Critical Approaches to Sex Determination in Sea Turtles
109
Collectively, these data indicate that the upper and lower limits of the TRT can
vary between sea turtle species and populations. Although data from previous studies
are far from comprehensive, they suggest general ranges for the lower limits (approximately 26.0 to 28.75 ∞ C) and upper limits (approximately 29.75 to 32.0 ∞ C) of TRTs,
and they provide a general range of widths for TRTs (approximately 1.0 to 3.0 ∞ C ).
This sort of information has major conservational and ecological implications. For
example, this suggests that a temperature change of 3.0 ∞ C or less could potentially
shift sex ratios from all male to all female (or vice versa). It also provides information
on thermal ranges where small changes should not make a difference for sex ratios
(e.g., extremely warm temperatures that are well above pivotal). In addition, these
data indicate that physiologically, the sex determination cascade switches from male
to female (or vice versa) over a relatively narrow temperature window.
4.2.4 M ETHODS FOR E STIMATING P IVOTAL T EMPERATURE AND TRT
IN A S EA T URTLE P OPULATION
Before characterizing TSD in a sea turtle population, one should first address the
reasons for doing so. Knowledge of pivotal temperatures and TRTs has a variety of
uses ranging from highly applied to theoretical. First, it has direct applications to
the conservation of endangered populations. If you know the TRT and pivotal
temperature in a population, you can then predict hatchling sex ratios by simply
monitoring incubation temperatures in nests. Furthermore, this knowledge provides
the basis for potentially manipulating hatchling sex ratios in an effort to enhance
reproductive output in a population. 40,41 Pivotal temperatures and TRTs are also of
ecological, physiological, and evolutionary interest because they allow researchers
to begin addressing questions regarding variation in TSD and how it might affect
the reproductive ecology in a population.
Conservational considerations must also be taken into account before beginning
a study of pivotal temperatures and TRTs. In most cases the study will involve an
endangered or threatened sea turtle population. Currently, it is impossible to verify
the sex of a sea turtle hatchling using external characteristics. The definitive method
of determining sex is by histological analysis of the gonads, which requires that the
hatchling be killed. Although this sounds counterintuitive to enhancing the recovery
of an endangered population, it is quite possible that the knowledge gained will
greatly enhance the recovery of the population.
When the effects of temperature on sex in sea turtles are studied, the resolution
and accuracy of temperature recordings is of major importance. For example, data
from previous studies suggest that pivotal temperatures can vary by tenths of degrees
between populations and even between clutches (reviewed by Ref. 31). Therefore,
accurate temperature measurements are a prerequisite to obtaining meaningful
results. A variety of temperature recording devices are available commercially. For
automated recording of temperatures, one of the most commonly used devices is
the temperature data logger. These units are battery operated, have an internal and/or
external temperature probe, and can be programmed to take temperature recordings
at a wide variety of intervals. Many of these data loggers are small enough to be
placed among the eggs in a nest or incubator. 42 The data loggers are connected to a
1123 book.book Page 109 Monday, November 11, 2002 11:11 AM
109
Collectively, these data indicate that the upper and lower limits of the TRT can
vary between sea turtle species and populations. Although data from previous studies
are far from comprehensive, they suggest general ranges for the lower limits (approximately 26.0 to 28.75 ∞ C) and upper limits (approximately 29.75 to 32.0 ∞ C) of TRTs,
and they provide a general range of widths for TRTs (approximately 1.0 to 3.0 ∞ C ).
This sort of information has major conservational and ecological implications. For
example, this suggests that a temperature change of 3.0 ∞ C or less could potentially
shift sex ratios from all male to all female (or vice versa). It also provides information
on thermal ranges where small changes should not make a difference for sex ratios
(e.g., extremely warm temperatures that are well above pivotal). In addition, these
data indicate that physiologically, the sex determination cascade switches from male
to female (or vice versa) over a relatively narrow temperature window.
4.2.4 M ETHODS FOR E STIMATING P IVOTAL T EMPERATURE AND TRT
IN A S EA T URTLE P OPULATION
Before characterizing TSD in a sea turtle population, one should first address the
reasons for doing so. Knowledge of pivotal temperatures and TRTs has a variety of
uses ranging from highly applied to theoretical. First, it has direct applications to
the conservation of endangered populations. If you know the TRT and pivotal
temperature in a population, you can then predict hatchling sex ratios by simply
monitoring incubation temperatures in nests. Furthermore, this knowledge provides
the basis for potentially manipulating hatchling sex ratios in an effort to enhance
reproductive output in a population. 40,41 Pivotal temperatures and TRTs are also of
ecological, physiological, and evolutionary interest because they allow researchers
to begin addressing questions regarding variation in TSD and how it might affect
the reproductive ecology in a population.
Conservational considerations must also be taken into account before beginning
a study of pivotal temperatures and TRTs. In most cases the study will involve an
endangered or threatened sea turtle population. Currently, it is impossible to verify
the sex of a sea turtle hatchling using external characteristics. The definitive method
of determining sex is by histological analysis of the gonads, which requires that the
hatchling be killed. Although this sounds counterintuitive to enhancing the recovery
of an endangered population, it is quite possible that the knowledge gained will
greatly enhance the recovery of the population.
When the effects of temperature on sex in sea turtles are studied, the resolution
and accuracy of temperature recordings is of major importance. For example, data
from previous studies suggest that pivotal temperatures can vary by tenths of degrees
between populations and even between clutches (reviewed by Ref. 31). Therefore,
accurate temperature measurements are a prerequisite to obtaining meaningful
results. A variety of temperature recording devices are available commercially. For
automated recording of temperatures, one of the most commonly used devices is
the temperature data logger. These units are battery operated, have an internal and/or
external temperature probe, and can be programmed to take temperature recordings
at a wide variety of intervals. Many of these data loggers are small enough to be
placed among the eggs in a nest or incubator. 42 The data loggers are connected to a
1123 book.book Page 109 Monday, November 11, 2002 11:11 AM