Critical Approaches to Sex Determination in Sea Turtles
119
sex of live turtles. A variety of methods have been evaluated for sexing immature
sea turtles (reviewed by Ref. 84). Laparoscopy represents a definitive method for
verifying sex, but it is difficult in the field and requires surgical training. 88,93 The
most widely used method uses testosterone levels in the blood (determined by
radioimmunoassay) to predict the sex of individual turtles (reviewed by Ref. 84).
The advantage of this method is that the assay is performed in the laboratory, so the
field component is limited to obtaining and storing blood samples. Radioimmunoassays can vary among laboratories, so the accuracy of this method is dependent on
the validation of a particular radioimmunoassay using samples from turtles of known
sex (e.g., sex identified by laparoscopy). In that way, male and female ranges of
testosterone levels can be determined and then used to predict the sex of other turtles.
It is optimal to validate the assay with samples from turtles from the population
being studied. Thus, for accurate predictions, it is imperative to validate the radioimmunoassay that will be used in a particular study.
4.3.7 PREDICTING HATCHLING SEX RATIOS
4.3.7.1 Direct Methods for Predicting Hatchling Sex Ratios
Several difficulties are associated with estimating hatchling sex ratios. Again, a major
problem is identifying the sex of individual turtles because they do not possess
external characteristics that can be used to distinguish sex. Basic techniques used
to sex hatchlings have been reviewed in detail. 94 The traditional method has required
the dissection of hatchlings and evaluation of the gonads. The most resolute way of
doing this is through histological study, in which the gonads show sex-specific
characteristics. 94,95 Female hatchlings have a well-developed cortex (i.e., a thickened
cortical layer) and poorly organized medullary portion of the gonad, whereas males
have little or no cortex and show organization in the medulla (Figure 4.2). Some
researchers have suggested that gross morphology of the gonads can be used to
determine the sex of hatchlings. 13,15,96 Others have suggested that histology is preferred over gross morphology to prevent errors. 41,97
Although these methods can provide valuable data, they require either that
hatchlings be killed or that hatchlings found dead in nests be used. The availability
of an accurate and nonlethal sexing technique for hatchling sea turtles would greatly
enhance our ability to investigate sex ratios in sea turtle populations. One method
has been proposed for sexing hatchling loggerheads by examining both testosterone
and estrogen levels in the blood or chorioallantoic fluid via radioimmunoassay. 98
That study found significantly higher estrogen–testosterone ratios in female
hatchlings, with only a minor overlapping of male and female ranges. However, that
method did not prove to be effective for sexing pre- and post-hatchling olive ridley
turtles. 94 Thus, the accurate use of that technique requires further validation.
Ideally, a nonlethal hatchling sexing technique should be accurate and practical
for sexing large numbers of hatchlings. Because sea turtles have TSD, there may
not be underlying genetic differences in the DNA that can be used to identify
phenotypic sex. Therefore, physiological differences may be the best avenue. Furthermore, if physiological differences can be detected in the blood (e.g., sex-specific
1123 book.book Page 119 Monday, November 11, 2002 11:11 AM
119
sex of live turtles. A variety of methods have been evaluated for sexing immature
sea turtles (reviewed by Ref. 84). Laparoscopy represents a definitive method for
verifying sex, but it is difficult in the field and requires surgical training. 88,93 The
most widely used method uses testosterone levels in the blood (determined by
radioimmunoassay) to predict the sex of individual turtles (reviewed by Ref. 84).
The advantage of this method is that the assay is performed in the laboratory, so the
field component is limited to obtaining and storing blood samples. Radioimmunoassays can vary among laboratories, so the accuracy of this method is dependent on
the validation of a particular radioimmunoassay using samples from turtles of known
sex (e.g., sex identified by laparoscopy). In that way, male and female ranges of
testosterone levels can be determined and then used to predict the sex of other turtles.
It is optimal to validate the assay with samples from turtles from the population
being studied. Thus, for accurate predictions, it is imperative to validate the radioimmunoassay that will be used in a particular study.
4.3.7 PREDICTING HATCHLING SEX RATIOS
4.3.7.1 Direct Methods for Predicting Hatchling Sex Ratios
Several difficulties are associated with estimating hatchling sex ratios. Again, a major
problem is identifying the sex of individual turtles because they do not possess
external characteristics that can be used to distinguish sex. Basic techniques used
to sex hatchlings have been reviewed in detail. 94 The traditional method has required
the dissection of hatchlings and evaluation of the gonads. The most resolute way of
doing this is through histological study, in which the gonads show sex-specific
characteristics. 94,95 Female hatchlings have a well-developed cortex (i.e., a thickened
cortical layer) and poorly organized medullary portion of the gonad, whereas males
have little or no cortex and show organization in the medulla (Figure 4.2). Some
researchers have suggested that gross morphology of the gonads can be used to
determine the sex of hatchlings. 13,15,96 Others have suggested that histology is preferred over gross morphology to prevent errors. 41,97
Although these methods can provide valuable data, they require either that
hatchlings be killed or that hatchlings found dead in nests be used. The availability
of an accurate and nonlethal sexing technique for hatchling sea turtles would greatly
enhance our ability to investigate sex ratios in sea turtle populations. One method
has been proposed for sexing hatchling loggerheads by examining both testosterone
and estrogen levels in the blood or chorioallantoic fluid via radioimmunoassay. 98
That study found significantly higher estrogen–testosterone ratios in female
hatchlings, with only a minor overlapping of male and female ranges. However, that
method did not prove to be effective for sexing pre- and post-hatchling olive ridley
turtles. 94 Thus, the accurate use of that technique requires further validation.
Ideally, a nonlethal hatchling sexing technique should be accurate and practical
for sexing large numbers of hatchlings. Because sea turtles have TSD, there may
not be underlying genetic differences in the DNA that can be used to identify
phenotypic sex. Therefore, physiological differences may be the best avenue. Furthermore, if physiological differences can be detected in the blood (e.g., sex-specific
1123 book.book Page 119 Monday, November 11, 2002 11:11 AM