Why sexing a sea turtle hatchling is important!

If you walk the beach in South Florida during a summer night, you might get the impression that sea turtles are everywhere. You might very well see hatchlings scampering to the ocean and adults nesting! That’s because we have the largest nesting population of loggerhead turtles (Caretta caretta) in the world (Figure 1), about 48,000 nests just last year! Additionally, the numbers of nesting green turtles (Chelonia mydas; ~ 28,000 nests) and leatherbacks (Dermochelys coriacea; 650 nests) have also increased over the past few years.

Although these numbers are encouraging, currently the six extant species of sea turtle found in U.S. waters are considered threatened or endangered. Together, the stresses introduced by humans have increased pollution, disease, habitat degradation and habitat destruction as risks for marine turtle survival. Humans have also hunted turtles and harvested their eggs. All of these activities have contributed to significant population declines, especially compared to historical numbers which were several orders of magnitude greater.

Figure 2. Sex ratio-temperature response curve. This graph shows the theoretical expected sex ratio response to incubation temperatures derived from laboratory experiments. Note that temperatures of 29°C produce a 50:50 sex ratio. Lower temperatures produce mostly males while higher temperatures produce mostly females. (Graph by J. Wyneken & B. Tezak)

In sea turtles, this pattern of temperature-dependent sex determination (TSD) is informally characterized by the “hot chicks, cool dudes” rule. That translates formally into the now well substantiated observation that when nest temperatures are relatively warm, most of the hatchlings will be female and when relatively cool, male. At intermediate temperatures, both sexes will be represented. But recent research at Florida Atlantic University suggests that while temperature is an important factor, it isn’t the only factor (Figure 3).

Rainfall, and its effect on moisture conditions inside the nest, also modifies sex ratios by promoting the production of proportionally more male hatchlings in the nest than predicted by temperature, alone. This effect, which is slight but none-the-less significant, was only revealed by field studies and careful measurements carried out over the past 14 years (Figures 4). Measurements of turtle sex proportions were documented from nests exposed to known conditions of temperature and humidity by actually looking inside young turtles that came from those nests. The “look inside” was done using a laparoscope to see the immature ovaries and testes. This labor-intensive technique was developed by Dr. Jeanette Wyneken (Figure 5) and until recently, was the only reliable way to determine turtle sex in juvenile marine turtles without sacrificing the animal. (Obviously, killing the turtle to find out its sex defeated the purpose of promoting the recovery of marine turtle populations!)

However, this technique has shortcomings. Hatchlings are too small for laparoscopic surgery so the turtles must be raised in captivity for at least three months, making the entire procedure a very expensive and labor intensive method for determining sex and nest sex ratios. Additionally, an expert must perform the surgery and make the identification.

Those requirements make it impractical to obtain data on a large scale, for example, to determine how many males and females are being produced from the thousands of nests deposited each year on Florida’s beaches. What is needed, instead, is a simple and relatively inexpensive procedure for estimating nest sex ratios from hatchlings, without harming the turtles.