114
The Biology of Sea Turtles, Vol. II
4.3.3 SEASONAL VARIATION IN HATCHLING SEX RATIOS
A number of previous studies also indicate that hatching sex ratios can vary over the
nesting season. An initial study by Mrosovsky et al. 73 indicated that sex ratios of
hatchling loggerheads in South Carolina and Georgia could vary from less than 10%
female during the cooler portions of the nesting season to 80% female during the
warmer months. Standora and Spotila 46 reviewed incubation duration data from
Hendrickson 57B and predicted seasonal changes in sex ratios of hatchling green turtles
at Sarawak, with monsoon season producing predominantly males, whereas nests laid
from April through November produced predominantly females. Other examples of
seasonal changes in hatchling sex ratios from nesting beaches include loggerheads
from Florida, 54 Brazil, 37 and Cyprus, 58 hawksbills from Brazil, leatherbacks from
Suriname, 12,53 leatherbacks from French Guiana, 39 and green turtles from Ascension
Island 59 and Suriname. 12 Of particular interest, several of these past studies have
predicted that the temperature decreases associated with periods of rain can have a
profound effect on sex ratios, resulting in the production of male biases. 12,53
Although seasonal variation in sex ratios may occur on many nesting beaches,
it may not occur on all beaches, depending on weather conditions and length of
nesting season. For example, beach temperatures were relatively constant at Tortuguero, Costa Rica, during the 1980 nesting season for green turtles, suggesting no
seasonal variation in sex ratios. 47 Furthermore, in some situations, incubation temperatures may be high enough that minor temperature fluctuations have little or no
effect, and 100% females are produced, as suggested for the 1993–1994 nesting
season of leatherbacks at Playa Grande, Costa Rica. 17
4.3.4 YEARLY VARIATION IN HATCHLING SEX RATIOS
As one might predict, hatching sex ratios from a given nesting beach can also vary
from year to year depending on factors such as weather and timing of nesting.
Although yearly variations have been reported, these variations are often relatively
small for a given nesting beach. For example, sex ratios of hatchling loggerheads
from a Florida nesting beach were predicted to be 92.6–96.7% female during 1985,
94.7–99.9% female during 1986, and 87–89% female during 1987. 54 Godfrey et al. 53
predicted a 10% variation between two nesting seasons (1982 vs. 1993) for hatchling
sex ratios of both leatherback and green turtles. A study of green turtle nesting
beaches on Ascension Island predicted only small interannual differences in beach
temperatures, on the basis of 14 years of data. 56
Variations in annual rainfall have been suggested to affect yearly variations in
sex ratios projected during 14 different nesting seasons of leatherback and green sea
turtles in Suriname, with annual differences as great as 20–90% female for green
turtles. 53 It also is possible that physical changes in a nesting beach can affect yearto-year variation. Because of displacement of beach sand by ocean currents, green
and leatherback nesting beaches in Suriname and French Guiana have been moving
to the west at approximately 2 km per year. 60 This change in beach location could
obviously contribute to yearly variations reported for hatchling sex ratios. 53 Human
alteration of beach characteristics could also affect sex ratios produced from a nesting
1123 book.book Page 114 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
4.3.3 SEASONAL VARIATION IN HATCHLING SEX RATIOS
A number of previous studies also indicate that hatching sex ratios can vary over the
nesting season. An initial study by Mrosovsky et al. 73 indicated that sex ratios of
hatchling loggerheads in South Carolina and Georgia could vary from less than 10%
female during the cooler portions of the nesting season to 80% female during the
warmer months. Standora and Spotila 46 reviewed incubation duration data from
Hendrickson 57B and predicted seasonal changes in sex ratios of hatchling green turtles
at Sarawak, with monsoon season producing predominantly males, whereas nests laid
from April through November produced predominantly females. Other examples of
seasonal changes in hatchling sex ratios from nesting beaches include loggerheads
from Florida, 54 Brazil, 37 and Cyprus, 58 hawksbills from Brazil, leatherbacks from
Suriname, 12,53 leatherbacks from French Guiana, 39 and green turtles from Ascension
Island 59 and Suriname. 12 Of particular interest, several of these past studies have
predicted that the temperature decreases associated with periods of rain can have a
profound effect on sex ratios, resulting in the production of male biases. 12,53
Although seasonal variation in sex ratios may occur on many nesting beaches,
it may not occur on all beaches, depending on weather conditions and length of
nesting season. For example, beach temperatures were relatively constant at Tortuguero, Costa Rica, during the 1980 nesting season for green turtles, suggesting no
seasonal variation in sex ratios. 47 Furthermore, in some situations, incubation temperatures may be high enough that minor temperature fluctuations have little or no
effect, and 100% females are produced, as suggested for the 1993–1994 nesting
season of leatherbacks at Playa Grande, Costa Rica. 17
4.3.4 YEARLY VARIATION IN HATCHLING SEX RATIOS
As one might predict, hatching sex ratios from a given nesting beach can also vary
from year to year depending on factors such as weather and timing of nesting.
Although yearly variations have been reported, these variations are often relatively
small for a given nesting beach. For example, sex ratios of hatchling loggerheads
from a Florida nesting beach were predicted to be 92.6–96.7% female during 1985,
94.7–99.9% female during 1986, and 87–89% female during 1987. 54 Godfrey et al. 53
predicted a 10% variation between two nesting seasons (1982 vs. 1993) for hatchling
sex ratios of both leatherback and green turtles. A study of green turtle nesting
beaches on Ascension Island predicted only small interannual differences in beach
temperatures, on the basis of 14 years of data. 56
Variations in annual rainfall have been suggested to affect yearly variations in
sex ratios projected during 14 different nesting seasons of leatherback and green sea
turtles in Suriname, with annual differences as great as 20–90% female for green
turtles. 53 It also is possible that physical changes in a nesting beach can affect yearto-year variation. Because of displacement of beach sand by ocean currents, green
and leatherback nesting beaches in Suriname and French Guiana have been moving
to the west at approximately 2 km per year. 60 This change in beach location could
obviously contribute to yearly variations reported for hatchling sex ratios. 53 Human
alteration of beach characteristics could also affect sex ratios produced from a nesting
1123 book.book Page 114 Monday, November 11, 2002 11:11 AM