Serum gonadotropin and steroids associated with breeding activities in the green sea turtle, Chelonia mydas: II. Mating and nesting in natural populations

Abstract

Circulating levels of gonadotropins (FSH and LH) and sex steroids (estrogen, testosterone, and progesterone) were examined in mating and nesting animals from three breeding aggregations of green sea turtles ( Chelonia mydas). Copulating males were characterized by relatively low levels of both gonadotropins and high androgen levels, but the latter did not differ from values previously observed in prebreeding males. Females showed significant changes in all hormones studied, except estradiol. The patterns of changes in gonadotropins and steroids as well as their absolute values were similar among the three natural breeding aggregations and captive turtles studied previously. Serum or plasma LH showed slight but variable increases between mating and nesting animals, whereas FSH was consistently elevated at the time of nesting. Field studies demonstrate that this rise in FSH is a transient surge of no more than a few hours' duration: FSH does not rise until the middle of the nesting process, when oviposition begins, and FSH declines before or immediately after oviposition is completed. Thus, this peak in FSH apparently does not initiate nesting behavior. Testosterone did not differ between mating and nesting in females, but it dropped significantly in the first days following nesting (i.e., in the internesting interval when ovulation occurs). Progesterone values were also similar between mating and nesting samples, but this steroid along with LH frequently showed a pronounced elevation in the 1–2 days postnesting; their elevation was considered to represent the “ovulatory surge”. Since this progesterone and LH surge only occurs if a nesting turtle is allowed to oviposit, the presence of shelled oviducal eggs probably suppresses ovulation.