Why sexing a sea turtle hatchling is important!
Boris Tezak and Itzel Sifuentes Romero
Department of Biological Sciences
Florida Atlantic University
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)
Figure 1. Loggerhead Turtle laying eggs in a Florida nesting beach (B.E. Witherington, photo)
The fragile nature of all present sea turtle populations makes it important to understand and assess the different factors that affect sea turtle populations now, and those that will remain important in the years to come. By knowing those factors we can hope to successfully promote the recovery of marine turtle populations. One particularly important factor that could influence the survival of sea turtles is climate change. Globally, we are already experiencing some of the effects of climate change with extremely hot summers, incredibly cold winters, as well as increased frequency and severity of storms. Although we can escape some of these changes through the use of technology (Air conditioning during the summer is a MUST in South Florida!), most animals are not so lucky and may soon be faced with environmental conditions that are not suitable for many key biological processes. For example, climate change may very well affect the proportion of male and female offspring produced by marine turtles and those proportions, in turn, the ability of each species to successfully reproduce. There can be no successful recovery of marine turtle populations without successful reproduction!
In sea turtles, unlike mammals (That’s us!), the sex of an individual is not determined at the moment of fertilization. This is because sea turtles lack sex chromosomes (X and Y in humans) and therefore, they don't have sex-specific genes (genes present only in a male or a female) that direct an embryo to become one or the other sex. Instead, sex in turtles is determined by the environment that the embryos experience during incubation inside the nest, and in particular by nest temperature (Figure 2). 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