Sex ratio

The reproductive behavior of migratory organisms is difficult to characterize as
the mating behavior can be difficult to observe. For some species, one sex can be readily
observable, but the other may remain hidden, confounding attempts to assess population
demographics. For such species, it can be difficult to determine the sex ratio of the
population. Without accurate accounts of the numbers of males and females, conservation
methods may be insufficient and their performance unclear. Alternative methods of
measuring sex ratios therefore must be used to estimate the number of individuals and
assess breeding behavior. Here I identified breeding sex ratios (BSR) measured using
paternity analysis of offspring through exclusion analysis to quantify the numbers of
males contributing. Here I discuss the mating behavior of three species of marine turtle
that nest in southern Florida: the loggerhead (Caretta caretta), the leatherback (Dermochelys coriacea), and the green sea turtle (Chelonia mydas) at three beaches
(Boca Raton, Juno Beach, and Sanibel Island) from 2013-2017; over 400 nesting females
were identified and genotyped and almost 7,000 hatchlings were collected and
genotyped. Females from all three species successfully mated with more than one male in
all years analyzed demonstrating multiple paternity of clutches. For loggerheads, many
male genotypes were identified, suggesting that females likely mate en route to their
nesting grounds, inducing a male-biased BSR. Examination of females that were sampled
more than once per season (repeats), evidence of sperm storage was found for all nests
and some turtles might mate in between nesting events. Leatherback females displayed a
higher rate of multiple paternity than was previously published for other Caribbean
nesting sites; I hypothesize that this result may be due to a mainland nesting beach effect.
The leatherback BSR over all years was approximately 1:1, and one male was identified
fathering than two different females’ nests (polygyny). For green turtles, multiple
paternity was found and there was evidence of polygyny. Across all three species,
evidence for indirect benefits of mating multiply (hatching success or larger hatchlings)
was weak or not supported. Together, the four studies contribute to the overall body of
reproductive behavior studies.

Frontier areas and rural areas historically have
been predominantly male (high sex ratio); urban areas
were predominantly female (low sex ratio). In 1970 the
Middle West scored average on the sex ratio scale for
all age cohorts; the West was high, the South low. Highest
sex ratio was in port and military locales; lowest was
in urban areas of the Northeast and South. Two regression
models reveal that percentage urban, net migration,
and percentage Black were most important explanators of
sex ratio. The greater life expectancy of females and
their tendency to concentrate in the cities, the larger
net migration rate of males, and the lower sex ratio
of Blacks, were causal. Analysis by age cohort revealed
regional differences exist only after age 34. After
adjusting for the above independent variables, the West
still scored highest and the South lowest.

South Florida's loggerhead (Caretta caretta), green (Chelonia mydas) and leatherback (Dermochelys coriacea) sea turtles hatchling have environmentally determined sex. The in situ nest mean hatchling sex ratios (SR) were highly female-biased : loggerhead F=0.89) and green turtle F=0.81; leatherback's SR was nearly balanced (0.55F). Nest temperatures and SRs differed between leatherbacks and loggerhead and green turtles. The latter two did not differ. The loggerhead response parameters were estimated within biological limitations by both 50-65% of incubation and mean middle 1/3 temperature. The maximum middle 1/3 temperature was the best-fit predictor for green turtles. No best-fit sex ratio-temperature response could be identified for leatherbacks. Clutches incubating under natural conditions can vary greatly in SR ; TRT differences may account for differences among species' sex ratios.