Sex determination and sex ratios of Pacific leatherback turtles, Dermochelys coriacea

Laboratory incubation of eggs and histology of gonads indicated that the pivotal temperature for leatherback turtles nesting on the Pacific coast of Costa Pica at Playa Grande was 29.4 C and not biologically different from that determined for Atlantic leatherbacks. We calculated sex ratios of leatherback hatchlings by monitoring beach and nest temperatures for eight thermal profiles and 47 nests during the 1994-1995 nesting season. Leatherbacks at Plays Grande nest mainly in the open beach zone (86.8-90.0%) and in the middle beach section (68.6-80.9%). Sand temperature increased as the nesting season progressed and reached 30.0 C (100% female) by the middle of November. Monitored nests that produced hatchlings had temperatures above 30.0 C during the critical sex determining period. The sex of all hatchlings determined by gonad histology for 18 monitored nests (n = 10-20 per nest) were 100% female. Estimated sex ratio for the 1993-1994 season was Oro male:100% female, for the 1994-1995 nesting season was 6.5% male:93.5% female, and for the 1995-1996 season was 25.7% male:74.3% female. These ratios were more female biased than sex ratios reported for the past 25 years in Suriname on the Atlantic coast of South America. Gene flow between populations and the response of leatherback populations to thermally different nesting areas may be responsible for the lack of intraspecific variation in pivotal temperature.