake Thamnophis elegans

In some mammals the development of embryos is affected by the sex of adjacent fetuses through increased or decreased levels of sex steroids present in the uterus. The effects of masculinization and de-feminization can be observed in behavior as well as in morphology. In this study we examined whether similar effects occur in a viviparous reptile, the garter snake Thamnophis elegans. We tested two populations of T. elegans for the effect of prenatal sex ratio on the development of a sexually dimorphic tail scale count, in which males have significantly more subcaudal scales than females. The results show that females from one population have significantly fewer subcaudal scales in litters with a higher proportion of males. No such trend was observed in males. The other population shows a significant difference in regression slopes comparing females to males, but the slopes cannot be shown to differ from zero in either sex. Unlike in mammals where testosterone is the organizing hormone for sexual differentiation, the results in T. elegans suggest that estrogen, after having been aromatized from testosterone, may influence the development of the subcaudal scale count in female garter snakes.