Manipulating Sex Ratios: Turtle Speed Ahead!

Mrosovsky, N; Godfrey, Matthew

Vogt' s ( 1994) commentary in this journal draws attention to the need for more awareness of the scientific literature on sex ratios by those managing sea turtle populations. In particular, he proposes more use of artificial incubation of turtle eggs for two reasons: first, to avoid incubation near the pivotal [= threshold] temperature (the constant temperature giving 50% of each sex) and thereby reduce the numbers of intersexes, and second, to produce female-biased sex ratios to boost population levels. Few would dispute that the influence of incubation temperature on sexual differentiation has implications for conservation practices. Unfortunately, however, the data base is not as solid as one might wish; on a number of points there is frank disagreement. Fundamental to any scientific assessment of the influence of temperature on sex ratio is a reliable method of sexing. The method of soaking gonads in glycerine to render them transparent and make internal structures visible (van der Heiden et al., 1985) is a clever idea and evidently works in some circumstances for some species. For sea turtles, however, the method has not been adequately validated, and there are two published reports of failure to validate this technique (Jackson et al., 1987; Mrosovsky and Benabib, 1990). Vogt indicates that one of these validation attempts (Mrosovsky and Benabib, 1990) was flawed because gonads were not preserved properly. But the authors of that study noted that "histology carried out on the gonad from one side of each turtle confirmed that fixation had been good." According to van der Heiden et al. ( 1985), the method works on fixed tissue; no qualifications about fixation methods were given by these authors. On a personal note, from a laboratory that has contributed to three papers on methods of sexing turtles (Yntema and Mrosovsky, 1980; Whitmore et al., 1985; Mrosovsky and Benabib, 1990), we add that we had ardently hoped to be able to use the glycerine method: it would have saved much time and effort.

Manipulating Sex Ratios: Turtle Speed Ahead!

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