The “Imaginary Earths” series speculates what the world might look like if an important aspect of life changed, whether related to the planet or humanity.
The gender of people is largely determined by the X and Y gender chromosomes. However, in many reptiles and fish, sex is affected by how warm or cool the eggs are before hatching. What could life be like for people if sex were also influenced by temperature?
That it was even possible to control the sex of animals with heat or cold first discovered in the rainbow agama lizard in 1966 by the French zoologist Madeline Charnier at the University of Dakar in Senegal. She found that fry from eggs incubated at lower temperatures were female, while the fry that developed at higher temperatures were male.
Since then, scientists have discovered other patterns of temperature-dependent sex determination. For example with the Hawaiian Green Sea Turtle, females emerge when incubated above a certain temperature and males when they are below a certain temperature, and when temperatures in nests fluctuate between those extremes, a mix of males and females is seen, according to a 2020 study in the magazine has been published Bionatura. Unlike the American alligator, females develop through extreme heat and cold and males through intermediate temperatures.
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The temperature controls sex determination in total crocodilians, most turtles, many fish and some lizards, according to Arizona State University biologist Karla Moeller. Within a certain time frame 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 boy.
Moeller noted that one of the causes of temperature-dependent sex determination is an enzyme known as aromatase, which can convert male sex hormones into female sex hormones. In animals such as the red-eared slider, heat during a specific stage of development can increase levels of this enzyme, leading to more females.
Evolutionary Mysteries
It remains uncertain why these animals employ temperature-dependent sexing, although a myriad of theories exist, Jennifer Graves, a geneticist at La Trobe University in Melbourne, Australia, told Live Science in a telephone interview.
“Our best guess is that temperature-dependent sex determination came about because reptiles lack parental care and the eggs interact closely with the environment,” Diego Cortez, a biologist at the National Autonomous University of Mexico in Mexico City, told Live Science. .com in an email. “We also know that increased incubation temperatures accelerate embryo development. Thus, the sex associated with higher incubation temperatures will hatch earlier.”
Because in reptiles, hatching is often related to the rainy season, when life is blooming, any young that emerge early will likely be given more food, Cortez said. “With more food, it will grow faster and have a better chance of surviving to maturity,” he said.
According to this idea, known as the survival-to-maturity hypothesis, ‘if for some reason it is better for a species to have larger females or larger males in adulthood, then this sex will be linked to high incubation temperatures so it can come true. earlier during the season, ”said Cortez.
Another possibility is that temperature-dependent sexing could provide mothers with a way to control the sex of their offspring. Scientists have suggested that female alligators can choose cooler nests to have more female young, so when populations are low, “females can nest near the water so that more females hatch,” Graves said. On the other hand, once populations have reached a stable level, females may opt for warmer nests “so that there are many more males, who get more male aggression and competition.” The next generation of females could then choose from the best males, Graves suggested.
Unlikely in humans?
All known temperature-dependent sexed species are both oviparous, or egg-layers, and cold-blooded, meaning that their body temperature changes with that of their environment. However, humans are neither.
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As such, “temperature-dependent sexing in humans is unlikely because you would need at least two different body temperatures – one that would stimulate female development and one that would stimulate male development,” Cortez said. “But the human body is always at 37 degrees Celsius (98.6 degrees Fahrenheit).”
Still, if women could somehow experience a range of body temperatures, Cortez said he could imagine a way to make temperature-dependent sex determination happen in humans. He noted that some of the proteins that help regulate circadian rhythms in humans – our internal clocks – are also related to temperature-dependent sex determination in reptiles. These proteins, known as CLK kinases, are found throughout the body and can detect very small fluctuations in body temperature.
“It wouldn’t be impossible to think that if CLK kinases are involved in temperature-dependent sex determination in reptiles, where they observe large changes in incubation temperatures – usually between 3 and 7 degrees Celsius. [5.4 to 12.6 degrees F] – that the system can be modified to detect smaller temperature changes that, hypothetically speaking, could then be linked to the sex of the embryo, “Cortez said.
For temperature-dependent sex determination to exist in humans, Graves suggested that one possibility is that we somehow become poikilotherms – that is, we can’t control our body temperature – just like the naked mole rat. Another possibility is that instead of live births, we could somehow lay eggs like one platypus, she added.
Controlling sex
So what would humanity look like if temperature could determine the sex of our offspring? The main consequence would likely be that it would then be trivial for parents to decide the sex of their children, Graves said.
A major risk is the potential for a major gender imbalance in a society.
“A lot of people like to determine the sex of their children,” Cortez said. “Unfortunately, in many places on this planet, men would prefer gender. So if humans could determine the sex of their offspring using a non-complicated technique, such as changing their body temperature during a specific week during pregnancy , the incubation temperature should only be changed during the week when sex is determined – I am convinced that this would create many societies that are biased toward men. “
That would be a problem.
“We know that excess of one specific gender in adult populations creates an imbalanced population associated with more violence, more sexual conflict because it is not easy for one gender to mate, less parental care, and so on. , ”Cortez added. . “In other words, a less harmonious society.”
You could imagine that governments could intervene to ensure that a particular gender is not favored too much. But “we can then begin to speculate what might happen if the choice of gender is not up to the parents – what forces might interest the state in somehow skewing the gender ratio,” Graves said.
Originally published on Live Science.