The series “Imaginary Lands” speculates what the world would be like if a key aspect of life changes, whether related to the planet or humanity.
Human sex is largely controlled by sex X and Y chromosomes. However, in many reptiles and fish, sex is influenced by the temperature of the eggs before hatching. What would life be like for humans if sex was also under the influence of temperature?
The fact that it was possible to control the sex of the animals using heat or cold was first discovered in the rainbow agama lizard in 1966, by the French zoologist Madeline Charnier, from the University of Dakar, in Senegal. She found that hatchlings incubated at lower temperatures were female, while those that grew at higher temperatures were male.
Since then, scientists have discovered other patterns of temperature-dependent sexual determination. For example, with the Hawaiian green turtle, females emerge if incubated above a certain temperature and males if below a certain temperature, and if nest temperatures fluctuate between these extremes, a mixture of males and females is seen, according to a 2020 study published in magazine Bionatura. In contrast, with the American Alligator, females develop in extremes of heat and cold and males at intermediate temperatures.
Related: What if humans had photosynthetic skin?
Temperature controls sex determination in all crocodilians, most turtles, lots of fish and some lizards, according to biologist Karla Moeller, of Arizona State University. Within a specific time window during the embryonic development of these animals, heat or cold can influence the production of sex hormones, which in turn can influence a puppy’s fate.
Moeller noted that one of the causes of temperature-dependent sex determination is an enzyme known as aromatase, which can convert male sex hormones to female ones. In animals such as the red-eared turtle, heat during a specific stage of development can increase levels of this enzyme, leading to more females.
Evolutionary mysteries
It is not yet known exactly why these animals practice temperature-dependent sexual determination, although there are a number of theories, 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 originated because reptiles are not caring for their parents and eggs are in close interaction with the environment,” said Diego Cortez, a biologist at the National Autonomous University of Mexico in Mexico City. Mexico, to Live Science. in an email. “We also know that high incubation temperatures accelerate the development of embryos. Therefore, sex that is related to higher incubation temperatures will be born earlier.”
Because, among reptiles, hatching is often linked to the rainy season, when life blooms, any baby that appears early is likely to receive more food, said Cortez. “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 hypothesis of survival to maturity, “if for some reason it is better for a species to have larger females or larger males at maturity, then this sex will be linked to high incubation temperatures so that it can hatch early of the season, “said Cortez.
Another possibility is that temperature-dependent sex determination may give mothers a means of controlling the sex of their children. Scientists have suggested that female crocodiles may choose cooler nests to have more female chicks, so when populations are low, “females can make their nests close to the water so that more females will hatch,” Graves said. In contrast, when populations reach a stable level, females can choose warmer nests “so that there are many more males, obtaining more aggression and male competition”. The next generation of females could then choose among the best males, suggested Graves.
Unlikely in humans?
All known species with temperature-dependent sex determination are oviparous, or eggs, and cold-blooded, which means that the temperature of their bodies changes with that of their surroundings. However, humans are none of those things.
Related: Why do animals hibernate?
As such, “determining temperature-dependent sex 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,” said Cortez. “But the human body is always at 37 degrees Celsius (98.6 degrees Fahrenheit).”
Still, if women could somehow experience a variety of body temperatures, Cortez said he could think of a way to determine temperature-dependent sex in humans. He noted that some proteins that help regulate circadian rhythms in humans – our internal clocks – are also related to the determination of temperature-dependent sex in reptiles. These proteins, known as CLK kinases, are found throughout the body and can detect very small fluctuations in body temperature.
“It would not be impossible to think that if CLK kinases were involved in determining temperature-dependent sex in reptiles, where they experienced major changes in incubation temperatures – usually between 3 and 7 degrees Celsius [5.4 to 12.6 degrees F] – that the system could be adapted to detect minor changes in temperature that could, hypothetically speaking, be linked to the sex of the embryo, “said Cortez.
In order for temperature-dependent sexual determination to exist in humans, Graves suggested a possibility that we somehow become poikilothermic – that is, unable to control our body temperature – much like the naked mole rat. Another possibility is that, instead of being born alive, we should somehow lay eggs as a platypus, she added.
Sex control
So, what would humanity be like if temperature could decide the sex of our offspring? The most important consequence would probably be that it would be trivial for parents to decide the sex of their children, Graves said.
A big risk is the potential for a big imbalance between the sexes in a society.
“Many humans like to decide the sex of their children,” said Cortez. “Unfortunately, in many places on this planet, the preferred sex would be male. Therefore, if humans could decide the sex of their offspring using an uncomplicated technique, such as changing the body temperature during a specific week during pregnancy – the temperature of incubation would need to be changed only during the week in which sex is determined – I am confident that this would create many biased societies for men. “
that would be a problem.
“We know that the excess of a specific sex in adult populations creates an unbalanced population that has been associated with increased violence, more sexual conflict because it is not easy for a sex to obtain a partner, less care for the parents and so on,” he added. Cortez. . “So, in other words, a less harmonious society.”
One can imagine that governments can intervene to ensure that sex is not strongly favored. However, “we can then begin to speculate what could happen if the choice of sex did not depend on the parents – what forces might be of interest to the state in distorting the gender ratio in one way or another,” Graves said.
Originally published on Live Science.