Changes in Gonadal and Adrenal Steroid Levels in the Leatherback Sea Turtle (Dermochelys coriacea) during the Nesting Cycle

The reproductive endocrinology of nesting leatherback turtles (Dermochelys coriacea) was studied during the 1996–1997 and 1997–1998 nesting seasons at Los Baulas National Park, Playa Grande, Costa Rica. Blood samples were collected from nesting females throughout the season. Females were observed to nest up to 10 times during the nesting season. Plasma steroids were measured by radioimmunoassay and total plasma calcium was measured by flame atomic absorption spectrophotometry. Plasma testosterone and plasma estradiol levels declined throughout the nesting cycle of the female. Testosterone declined from a mean of 10.18 ± 0.77 ng/ml at the beginning of the nesting cycle to 1.73 ± 0.34 ng/ml at the end of the nesting cycle. Estradiol declined in a similar manner, ranging from a mean of 190.95 ± 16.80 pg/ml at the beginning of the nesting cycle to 76.52 ± 12.66 pg/ml at the end of the nesting cycle. Plasma progesterone and total calcium levels were relatively constant throughout the nesting cycle. Lack of fluctuation of total calcium levels, ranging from a mean high of 97.46 ± 11.37 μg/ml to a mean low of 64.85 ± 11.20 μg/ml, further suggests that vitellogenesis is complete prior to the arrival of the female at the nesting beach. Clutch size (both yolked and yolkless eggs) did not vary over the course of the nesting cycle.