Critical Approaches to Sex Determination in Sea Turtles
107
interclutch variation was detected between two clutches from the same nesting
beach during that study. 34 A study of loggerhead turtles in Australia estimated
pivotal temperatures of 27.7 and 28.7 ∞ C for two different nesting beaches, and a
significant interclutch variation was detected from clutches from one of those
beaches. 33 A later study suggested a pivotal temperature of approximately 29.0 ∞ C
for one of those same nesting beaches. Other estimates for loggerhead pivotal
temperatures include 29.2 ∞ C for a Brazilian nesting beach 37 and 29.7 ∞ C for a South
African nesting beach. 38 Thus, pivotal temperatures within this species have been
reported to vary by up to 1.0 ∞ C or more.
Although some of this variation may be associated with experimental error,
differences in protocols, or divergences in statistical analyses of the data, these
findings suggest that pivotal temperatures vary among and even within a population.
A reported pivotal temperature simply represents an estimate of the mean pivotal
temperature for a sample from a population. Considering the large number of sea
turtle populations in the world, there have been only a limited number of pivotal
temperature studies. Because of the endangered or threatened status of many of these
populations, pivotal temperature studies have usually used a limited number of
hatchlings and clutches. To better understand the variations in pivotal temperatures
within and between populations, more comprehensive studies using large numbers
of clutches would be optimal.
If one is interested in studying sex determination or estimating hatchling sex ratios
in a given sea turtle population, it is advantageous to determine the pivotal temperature
for that particular population, rather than estimating it based on data from other
populations. Because interclutch variation can occur, the accuracy of the estimated
pivotal temperature will be dependent on the number of clutches examined. Furthermore, because the estimated pivotal temperature will represent a mean, it should
include statistics to describe the variance associated with the mean (see Section 4.2.4).
4.2.3 TRT OF S EA T URTLES
In addition to pivotal temperature, the TRT (Figure 4.1) is a useful parameter for
characterizing sex determination in sea turtles. As defined earlier, the TRT represents
the range of temperatures in which the sex ratio shifts from 100% male to 100%
female. 30 For sea turtles (which have an MF pattern of sex determination), temperatures below the lower limit of the TRT will produce only males and temperatures
above the upper limit of the TRT will produce only females. A TRT is more difficult
to accurately estimate than a pivotal temperature because of the greater number of
incubation temperatures necessary to clearly pinpoint the lower and upper bounds
of the TRT curve. Most studies have not ruled out the possibility that the TRT could
be larger or smaller than reported.
A general characterization of the TRT can be gleaned from past studies by examining maximum incubation temperatures that produce all males and minimum incubation temperatures that produce all females in a particular study (Table 4.2). Many
past studies did not precisely determine the TRT because of the number of incubation
temperatures and the specific temperatures utilized in the study. However, some studies
included a reasonable number of appropriate incubation temperatures, and the results
1123 book.book Page 107 Monday, November 11, 2002 11:11 AM
107
interclutch variation was detected between two clutches from the same nesting
beach during that study. 34 A study of loggerhead turtles in Australia estimated
pivotal temperatures of 27.7 and 28.7 ∞ C for two different nesting beaches, and a
significant interclutch variation was detected from clutches from one of those
beaches. 33 A later study suggested a pivotal temperature of approximately 29.0 ∞ C
for one of those same nesting beaches. Other estimates for loggerhead pivotal
temperatures include 29.2 ∞ C for a Brazilian nesting beach 37 and 29.7 ∞ C for a South
African nesting beach. 38 Thus, pivotal temperatures within this species have been
reported to vary by up to 1.0 ∞ C or more.
Although some of this variation may be associated with experimental error,
differences in protocols, or divergences in statistical analyses of the data, these
findings suggest that pivotal temperatures vary among and even within a population.
A reported pivotal temperature simply represents an estimate of the mean pivotal
temperature for a sample from a population. Considering the large number of sea
turtle populations in the world, there have been only a limited number of pivotal
temperature studies. Because of the endangered or threatened status of many of these
populations, pivotal temperature studies have usually used a limited number of
hatchlings and clutches. To better understand the variations in pivotal temperatures
within and between populations, more comprehensive studies using large numbers
of clutches would be optimal.
If one is interested in studying sex determination or estimating hatchling sex ratios
in a given sea turtle population, it is advantageous to determine the pivotal temperature
for that particular population, rather than estimating it based on data from other
populations. Because interclutch variation can occur, the accuracy of the estimated
pivotal temperature will be dependent on the number of clutches examined. Furthermore, because the estimated pivotal temperature will represent a mean, it should
include statistics to describe the variance associated with the mean (see Section 4.2.4).
4.2.3 TRT OF S EA T URTLES
In addition to pivotal temperature, the TRT (Figure 4.1) is a useful parameter for
characterizing sex determination in sea turtles. As defined earlier, the TRT represents
the range of temperatures in which the sex ratio shifts from 100% male to 100%
female. 30 For sea turtles (which have an MF pattern of sex determination), temperatures below the lower limit of the TRT will produce only males and temperatures
above the upper limit of the TRT will produce only females. A TRT is more difficult
to accurately estimate than a pivotal temperature because of the greater number of
incubation temperatures necessary to clearly pinpoint the lower and upper bounds
of the TRT curve. Most studies have not ruled out the possibility that the TRT could
be larger or smaller than reported.
A general characterization of the TRT can be gleaned from past studies by examining maximum incubation temperatures that produce all males and minimum incubation temperatures that produce all females in a particular study (Table 4.2). Many
past studies did not precisely determine the TRT because of the number of incubation
temperatures and the specific temperatures utilized in the study. However, some studies
included a reasonable number of appropriate incubation temperatures, and the results
1123 book.book Page 107 Monday, November 11, 2002 11:11 AM