Hot stuff: how climate change is affecting sex ratios
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Like many of the Greeks of his time, Aristotle thought men were hotter than women. Now, what Aristotle got up to in the bath houses of Athens is no concern of ours, but he did think that heat gives rise to males, and cold gives rise to females, to the extent that he thought the temperature of the man during sexual intercourse affected the sex of the baby. He even went so far as to tell men who wanted a male heir to try and conceive during the summer when the weather was warmer.
If you know anything about Aristotle, you probably know that he was wrong about an awful lot of things. This was a man who thought that the Sun revolved around the Earth, and even for some reason thought women had fewer teeth than men.
But for all his mistakes, Aristotle was kind of right about the temperature thing, if you’re a reptile, that is. In alligators, crocodiles, turtles and some fish and lizard species, your sex isn’t determined by your chromosomes, but by the temperature of the egg during incubation.
Take for example, sea turtles. There are seven species of sea turtle and all of them have temperature-dependent sex determination, meaning that they produce more females the warmer it gets. As I’m sure you’ve seen in David Attenborough documentaries, when a female turtle is ready to lay eggs, she hauls herself up onto a sandy beach in the dead of night and digs a hole in the sand with her hind flippers about as deep as your arm. In here she’ll deposit her clutch of about 100 eggs and carefully cover the nest by burying it with more sand, leaving her precious eggs to incubate for the next two months. If you cast your mind back to the last time you built sandcastles at the beach, you’ll remember that sand gets slightly cooler the deeper down you dig, so each of her eggs incubates at a slightly different temperature.
Inside each egg, the enzyme aromatase is busy at work converting testosterone into oestrogen. But here’s the crucial bit: it’s more active when the temperature is warmer. If the egg incubates at 31°C or warmer, enough testosterone is converted into oestrogen to produce a female turtle. If it’s 27.7° C or cooler, you’ll get a male turtle, and you get an assortment of female and male turtles from the eggs that incubate between the two temperatures.
When you think about it, it’s pretty astonishing that turtles have evolved to have such fine control over the temperatures of their nests, keeping them within that 3 degree range. As a result, the sex ratio of the species, or the relative number of males and females, has remained fairly stable for thousands of years. That is, unfortunately, until the last few years.
As you can probably guess, global warming is a bit of a nightmare if you’re a species that relies on temperature to control its sex ratio. As the temperature of the sand rises, more and more turtles hatching are females and this is happening all around the world. For green sea turtles on the Great Barrier Reef in Australia, hatchlings at the more northern, and therefore warmer beaches were 99% female. In Florida, one turtle hospital reported that every single turtle they have tested for the last four years has been female. They haven’t found one male. And if there aren’t enough males to go round, that means fewer and fewer females will get to mate and lay eggs until the population dwindles into extinction.
Natural selection loves individuals who can ‘beat the system’, and being a male in such a female skewed population is like winning the natural selection lottery. Imagine being the one male turtle that gets to mate with 99 female turtles - that’s a lot of opportunities to pass your genes on to the next generation! And if you’re a female that is genetically predisposed to dig your nest a little deeper where the sand is cooler, or even lay your eggs earlier in the year before the beach gets too hot, and you produce more males as a result, think how successful your sons will be!
Any genes that make you more likely to produce males will be super successful, and we could see turtles adapt to warming temperatures, and the species returning to a more even sex ratio . But unfortunately, natural selection takes time, and it’s now a race between how fast climate change progresses and how fast the turtles can adapt in response.
There’s a small relief to be found in the direction of this skew. Having only 1% of your population being male isn’t great, but one male can fertilise an awful lot of females, albeit causing a massive decrease in genetic diversity, which isn’t great for the species overall.
If it had skewed the other way, and 99% of the species were male, well there’s only so many eggs those female 1% can produce, and the species would be nose diving into extinction. American alligators might face this problem as for them, warmer eggs leads to more males. Imagine being one of the few female alligators having to fight off hundreds of male suitors all clamouring for your affection. Not a pretty sight.
What’s even more unsettling is that climate change might not only affect species that use temperature to determine their sex, but even species that have genetic sex determination. Higher temperatures during embryonic development can lead to something called ‘sex reversal’ where an animal is born with the physical characteristics of one sex but the genetic code for the other.
Take, for example, the common toad. Instead of having X and Y chromosomes, they’ve got Z and W sex chromosomes. Males have two copies of the Z chromosome, ZZ, and females are the heterogametic sex, that is they have one copy of each, ZW.
You could have an embryo with ZW chromosomes, i.e. genetically female, but if it develops under warmer conditions, the higher temperature affects the activity of the male hormones and the embryo becomes masculinised, developing the physical characteristics of a male toad. This genetically female but physiologically male toad could then mate with another female and have offspring.
Now, if toads were using the XY chromosomal sex determination system, two genetic females mating would be XX and XX, so the only offspring they could produce would have XX chromosomes, i.e. females. So even though warming would masculinise one generation, there would be an increase in the number of females born to balance it out in the next generation.
But with the ZW system, a masculinised ZW toad mating with a normal female ZW would still produce ZZ male offspring. So because of the sex chromosomes they use, the toads can’t counteract the masculinising effect of warmer temperatures as easily, so the proportion of physiological males increases. And one study compared 6 different amphibian species over the last 60 years and found that indeed, the two species that had the ZW system were becoming half a percent more male dominated each year, but the four species that had the XY system remained stable. If the toads are unable to adapt and the proportion of males continues to increase, there’s a small possibility that physiologically female toads could cease to exist, and with no females to lay eggs, the whole species could ultimately croak.
https://www.cell.com/current-biology/fulltext/S0960-9822(17)31539-7
99% of Australian Green Sea Turtles Studied Turning Female From Climate Change
