A New Approach to Identifying Sex in Hatchling Turtles, and a Potential Game-Changer for Assessing Marine Turtle Hatchling Sex Ratios

One of the most fascinating features of sea turtle biology is that these animals do not have sex chromosomes, nor do they produce an almost identical ratio of boys to girls (as do we and many other animals); instead, they display a mechanism known as temperature dependent sex determination (or TSD) and an unbalanced sex ratio that normally favors females. TSD means that the temperature of the sand surrounding the eggs during incubation determines whether the embryos will develop into a male or female (Figure 1). Although how this works is still unresolved, we know that warm temperatures produce females and cool temperatures produce males (think hot chicks and cool dudes!).

Lab experiments over the past two decades show that under constant incubation conditions, temperatures below 27° C (80.6° F) produce all-male hatchlings, temperatures above 31° C (87.8° F) produce all female hatchlings,

Figure 2. Laboratory based sex ratio-temperature response curve for loggerhead turtles. The graph shows the relationship between temperature and the resulting sex ratios they produce in sea turtle nests. At low temperatures, all of the hatchlings are male; at high temperatures, all are female. At intermediate temperatures, the ratio of males to females varies and is 50:50 at 29° Centigrade, the “pivotal” temperature (PT).

Figure 1. Temperature dependent sex determination (TSD) in sea turtles. Cool temperature promote the development of males; warm temperatures promote the development of females.

and tmperatures in between produce a mixture of male and female hatchlings from that nest (Figure 2). This means that one of the most important aspects of sea turtle biology (their sex!) is dangerously vulnerable to changes in climate. Recent studies have highlighted that with increasing temperature, the sex ratios of sea turtle populations could gradually become even more, and dangerously, biased towards females. However, determining whether this prospect will happen is difficult to assess for two reasons. First, it takes marine turtles ~ 25 years (a bit sooner in some species; a bit later in others) to reach sexual maturity, so estimating sex ratio shifts in the population can’t be done immediately. A second problem is that there is no convenient way to estimate how many male and female turtles are being produced at the major nesting beaches across the globe because neither the hatchlings nor the juveniles differ in appearance until they reach sexual maturity.

Currently, there are only two techniques that can be used to identify the sex of young turtles. One technique requires microscopic examination of their gonads (reproductive organs). This technique is very reliable but it has huge drawback; the turtle must be sacrificed which is the last thing we want to do when trying to protect an endangered species. The second technique requires “looking inside” with a small camera (or laparoscope) to see the immature testes and ovaries (Figure 3). This is a labor-intensive technique that allows for reliable identification of juvenile turtles without sacrificing the animal. However, it too has some important shortcomings.