The Imaginary Earths series speculates what the world might look like if a key aspect of life were to change, be it in relation to the planet or humanity.
The sex of humans is largely controlled by the X and Y sex chromosomes. For many reptiles and fish, however, sex is affected by how warm or cool the eggs are before they hatch. What could life look like for humans if sex were also subject to temperature fluctuations?
The fact that it was even possible to control the sex of animals through heat or cold was first discovered in 1966 by the French zoologist Madeline Charnier at the University of Dakar in Senegal in the rainbow agama lizard. She found that pups from eggs incubated at lower temperatures were female, while those that developed at higher temperatures were male.
Since then, scientists have discovered other patterns of temperature-dependent sex determination. For example, the Hawaiian green sea turtle occurs when females are incubated above a certain temperature and males when they are below a certain temperature, and when temperatures in nests fluctuate between these extremes, a 2020 study suggests that it becomes one Mixture of men and women observed published in the magazine Bionatura. In contrast, in the American alligator, women develop from extremely hot and cold temperatures and men from intermediate temperatures.
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According to Arizona State University organism biologist Karla Moeller, temperature controls sex determination in all crocodiles, most turtles, many fish, and some lizards. Within a certain time window during the embryonic development of these animals, heat or cold can affect the production of sex hormones, which in turn can affect the fate of a young animal.
Moeller found that one cause of temperature-dependent sex determination is an enzyme known as aromatase, which can convert male sex hormones into female sex hormones. In animals like the red-eared water turtle, heat during some developmental stage can increase levels of this enzyme, resulting in more females.
It remains uncertain why these animals practice temperature-based sexing, although there are a myriad of theories, Jennifer Graves, a geneticist at La Trobe University in Melbourne, Australia, told Live Science in a telephone interview.
“We assume that temperature-dependent sexing is due to the fact that reptiles have no parental care and the eggs are in close interaction with the environment,” said Diego Cortez, a biologist at the National Autonomous University of Mexico in Mexico City Live Science in one email. “We also know that increased incubation temperatures accelerate embryo development. The sex associated with higher incubation temperatures will therefore hatch earlier.”
Because hatching in reptiles is often associated with the rainy season, when life is in bloom, any hatchling that shows up early is likely to get more food, Cortez said. “With more food, it grows faster and has a higher chance of surviving until it is ripe,” he said.
According to this idea, known as the survival-to-maturity hypothesis, “if for any reason it is better for a species to have taller women or taller men at the time of maturity, that sex will be associated with high temperatures of incubation it can hatch. ” earlier in the season, “said Cortez.
Another possibility is that temperature-dependent sexing could allow mothers to control the sex of their offspring. Scientists have suggested that female alligators might choose cooler nests in order to have more female hatchlings. So when populations are low, “women can build their nests near the water so more women can hatch,” Graves said. In contrast, once populations reached stable levels, women might choose warmer nests, “so that there are many more men who get more male aggression and competition”. The next generation of women could then choose from the best men, Graves suggested.
Unlikely in humans?
All known species with temperature-dependent sex determination are both egg-shaped or egg-layered and cold-blooded, which means that their body temperatures change with those of their surroundings. However, humans are none of these things.
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Therefore, “temperature-dependent sexing in humans is not very likely because you would need at least two different body temperatures – one that would trigger female development and one that would trigger male development,” Cortez said. “But the human body is always 37 degrees Celsius (98.6 degrees Fahrenheit).”
However, if women could experience a range of body temperatures, Cortez could envision a way for temperature-based sex determination in humans. He noted that some proteins that help regulate the circadian rhythm in humans – our internal clocks – are also related to temperature-dependent sexing in reptiles. These proteins, known as CLK kinases, are found throughout the body and can sense very small fluctuations in body temperature.
“It’s not impossible to believe that CLK kinases are involved in temperature-dependent sex determination in reptiles, where they sense large changes in incubation temperatures – usually between 3 and 7 degrees Celsius [5.4 to 12.6 degrees F] – that the system could be adjusted to capture minor temperature changes that could hypothetically be related to the sex of the embryo, “said Cortez.
In order to achieve temperature-dependent sex determination in humans, Graves suggested that we somehow become poikilotherms – that is, we cannot control our body temperature – similar to what happens in the naked mole rat. Another possibility is that instead of live births, we should lay eggs like platypus, she added.
So what would humanity look like if temperature could determine the sex of our offspring? The most important consequence would probably be that it would then be trivial for parents to decide on the gender of their children, said Graves.
One major risk is the potential for major gender imbalance in a society.
“A lot of people like to decide the gender of their children,” said Cortez. “Unfortunately, in many places on this planet, men would be the preferred sex. So if humans could determine the sex of their offspring using a straightforward technique, such as changing their body temperature during a specific week of pregnancy – the incubation temperature would only have to do during The week that gender is determined – I am confident that this would create many societies geared towards men. “
That would be a problem.
“We know that exceeding a certain gender in adult populations leads to an unbalanced population that has been linked to increased violence, more sexual conflict, as it is not easy for a sex to find a partner, less parental care and so on, “added Cortez. “In other words, a less harmonious society.”
One could imagine that governments could step in to ensure that one gender is not overly favored. “We could then, however, begin to speculate about what might happen if the choice of gender is not left to the parents – what forces might interest the state to distort gender relations in one way or another,” Graves said.
Originally published on Live Science.