What if temperature determines the sex of a baby?

The “Imaginary Lands” series speculates what the world might look like if a key aspect of life changes, whether it is related to the planet or to humanity.

The sex of humans is largely controlled by sex X and Y chromosomes. However, in many reptiles and fish, sex is rather influenced by the temperature or freshness of the eggs before they hatch. What would human life be like if sex was also under the influence of temperature?

The fact that it was even possible to control the sex of animals using heat or cold was first discovered in the rainbow lizard agama in 1966 by the French zoologist Madeline Charnier at the University of Dakar in Senegal. She found that hatchlings from eggs incubated at lower temperatures were females, while those who developed at higher temperatures were males.

Since then, scientists have discovered other models of temperature-dependent sex determination. For example, with the Hawaiian green turtle, females emerge if incubated above a certain temperature and males below a certain temperature, and if temperatures in the nests fluctuate between these extremes, a mixture of males and females is observed, according to a study of 2020 published in the newspaper Bionature. On the other hand, with the american alligator, females develop from extreme hot and cold temperatures and males from intermediate temperatures.

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The temperature controls the determination of sex, in all crocodilians, more turtles, many fish and a few lizards, according to organism biologist Karla Moeller of Arizona State University. Within a specific window of time during the embryonic development of these animals, heat or cold can influence the production of sex hormones, which in turn can influence the fate of a newborn baby.

Moeller noted that one of the causes of temperature 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 turtle, heat during a specific developmental stage can increase the levels of this enzyme, resulting in more females.

Mysteries of evolution

It is not known exactly why these animals perform sex determination based on temperature, although a large number of theories exist, Jennifer Graves, a geneticist at La Trobe University in Melbourne, Australia, told Live Science. during a telephone interview.

“Our best guess is that the determination of sex as a function of temperature is due to the fact that reptiles do not have parental care and the eggs interact closely with the environment,” Diego Cortez told Live Science. , biologist at the National Autonomous University of Mexico in Mexico City. in an email. “We also know that high incubation temperatures accelerate the development of embryos. Thus, sex associated with higher incubation temperatures will hatch earlier.”

Because hatching among reptiles is often linked to the rainy season, when life thrives, any newborn that emerges early will likely have more food, Cortez said. “With more food, it will grow faster and have a better chance of surviving until it reaches 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 at maturity, then that sex will be linked to high incubation temperatures so it can hatch earlier in the season, ”Cortez said.

Another possibility is that determining sex based on temperature could allow mothers to control the sex of their offspring. Scientists have suggested that female alligators might choose cooler nests to have more female hatchlings, so when populations are low, “females can nest near water so more females will hatch.” Graves said. On the other hand, when populations have reached a stable level, females can choose warmer nests “so there are a lot more males, which increases the aggressiveness and competition of the males”. The next generation of females could then choose from the best males, Graves suggested.

Unlikely in humans?

All known species whose sex determination depends on temperature are both oviparous, or laying, and cold-blooded, which means that their body temperature changes with that of their environment. However, humans are neither of these things.

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As such, “determining sex based on temperature in men is not very likely because you would need, at a minimum, two different body temperatures – one that would trigger female development and one that would trigger female development. male development, ”Cortez said. “But the human body is still 37 degrees Celsius (98.6 degrees Fahrenheit).”

Still, if women could somehow know a range of body temperatures, Cortez said he could imagine a way for temperature-based sex determination to occur in humans. He noted that certain proteins that help regulate circadian rhythms in humans – our internal clocks – are also linked 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 detect large changes in incubation temperatures – typically between 3 and 7 degrees Celsius [5.4 to 12.6 degrees F] – that the system could be adapted to detect smaller changes in temperature which could, hypothetically speaking, then be related to the sex of the embryo, ”Cortez said.

For temperature-dependent sex drive to exist in humans, Graves suggested that one possibility is that we somehow become poikilotherms – that is, unable to control our body temperature – much like the naked mole rat. Another possibility is that instead of live births, we somehow had to lay eggs like a platypus, she added.

Control sex

So what would humanity be like if the temperature could decide the sex of our offspring? The most important consequence would probably be that it would then be trivial for parents to decide on the sex of their children, Graves said.

A big risk is the potential for a major gender imbalance in a society.

“A lot of humans like to decide the sex of their children,” Cortez said. “Unfortunately, in many places on this planet, the preferred sex would be males. So if humans could decide the sex of their offspring using a simple technique, like changing their body temperature for a specific week during pregnancy – the incubation temperature would need to be changed only during the week that gender is determined – I am convinced that this would create many male biased societies. “

It would be a problem.

“We know that the excess of a specific sex in adult populations creates an imbalanced population which has been linked to an increase in violence, more sexual conflicts because it is not easy for one sex to have a partner. , less parental care, and so on, ”Cortez added. . “So, in other words, a less harmonious society.”

One could imagine that governments could intervene to ensure that a gender is not too strongly favored. However, “we might then start to speculate on what might happen if the choice of sex was not up to the parents – what forces might be on the state’s side to skew the sex ratio in some way or another,” Graves said.

Originally posted on Live Science.

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