February 2018

Beach development, hunting, pollution, and fishing all threaten the survival of the endangered green sea turtle. Now, new research has uncovered a more unusual risk they face: lack of males. At some beaches in Australia, more than 99% of new hatchlings are now female. Older cohorts of turtles are also female-biased, though less severely, suggesting that the feminization of this population has been going on for decades. Since it’s the female sea turtles that invest heavily in producing eggs and one male can mate with many females, a dip in the male segment of the population doesn’t mean immediate doom, but what happens if the species skews so far female that there aren’t enough would-be dads to go around? Scientists are concerned about how these all-girl generations of turtles will affect the long term prospects for the species. Why is this occurring, and what can be done to nudge the turtles back towards gender parity?

Where's the evolution?

In humans, sex at birth is determined by which chromosomes we inherit from our parents. XX individuals are biologically female and XY individuals are male.  This system holds for most mammals, but it is far from universal. In birds for example, it’s the females that carry non-matching sex chromosomes.  In other species, sex is not dictated by genetics. Many fish switch sexes in the middle of their lives. And in some tubeworms, where the larvae settle makes all the difference: land on the ocean floor and you become female; land on another tubeworm and you become male. In turtles and most reptiles, an individual’s sex is decided by how hot or cold its egg was during incubation. Eggs that experienced higher temperatures produce female hatchlings, while cooler conditions produce males.

Therein lies the explanation for the female dominated turtle generations. Because of human emission of greenhouse gases, the climate has warmed over the past 35 years — and so too have the beach sands in which turtles lay their eggs. This explanation fits well with the results of the new research. The beaches with the most skewed sex ratios (the beaches that produced 99% female hatchlings) were the warmer beaches, while cooler nesting grounds produced only about 65% females. Furthermore, older generations of turtles, whose eggs would have incubated before global temperatures had risen as much as they have today, had less biased sex ratios.

So where’s the evolution? Because humans (and most other species with separate sexes) have about a 50/50 sex ratio, it might not be something that one would think could evolve. But in fact, sex ratio can evolve, and that common 50/50 sex ratio is no accident. The following thought experiment is a good way to understand the evolutionary process that usually leads to a 50/50 sex ratio:

1. Imagine a population where it is equally costly to produce male and female offspring, but the sex ratio is skewed, with three females born for every one male.
2. In a population that’s 75% gal, a guy’s odds of becoming a father are unusually good. On average, any individual male will father about three times as many offspring as any single female will mother.
3. In this situation, what will happen to a random mutation that happens to cause an individual to produce fewer daughters and more sons? Since males have more reproductive success than females, this mutation will lead to more grandchildren and will be favored, becoming more common through natural selection.
4. And as this mutation spreads through many generations, more and more sons will be produced, and the population will start tipping back towards a 50/50 sex ratio.
5. What happens if this process continues for so long that males begin to outnumber females? In that case, the same reasoning holds, but with the opposite effect: any mutation that caused the production of more daughters would be favored and spread.

Whenever one sex becomes more common, mutations that lead to biased production of the other sex are favored. Ultimately, we’d expect this process to produce a population with a 50/50 sex ratio. For most species, the 50/50 sex ratio is what is known as an evolutionarily stable strategy, meaning that any mutation that causes the population to deviate from this ratio is acted against by natural selection. So sex ratio, whether genetically or environmentally determined, is something that can (and does) evolve; it just usually winds up about the same across species, not because there’s no mechanism for producing more of one sex, but because a 50/50 sex ratio is evolutionarily stable in the long term.

Based on the reasoning above, what should we expect to happen to female-biased populations of the green sea turtle? Clearly, selection should favor traits that cause more males to be produced, for example, mutations that bumped up the temperature threshold at which an egg produces a female hatchling, that cause mother turtles to seek cooler sands in which to lay eggs, or that prompt egg laying earlier in the season before extreme heat sets in. Over many generations, we would expect such mutations to be favored, pushing the population back towards a more even sex ratio. However, the key words there are “over many generations.” Evolutionarily stable strategies are what we expect to observe over large swaths of evolutionary time. If genetic variants that lead to more males do not already exist in turtles, random mutation may eventually generate them, but that process takes time. And once a favorable mutation exists, it spreads on generational timescales. For sea turtles, that is the 35-40 years that it takes them to reach sexual maturity, compounded over many generations.

Short term, more female sea turtles likely means more egg laying and larger populations, which is a good thing. But over slightly longer timescales, once the number of males is so small that many females are left mate-less, populations will begin to decline. Can evolution help turtles avoid that fate and lead the species back towards sexual parity and the evolutionarily stable strategy that natural selection favors? There may simply not be time. Scientists expect that the climate will continue heating up at a rate 20 times faster than at any time in the last two million years of Earth history. It is unlikely that we can depend on the slow pace of turtle evolution to keep up with this breakneck warming. Instead, scientists need to study the turtles further to understand how mixing between populations from warmer and cooler beaches might factor into the species’ future and what steps might be taken (such as moving eggs to cooler sands or beaches) to safeguard these endangered animals.