Temperature-dependent Sex Determination of a Sea Turde, Caretta ...

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152 Shoji Tokunaga, Yuji Iwakiri and Yoshito Nakajima The two incubation groups (B5 and CI) of the temperature shift experiment showed no significant difference in sex ratio. The overallsex ratio (95% CI) by the temperature shift experiment was 0.11 (0.03-0.25). Considering only the duration of incubation period at each temperature, the mean incubation temperature was calculated to be 29.7°C,at which temperature the expected sex ratio was0.35. When the difference of developmental speed by incubation temperature was also consid ered, the weighted mean incubation temperature was 30.1°C, and the expected sex ratio was 0.20. Discussion In this series of experiments, all eggswere kept at uncontrolled temperature for 4 or 5 daysduring transportation. If the uncontrolled temperature during transpor tation affected sex determination, interpretation of the present results on sex deter mination would be difficult. However, the presence of the critical period among TSD species should be noted. Before and after this critical period, sex determina tion is insensitive to temperature. This was demonstrated in an experiment on the critical period of sex differentiation in whichYntema and Mrosovsky (1982) temporarily alternated the incubation temperature either upwards or downwards from the critical temperature. Theyshowed that the incubation temperature in the first 11 days of incubation had no effecton sexual differentiation of the loggerhead sea turde. Therefore, the results of the present studyare not likelyto be attributable to temperatures experienced prior to the controlled incubation. The temperatures during transportation might have affected the incubation period. Consequently, both the relationship between incubation temperature and incubation period and the critical period of incubation might have some degree of error. However, the duration under uncontrolled temperature waslessthan 10% of the total incubation period. Furthermore, the difference of incubation tempera ture changes the incubation duration by 24% (from 27.7 to 30.7°C, thisstudy) or by 44% (from 25 to 32°C,Limpus etal, 1985). This means that even if the temperature during transportation differed by 7°C among clutches, the overall incubation duration was affected bylessthan 5%. Thus, the effectof uncontrolled temperature duringtransportation on incubation duration should have beenwithin the allowable margin of error. The critical temperature has been reported for C. caretta from some localities. Limpus et al (1985) examined the critical temperature of C. caretta from Heron Island and Mon Repos, Australia. They estimated that the critical temperature with 95% confidence limits for the combined clutches from the two localities was 28.6 ± 0.5°C. Mrosovsky (1988) incubated the eggs from North Carolina, Georgia and Florida, USA. He estimated the critical temperature of the eggs from the three localities combined was 29.0°C. Kamezaki and Kuroyanagi (1991) estimated that
Sex determination of a sea turtle 153 the critical temperature ofa clutch of C. caretta from Atsumi, Japan exists between 28 and 30°C. The present estimate of critical temperature was 29.4°C, which is in the range of the critical temperature reported for the clutch from Atsumi. It was close to the overall estimate of 28.6°C given by Limpus et al (1985) and 29.0°C given by Mrosovsky (1988). In some turdes with TSD, dominance of one-sex-determining temperature over another-sex-determining temperature is known. In map turtles {Graptemys ouachitensis), for example, an isolated pulse ofmale-producing temperature can have a substantial male-inducing effect, although an isolated pulse of female-producing temperature in the sensitive period has litde female-producing effect (Bull and Vogt, 1981). Thus, sex is more readily influenced by male-determining tempera ture than female-determining temperature in this species. In snapping turtle (Chelydra serpentina), on the contrary, female determination is more easily induced than male determination. Incubation at female-producing temperature through either halfofthe sensitive period induces femaleness in most embryos; male determi nation requires incubation at male-producing temperature for nearly the entire duration of the sensitive period (Yntema, 1979). In this study, the sex ratio at fluctuating temperature was close to the expected sex ratio at crude mean tempera ture (0.53 vs. 0.57) and that at weighted mean temperature considering the develop mental speed (0.53 vs. 0.48), suggesting a lack of dominance ofone-sex-determining temperature over another-sex-determining temperature in C. caretta. The present study, along with other studies on sex determination of C. caretta, shows that the sex ratio of the hatchling turdes can vary in response to subde differ ences in the thermal environment of the eggs. Hence, a variety of environmental factors of the nest can cause significant differences in the hatchling sex ratio of C. caretta. For example, the brown sands of Mon Repos are consistendy warmer than the white sands of Heron Island. The hatchling sex ratio at Mon Repos was biased toward females by about 30% compared to that at Heron Island (Limpus et al, 1983). Seasonal change of hatchling sex ratio was also observed. Sex ratios of hatchling C. caretta taken from South Carolina and Georgia ranged from no females in nests laid in late May to 80% females in those laid in early July; the sex ratio decreased to 10% females in nests laid in early August (Mrosovsky etal, 1984). It has also been demonstrated that the hatchling sex ratios of C. caretta varygeographi cally. The overall hatchling sex ratio produced in South Carolina and Georgia from 1979 to 1982 was estimated to be 0.437 (Mrosovsky etal, 1984). At Florida beach, the estimated sex ratios (year) of hatchling turdes were 0.033-0.074 (1986), 0.001- 0.053 (1987) and 0.11-0.13 (1988), and strongly female-biased sex ratios were expected in 1989 and 1990, as well, by the observed high sand temperatures (Mrosovsky and Provencha, 1992). These highly female-biased sex ratios are noteworthy, because 90% of U.S. C. caretta were estimated to nest in Florida (Murphy and Hopkins, 1984).
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