Critical Approaches to Sex Determination in Sea Turtles
123
reviewed previously, a variety of sex ratios have been reported in sea turtle populations,
with many having female biases. It is not clear, however, it those data represent a random
sampling of all sea turtle nesting beaches. It is plausible that highly biased sex ratios are
more often reported because there is an inherent interest to find and publish extreme sex
ratios. As such, this issue is still speculative.
In summary, the manipulation of hatchling sex ratios can most certainly have
an effect on the reproductive ecology and recovery of sea turtle populations. Unfortunately, we do not understand all the ecological and evolutionary ramifications of
artificially skewing a sex ratio. Although it seems logical that female biases would
be preferred over male biases in a conservation program for a recovering population,
one should be cautious when manipulating sex ratios until more data are available
on this subject.
4.4 PHYSIOLOGY OF TSD
The majority of studies of sex determination in sea turtles have focused on pivotal
temperatures, TRTs, and sex ratios. Although some studies have addressed the
physiology underlying sex determination in sea turtles, the majority of information
regarding the physiology of TSD in turtles has been documented in freshwater turtles,
many of which have a similar pattern of sex determination (i.e., MF pattern) and
similar pivotal temperatures and TRTs. 110,111 As such, this section of the chapter will
focus on sea turtles, but also include information from freshwater turtles when
applicable.
4.4.1 THERMOSENSITIVE PERIOD AND GONADAL DIFFERENTIATION
Studies of loggerhead 44 and olive ridley sea turtles 103 indicate that temperature affects
sex determination during the approximate middle third of incubation. This is consistent with data reported for freshwater turtles. 45 Temperature appears to have both
a cumulative and a quantitative effect on sex determination. 112 That is, an egg must
be exposed to a male-producing or a female-producing temperature for an extended
time period before sex is determined, so a short-duration spike of male-producing
or female-producing temperature will not irreversibly determine sex. For turtles with
the MF pattern of sex determination, the warmer the temperature, the more potent
it is for producing females, and the cooler the temperature, the more potent it is for
producing males. 112
Gonadal differentiation has been described in the olive ridley sea turtle, and the
first signs of sexual differentiation occur toward the end of the thermosensitive period. 14
Sexual differentiation at female-producing temperatures includes the thickening of
cortical epithelial tissue (i.e., a proliferation of the gonad’s cortex) and degeneration
of medullary cords (i.e., regression of the gonad’s medulla). In contrast, at maleproducing temperatures, the cortex does not proliferate, and the medullary cords do
not regress and eventually will develop into seminiferous tubules. By the time of
hatching, an ovary can be histologically distinguished from a testis (Figure 4.2).
1123 book.book Page 123 Monday, November 11, 2002 11:11 AM
123
reviewed previously, a variety of sex ratios have been reported in sea turtle populations,
with many having female biases. It is not clear, however, it those data represent a random
sampling of all sea turtle nesting beaches. It is plausible that highly biased sex ratios are
more often reported because there is an inherent interest to find and publish extreme sex
ratios. As such, this issue is still speculative.
In summary, the manipulation of hatchling sex ratios can most certainly have
an effect on the reproductive ecology and recovery of sea turtle populations. Unfortunately, we do not understand all the ecological and evolutionary ramifications of
artificially skewing a sex ratio. Although it seems logical that female biases would
be preferred over male biases in a conservation program for a recovering population,
one should be cautious when manipulating sex ratios until more data are available
on this subject.
4.4 PHYSIOLOGY OF TSD
The majority of studies of sex determination in sea turtles have focused on pivotal
temperatures, TRTs, and sex ratios. Although some studies have addressed the
physiology underlying sex determination in sea turtles, the majority of information
regarding the physiology of TSD in turtles has been documented in freshwater turtles,
many of which have a similar pattern of sex determination (i.e., MF pattern) and
similar pivotal temperatures and TRTs. 110,111 As such, this section of the chapter will
focus on sea turtles, but also include information from freshwater turtles when
applicable.
4.4.1 THERMOSENSITIVE PERIOD AND GONADAL DIFFERENTIATION
Studies of loggerhead 44 and olive ridley sea turtles 103 indicate that temperature affects
sex determination during the approximate middle third of incubation. This is consistent with data reported for freshwater turtles. 45 Temperature appears to have both
a cumulative and a quantitative effect on sex determination. 112 That is, an egg must
be exposed to a male-producing or a female-producing temperature for an extended
time period before sex is determined, so a short-duration spike of male-producing
or female-producing temperature will not irreversibly determine sex. For turtles with
the MF pattern of sex determination, the warmer the temperature, the more potent
it is for producing females, and the cooler the temperature, the more potent it is for
producing males. 112
Gonadal differentiation has been described in the olive ridley sea turtle, and the
first signs of sexual differentiation occur toward the end of the thermosensitive period. 14
Sexual differentiation at female-producing temperatures includes the thickening of
cortical epithelial tissue (i.e., a proliferation of the gonad’s cortex) and degeneration
of medullary cords (i.e., regression of the gonad’s medulla). In contrast, at maleproducing temperatures, the cortex does not proliferate, and the medullary cords do
not regress and eventually will develop into seminiferous tubules. By the time of
hatching, an ovary can be histologically distinguished from a testis (Figure 4.2).
1123 book.book Page 123 Monday, November 11, 2002 11:11 AM