When we consider the large daily fluctuations in temperature at the surface of the sand and throughout the nesting season from May through September, it is gratifying to find that the temperature in the nest about 15 inches beneath the surface remains remarkably stable.
We have found that the nest temperature on the Kiawah beach varies only about 1 C over a 24-hour period. Even over the extended period from June to September, our data showed the average daily nest temperature fluctuated by only six degrees, with an average temperature of 28.7 C or 84 F.
Therefore, the turtle nest is an excellent incubation chamber for the eggs. Not only is the temperature maintained remarkably stable on the Kiawah beach, but also it is maintained very close to what is termed the pivotal temperature of 29 C.
Some readers may recall that the temperature of the egg during incubation determines the sex of a sea turtle. The ratio of males to females is 1:1 at the pivotal temperature of 29 C.; higher temperatures favor the formation of females, and lower temperatures favor the formation of males.
Therefore, our data reassures us that the hatchlings that depart from the Kiawah beach are nearly equally males and females. By contrast, the hatchlings are mostly males on the cooler beaches of North Carolina, and mostly females from those hot Florida beaches.
Unfortunately, we do not understand the breeding population dynamics of sea turtles well enough to be sure that a male to female ratio of 1:1 is optimal. However, Mother Nature arranges it so that most species with the conventional X-Y set of chromosomes are produced in a male to female ratio of 1:1.
For the time being we must trust that she knows what she is doing, and we can take comfort that our hatchlings are departing our beach in matched numbers of boys and girls.