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Some animals get dangerously hot when they have sex—they might be the ones that survive in a warming world

The thermal traits that have evolved for sexual success may help them endure climate change.

Some animals get dangerously hot when they have sex—they might be the ones that survive in a warming world
[Source Photo: William L. Farr/Wiki Commons]

For the rose-bellied lizard of Mexico and Southern Texas, not all body parts are created equal. In some male lizards, the left testis has evolved to fill with melanin, so it becomes darker than the other. That allows it to absorb more heat from the sun and give the critter a head start in producing sperm in cooler temperatures in advance of their competitors, most of whom have to wait until the weather warms up. The luckily dark-testicled lizards fertilize more eggs, giving them an advantage in sexual selection, and for passing on their superior genes.

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Assessing multiple examples from across the animal kingdom, a new study shows that many organisms’ sexual traits and behaviors, from courtship rituals to self-defense weapons, require high heat expenditure—often to dangerous levels. But, many, like the rose-bellied lizard, have developed mechanisms to tolerate the heat, allowing them to reproduce in competitive environments, and still survive. In a likely much hotter future, species with such thermal tolerance for reproduction have a good shot at survival; while for others, global warming could be their downfall.

[Image: Noah Leith and Ecology Letters]
“When one suite of traits evolves to meet one demand, that can be beneficial for meeting other demands—or it can trade off with meeting other demands,” says Noah Leith, one of the paper’s coauthors from Washington University in St. Louis.

Reproduction is forcing organisms into “expressing these sexual traits that make their body temperatures super hot,” Leith says. Across the animal kingdom, and along different stages of mating, from attraction to fertilization, the researchers found countless instances of reproduction affecting body temperatures. In some cases, sexual preferences come at a thermal cost to the animal. Lionesses, for instance, tend to be more attracted to lions with darker manes. But, like that lizard’s melanized testis, darker substances absorb more heat, exposing lions with those alluring manes to more heat stress, making them less likely to adapt to warmer climates. And those fetching dark-maned beasts tend to produce a lower offspring output than lighter-maned lions.

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But at other times, animals have evolved to develop thermal protections in line with those preferences. Cicadas, insects whose males are known for their loud buzzing sound during mating, can generate metabolic heat and raise their body temperatures by 20 degrees, in order to keep buzzing. Even though that “seems like that would like cook them from the inside,” Leith says, it actually helps them engage in their “chorusing” in cooler times of the day when rivals aren’t as active, and most predators aren’t around.

[Image: Noah Leith and Ecology Letters]
As climate change brings hotter temperatures across the globe, animals that have adapted to tolerating heat may surely have an advantage. “There’s lots of really cool ways that sexual selection can enhance adaptations to climate change,” Leith says. The sharp and elegant horns of the bongo antelope are sexual ornaments built to attract females—and for fighting off rival males. But they also help dissipate heat. So, in a hotter future, they could help antelope survive. Perhaps the horns will get bigger to both help regulate temperatures and further attract mates.

Some scientists have predicted that organisms as a whole could get smaller because it’s easier to survive with less body mass to cool. But, in some species, larger members are more sexually successful, such as bulky elephant seals that can fight off smaller ones. As they do so, they generate excess heat, but their bodies have adapted to cooling by increasing cuticular blood flow during brawls.

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Even if organisms are currently tolerating heat in some capacity, they may find it harder in a warmer world. After sex, male Japanese beetles will “hang out on the female for a long time,” Leith says, standing on guard to prevent rival males from interfering with fertilization. But doing so exposes them to heat, preventing the chance to seek out watery plants to eat and cool down. “So it’s kind of at odds with the adaptations that are beneficial for climate change,” Leith says. In a hotter future, rising temperatures may be detrimental to reproduction for many in the animal kingdom, compromising their survival. (Or, as Leith suggests, they could develop other reproductive adaptations, such as the “mating plug,” produced by many male animals, from kangaroos to scorpions, which more quickly promotes his own fertilization success in promiscuous mating conditions.)

If the climate crisis gets so severe, less reproduction-adaptive organisms may succumb to hotter temperatures, ultimately rendering them extinct. But those with evolving heat tolerance could be strong suitors and produce more offspring. And, if their offspring have the same adaptation abilities, “then heat tolerance may evolve and track climate change more quickly in populations where competition for mates is more intense,” says Leith.

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