112
The Biology of Sea Turtles, Vol. II
Are there optimal sex ratios for the recovery of a population, and should sex ratios
be manipulated in an effort to enhance the recovery of an endangered sea turtle
population? These are just a few of the questions regarding TSD that confront sea
turtle biologists. A prerequisite to answering these questions is the examination of
natural sex ratios in sea turtle populations and in conservation programs.
4.3.1 HATCHLING SEX RATIOS FROM NESTING BEACHES
It is beyond the intended scope of this chapter to provide a critical review of all
previous reports of naturally occurring sea turtle sex ratios, however, a general
overview will be presented. Examples of hatchling sex ratios that have been predicted
for sea turtle nesting beaches are shown in Table 4.3. The reader should be cautioned
that there is great variability in the methodology and scope of these studies. For
example, some predictions are for a single nesting season, whereas others are for
up to 14 different nesting seasons, and the amount of temperature data recorded
and/or the amount of sexing data on hatchlings vary. Furthermore, some of these
studies rely on pivotal temperature data from other sea turtle populations for sex
ratio projections. Regardless, several general points can be derived from these data.
First and foremost, the great majority of the predicted sex ratios do not conform to
a 1:1 sex ratio suggested by evolutionary theory. Although a few of the sex ratios
approach 50% female, there is an obvious predominance of beaches that produce
female-biased sex ratios, and some of these biases are extreme (greater than 90%
female). No reports exist of extreme male biases over an entire nesting season. On
the basis of these data, we are confronted with the possibility that TSD in sea turtles
may not conform to the predictions of evolutionary theory by Fisher; 51 rather, a
predominance of female biases may exist. However, other explanations are possible.
It is plausible that some criteria are not fulfilled regarding fisherian sex ratios, such
as sex ratios not being at equilibrium. 31,53 Alternatively, because the sex ratios
reported represent only a small sampling of all sea turtle populations and nesting
beaches, the results could be affected by sampling bias. Thus, one should be cautious
in extrapolating from the limited database that is currently available.
4.3.2 NEST LOCATION AND HATCHLING SEX RATIOS
Several studies have shown that nest location can have a profound influence on
hatchling sex ratios. Nesting beaches can have several thermal zones (e.g., beach
slope near the water, open beach flat, dune bordering beach, dune with vegetation,
etc.) that can influence sex ratio. 46,47,54 Multiple nesting beaches on islands can also
provide a variety of thermal environments. Depending on the specific beach chosen
by green turtles nesting on a small island (Heron Island) on the Great Barrier Reef,
hatchling sex ratios were shown to vary from 29.5 to 63.1% female. 33,55 A similar
situation was predicted for green turtles on Ascension Island in which one nesting
beach was 2.6∞C warmer than another. 56 Indeed, sand color has been shown to be
directly related to the thermal properties of nesting beaches. 57A Thus, the specific
location of nesting can significantly affect sex ratio.
1123 book.book Page 112 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
Are there optimal sex ratios for the recovery of a population, and should sex ratios
be manipulated in an effort to enhance the recovery of an endangered sea turtle
population? These are just a few of the questions regarding TSD that confront sea
turtle biologists. A prerequisite to answering these questions is the examination of
natural sex ratios in sea turtle populations and in conservation programs.
4.3.1 HATCHLING SEX RATIOS FROM NESTING BEACHES
It is beyond the intended scope of this chapter to provide a critical review of all
previous reports of naturally occurring sea turtle sex ratios, however, a general
overview will be presented. Examples of hatchling sex ratios that have been predicted
for sea turtle nesting beaches are shown in Table 4.3. The reader should be cautioned
that there is great variability in the methodology and scope of these studies. For
example, some predictions are for a single nesting season, whereas others are for
up to 14 different nesting seasons, and the amount of temperature data recorded
and/or the amount of sexing data on hatchlings vary. Furthermore, some of these
studies rely on pivotal temperature data from other sea turtle populations for sex
ratio projections. Regardless, several general points can be derived from these data.
First and foremost, the great majority of the predicted sex ratios do not conform to
a 1:1 sex ratio suggested by evolutionary theory. Although a few of the sex ratios
approach 50% female, there is an obvious predominance of beaches that produce
female-biased sex ratios, and some of these biases are extreme (greater than 90%
female). No reports exist of extreme male biases over an entire nesting season. On
the basis of these data, we are confronted with the possibility that TSD in sea turtles
may not conform to the predictions of evolutionary theory by Fisher; 51 rather, a
predominance of female biases may exist. However, other explanations are possible.
It is plausible that some criteria are not fulfilled regarding fisherian sex ratios, such
as sex ratios not being at equilibrium. 31,53 Alternatively, because the sex ratios
reported represent only a small sampling of all sea turtle populations and nesting
beaches, the results could be affected by sampling bias. Thus, one should be cautious
in extrapolating from the limited database that is currently available.
4.3.2 NEST LOCATION AND HATCHLING SEX RATIOS
Several studies have shown that nest location can have a profound influence on
hatchling sex ratios. Nesting beaches can have several thermal zones (e.g., beach
slope near the water, open beach flat, dune bordering beach, dune with vegetation,
etc.) that can influence sex ratio. 46,47,54 Multiple nesting beaches on islands can also
provide a variety of thermal environments. Depending on the specific beach chosen
by green turtles nesting on a small island (Heron Island) on the Great Barrier Reef,
hatchling sex ratios were shown to vary from 29.5 to 63.1% female. 33,55 A similar
situation was predicted for green turtles on Ascension Island in which one nesting
beach was 2.6∞C warmer than another. 56 Indeed, sand color has been shown to be
directly related to the thermal properties of nesting beaches. 57A Thus, the specific
location of nesting can significantly affect sex ratio.
1123 book.book Page 112 Monday, November 11, 2002 11:11 AM