116
The Biology of Sea Turtles, Vol. II
Studies of sex ratios in the immature portion of a population (i.e., juvenile to
subadult turtles) may provide more accurate estimates of sex ratios, because these
studies may not be hampered by sampling bias problems associated with adult
breeding migrations (although it is possible that sex-specific behavior may be
expressed by juvenile and subadult turtles). A number of studies have examined sex
ratios in the immature portion of sea turtle populations, and the results provide
insight on the potential variability of sex ratios within a sea turtle population. For
loggerheads along the Atlantic coast of the U.S., one study used blood testosterone
levels to sex 218 immature turtles in the Atlantic waters off Florida and found a 2:1
(female–male) sex ratio. 69 A similar study used blood testosterone levels to sex
loggerheads from four different locations along the Atlantic coast of the U.S., and
also found an approximate 2:1 (female–male) sex ratio. 70 Another study used
necropsy to sex 139 immature loggerhead turtles that had stranded on the Georgia
coast, and also found an approximate 2:1 (female–male) sex ratio. 71 These studies
suggest that an approximate 2:1 (female–male) sex ratio may exist in the immature
portion of the loggerhead population along the Atlantic coast of the U.S. An approximate 2:1 (female–male) sex ratio has also been reported for immature loggerheads
in the Gulf of Mexico. 68B Although distinctly female-biased, this 2:1 sex ratio is far
less than the approximate 9:1 (female–male) hatchling sex ratios suggested for
Florida nesting beaches. 54,72
A number of suggested hypotheses could account for this discrepancy. First, the
two Florida beaches examined for hatchling sex ratios may not be representative of
all nesting beaches for the loggerheads in the U.S. It is possible that other loggerhead
nesting beaches produce greater numbers of males. For example, a study of hatchling
loggerhead sex ratios on South Carolina and Georgia beaches (which are part of the
northern nesting subpopulation) suggested a sex ratio of approximately 1:1
(female–male). 73 However, the two beaches examined in Florida (Cape Canaveral
and Hutchinson Island) represent major nesting beaches for the south Florida nesting
subpopulation, which produces approximately 90% of all hatchling loggerheads from
U.S. beaches. 74 If other beaches in the south Florida nesting subpopulation also
produce hatchling sex ratios of approximately 9:1 (female–male), then even strong
male biases on northern or Gulf of Mexico nesting beaches could not contribute
enough hatchlings to account for an overall 2:1 (female–male) sex ratio that was
suggested for the immature portion of the population. Therefore, other factors must
account for the difference. One possibility is that some nesting beaches of the south
Florida subpopulation may produce more males. It is also possible that some previous
years had relatively cool periods that produced more males. Another hypothesis
would be that males may be produced during the early portion of the nesting season 73
and have greater survival rates because of factors such as food availability or longer
growth period prior to first winter. An additional possibility is that turtles captured
off the U.S. Atlantic coast come from other nesting beaches (e.g., Mexico) that could
produce more male-biased ratios. No attempts were made to assign beaches of origin
to the samples of turtles in those studies. Adult sex ratios in this population have
not been adequately addressed because of the logistical difficulty of sampling adult
loggerheads in coastal waters, and also sampling biases associated with sex-specific
migration patterns of adult sea turtles. 75 Regardless, the discrepancy between the
1123 book.book Page 116 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
Studies of sex ratios in the immature portion of a population (i.e., juvenile to
subadult turtles) may provide more accurate estimates of sex ratios, because these
studies may not be hampered by sampling bias problems associated with adult
breeding migrations (although it is possible that sex-specific behavior may be
expressed by juvenile and subadult turtles). A number of studies have examined sex
ratios in the immature portion of sea turtle populations, and the results provide
insight on the potential variability of sex ratios within a sea turtle population. For
loggerheads along the Atlantic coast of the U.S., one study used blood testosterone
levels to sex 218 immature turtles in the Atlantic waters off Florida and found a 2:1
(female–male) sex ratio. 69 A similar study used blood testosterone levels to sex
loggerheads from four different locations along the Atlantic coast of the U.S., and
also found an approximate 2:1 (female–male) sex ratio. 70 Another study used
necropsy to sex 139 immature loggerhead turtles that had stranded on the Georgia
coast, and also found an approximate 2:1 (female–male) sex ratio. 71 These studies
suggest that an approximate 2:1 (female–male) sex ratio may exist in the immature
portion of the loggerhead population along the Atlantic coast of the U.S. An approximate 2:1 (female–male) sex ratio has also been reported for immature loggerheads
in the Gulf of Mexico. 68B Although distinctly female-biased, this 2:1 sex ratio is far
less than the approximate 9:1 (female–male) hatchling sex ratios suggested for
Florida nesting beaches. 54,72
A number of suggested hypotheses could account for this discrepancy. First, the
two Florida beaches examined for hatchling sex ratios may not be representative of
all nesting beaches for the loggerheads in the U.S. It is possible that other loggerhead
nesting beaches produce greater numbers of males. For example, a study of hatchling
loggerhead sex ratios on South Carolina and Georgia beaches (which are part of the
northern nesting subpopulation) suggested a sex ratio of approximately 1:1
(female–male). 73 However, the two beaches examined in Florida (Cape Canaveral
and Hutchinson Island) represent major nesting beaches for the south Florida nesting
subpopulation, which produces approximately 90% of all hatchling loggerheads from
U.S. beaches. 74 If other beaches in the south Florida nesting subpopulation also
produce hatchling sex ratios of approximately 9:1 (female–male), then even strong
male biases on northern or Gulf of Mexico nesting beaches could not contribute
enough hatchlings to account for an overall 2:1 (female–male) sex ratio that was
suggested for the immature portion of the population. Therefore, other factors must
account for the difference. One possibility is that some nesting beaches of the south
Florida subpopulation may produce more males. It is also possible that some previous
years had relatively cool periods that produced more males. Another hypothesis
would be that males may be produced during the early portion of the nesting season 73
and have greater survival rates because of factors such as food availability or longer
growth period prior to first winter. An additional possibility is that turtles captured
off the U.S. Atlantic coast come from other nesting beaches (e.g., Mexico) that could
produce more male-biased ratios. No attempts were made to assign beaches of origin
to the samples of turtles in those studies. Adult sex ratios in this population have
not been adequately addressed because of the logistical difficulty of sampling adult
loggerheads in coastal waters, and also sampling biases associated with sex-specific
migration patterns of adult sea turtles. 75 Regardless, the discrepancy between the
1123 book.book Page 116 Monday, November 11, 2002 11:11 AM