Critical Approaches to Sex Determination in Sea Turtles
121
resulted in a variety of studies adopting indirect methods for predicting hatchling
sex ratios. Beach temperatures, nest incubation temperatures, and incubation durations have been used in a variety of studies to predict hatchling sex
ratios. 12,37,48,49,53,54,56,58,59,72,100–102 Previous studies indicate that temperature during the
middle third of incubation determines sex in sea turtles. 44,103 Therefore, if the pivotal
temperature and TRT are known for the population being studied, nest incubation
temperature or even beach temperature (e.g., at nest depth) can be used to predict
hatchling sex ratios. For example, an average incubation temperature during the
middle third of incubation that is above the upper limit of the TRT would be predicted
to produce all females, whereas an average incubation temperature below the lower
limit of the TRT would be predicted to yield 100% males. Average incubation
temperatures between the pivotal temperature and the upper limit of the TRT would
be predicted to produce a female-biased sex ratio, and average temperatures between
the pivotal temperature and the lower limit of the TRT would be predicted to produce
a male-biased sex ratio.
The availability of small data loggers for recording temperatures provides a practical means of directly recording incubation temperatures within sea turtle nests. 48,72
If direct measurement of incubation temperature is not feasible, then incubation duration provides an alternative method for predicting sex ratios for a given nest. 37,101,102
The total duration of incubation will reflect temperatures during the middle third of
incubation, and assuming that temperatures are relatively constant throughout the entire
incubation period, incubation duration should be a reliable predictor of sex ratio. In
general, longer incubation durations are indicative of male-biased sex ratios, and
shorter incubation durations are indicative of female-biased sex ratios.
It is significant that the accuracy of these indirect methods is dependent on
validation data that are based on direct sexing techniques. For example, when
incubation temperatures are used, the accuracy of the sex ratio predictions is dependent on the validity of the pivotal temperature and TRT data for that population.
4.3.7.3 Predicting an Overall Sex Ratio for a Nesting Beach
One of the primary reasons for developing the sexing techniques discussed above
is to provide a means of examining the overall hatchling sex ratios produced from
nesting beaches. Once a method has been chosen for estimating sex ratios from
nests, you then need to develop an effective strategy for evaluating the sex ratio
produced from the nesting beach of interest. This subject has been reviewed previously in detail. 21 Major points that should be considered include the spatial and
temporal diversity of nests during a nesting season. In most situations, it is not
possible to monitor temperature in all nests, so subsets of nests that accurately
represent the diversity and abundance of nest locations on the nesting beach can be
used (e.g., beach flat, dune, and vegetation zone). In addition, nests should be
examined throughout the nesting season to evaluate any temporal variation in sex
ratio during a nesting season. As indicated previously, seasonal variation in hatchling
sex ratios has been detected in a number of studies. Weather parameters such as air
temperature, rainfall, and cloud cover should be recorded throughout the nesting
season for possible correlation with sex ratios. Studies have also shown annual
1123 book.book Page 121 Monday, November 11, 2002 11:11 AM
121
resulted in a variety of studies adopting indirect methods for predicting hatchling
sex ratios. Beach temperatures, nest incubation temperatures, and incubation durations have been used in a variety of studies to predict hatchling sex
ratios. 12,37,48,49,53,54,56,58,59,72,100–102 Previous studies indicate that temperature during the
middle third of incubation determines sex in sea turtles. 44,103 Therefore, if the pivotal
temperature and TRT are known for the population being studied, nest incubation
temperature or even beach temperature (e.g., at nest depth) can be used to predict
hatchling sex ratios. For example, an average incubation temperature during the
middle third of incubation that is above the upper limit of the TRT would be predicted
to produce all females, whereas an average incubation temperature below the lower
limit of the TRT would be predicted to yield 100% males. Average incubation
temperatures between the pivotal temperature and the upper limit of the TRT would
be predicted to produce a female-biased sex ratio, and average temperatures between
the pivotal temperature and the lower limit of the TRT would be predicted to produce
a male-biased sex ratio.
The availability of small data loggers for recording temperatures provides a practical means of directly recording incubation temperatures within sea turtle nests. 48,72
If direct measurement of incubation temperature is not feasible, then incubation duration provides an alternative method for predicting sex ratios for a given nest. 37,101,102
The total duration of incubation will reflect temperatures during the middle third of
incubation, and assuming that temperatures are relatively constant throughout the entire
incubation period, incubation duration should be a reliable predictor of sex ratio. In
general, longer incubation durations are indicative of male-biased sex ratios, and
shorter incubation durations are indicative of female-biased sex ratios.
It is significant that the accuracy of these indirect methods is dependent on
validation data that are based on direct sexing techniques. For example, when
incubation temperatures are used, the accuracy of the sex ratio predictions is dependent on the validity of the pivotal temperature and TRT data for that population.
4.3.7.3 Predicting an Overall Sex Ratio for a Nesting Beach
One of the primary reasons for developing the sexing techniques discussed above
is to provide a means of examining the overall hatchling sex ratios produced from
nesting beaches. Once a method has been chosen for estimating sex ratios from
nests, you then need to develop an effective strategy for evaluating the sex ratio
produced from the nesting beach of interest. This subject has been reviewed previously in detail. 21 Major points that should be considered include the spatial and
temporal diversity of nests during a nesting season. In most situations, it is not
possible to monitor temperature in all nests, so subsets of nests that accurately
represent the diversity and abundance of nest locations on the nesting beach can be
used (e.g., beach flat, dune, and vegetation zone). In addition, nests should be
examined throughout the nesting season to evaluate any temporal variation in sex
ratio during a nesting season. As indicated previously, seasonal variation in hatchling
sex ratios has been detected in a number of studies. Weather parameters such as air
temperature, rainfall, and cloud cover should be recorded throughout the nesting
season for possible correlation with sex ratios. Studies have also shown annual
1123 book.book Page 121 Monday, November 11, 2002 11:11 AM