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As a SciShow viewer, you can keep building your STEM skills with a 30 day free trial and 20% off an annual premium subscription at Brilliant.org/SciShow. We know the changing climate is messing with a lot of things around the world, like weather patterns and ocean temperatures.

And it’s no secret that a whole bunch of animals aren’t going to be too happy about that. But there’s one side effect of climate change that you might not have expected - it could also make a lot more reptiles female. [intro song] Now, we typically think of sex in animals as being largely determined by their genes, specifically the sex chromosomes. In mammals, females have two copies of an X chromosome, while males have one X and one Y.

In birds, there’s a system with Z’s and W’s, and it’s the male with two copies of the Z. But sex isn’t always determined by genetics. Instead of using sex chromosomes, some animals rely on an  egg’s incubation temperature to determine the sex of the hatchling!

This temperature-dependent sex determination is used by all species of crocodilians, most turtles, and even some lizards. Now, using chromosomes to determine sex produces about a 50/50 chance of an offspring being male or female, but in reptile species that use the temperature-based system, a whole clutch of eggs might end  up being mostly just one sex. In a lot of cases, the eggs that incubate at lower temperatures develop into males, and higher temperatures result in females.

So a nest of eggs that incubates at temperatures in the middle zone will likely differentiate t o an even sex split between males and females. And sometimes just a few degrees of difference results in an all-male or all-female clutch. But there are even some species where both temperature extremes result in female hatchlings - hotter or colder nests - with males being produced at intermediate temps.

Relying on temperature at all seems like a risky way to determine the sex ratios of offspring, but some scientists believe this is the way all reptiles originally did it. It’s possible that the W, X, Y, and Z chromosomes of birds and mammals evolved independently after those lineages split from early reptiles. Because it’s pretty hard to  determine how extinct genes worked, we can’t say for sure which method came first or why there’s such  variety among modern reptiles.

And to make things even more complicated, there are plenty of reptiles that do use sex chromosomes. What’s weirder still is that  there are even species of fish and amphibians that have sex chromosomes but where temperature extremes can override their chromosomes and determine the sex of the animal. The sheer variety of sex determination methods and the examples of what appears to be an evolutionary intermediate step between chromosomes and temperature suggest reptiles could find a way to avoid getting caught in an all-female world.

Whichever method came first, there’s no denying that relying on temperature is a risky move. And it’s getting riskier. As the planet becomes hotter, the nests in which these temperature-reliant eggs hatch are getting hotter, too.

And that means we’re likely to see more and more female crocodiles and turtles. If it only takes a few degrees difference to change the whole sex ratio of a nest, a rise in the average temperatures during peak breeding months could significantly change the population’s sex ratio - and the mating chances for  those individuals down the line. Take the painted turtle.

Painted turtle clutches along the Mississippi River produce nearly all males at 23 degrees Celsius, and almost all females at 25 degrees Celsius. A change in the average July temperature of just 1 or 2 degrees could mean the difference between nests with even sex ratios and nests that are 100% female. And since the Earth’s temperature is rising about 0.2 Celsius per decade, that one degree rise could  happen in just 50 to 100 years, which might not be enough time for evolution to fully catch up.

Some species, like sea turtles, live much longer than that, so they may not have generations and generations of time to evolve changes in genetics or behavior. In the short term, this all means that we’re likely to see more reptile clutches that are entirely or mostly female, at least from the croc and turtle species where temperature controls sex differentiation. It’s currently unclear if these reptiles will shift their breeding grounds or times to accommodate the changing temps, like some other animals have done in response to climate change.

Breeding earlier in the spring, for example, would mean cooler temperatures and thus, more equal sex ratios. And we know that mother reptiles do seem to try to select nest locations with temperatures to give their offspring the  best chance for survival. In the long term, perhaps more reptile species will do away with temperature-dependence and just move toward sex chromosomes.

It’s a move that plenty of reptile species have made, like snakes. We know reptiles have survived rapid climate change events in the past 280 million years, so we know they can adapt to rapid changes in temperature. The question is: how quickly can they do it this time?

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