For the pink-bellied lizard of Mexico and South 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 others. This allows it to absorb more heat from the sun and give the critter an edge in producing sperm at lower temperatures for their competitors, most of whom have to wait for the weather to warm up. The lizards with happily dark testicles fertilize more eggs, giving them an advantage in sexual selection and for passing on their superior genes.
Reviewing multiple examples from across the animal kingdom, a new study shows that the sexual traits and behaviors of many organisms, from courtship rituals to self-defense weapons, require high heat expenditure — often to dangerous levels. But many, like the pink-bellied lizard, have evolved mechanisms to tolerate the heat, allowing them to reproduce in competitive environments and still survive. In a likely much warmer future, species with such thermal tolerance to reproduction have a good chance of survival; while for others global warming could be their downfall.
“When a set of properties evolves to meet one demand, it can be beneficial to meet other demands — or it can alternate with meeting other demands,” said Noah Leith, one of the paper’s co-authors. from Washington University in St. Louis.
Reproduction forces organisms to express “these sexual traits that make their body temperature super hot,” Leith says. In the animal kingdom and at various stages of mating, from attraction to fertilization, the researchers found numerous examples of reproduction affecting body temperature. In some cases, sexual preferences have thermal costs for the animal. Lionesses, for example, are more attracted to lions with dark manes. But like that lizard’s melanized testis, dark fabrics absorb more heat, exposing lions with those seductive manes to more heat stress, making them less likely to adapt to warmer climates. And those dark-maned beasts tend to produce fewer offspring than lighter-maned lions.
But at other times, animals have evolved to develop thermal protections consistent 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 to keep buzzing. While that “seems like they want to cook them from the inside,” Leith says, it actually helps them join in their “chorus” during cooler times of the day when rivals aren’t as active and most predators aren’t around. to be .
As climate change brings higher temperatures globally, animals that have adapted to tolerate heat could certainly have an advantage. “There are a lot of really cool ways that sexual selection can improve adaptations to climate change,” Leith says. The sharp and elegant horns of the bongo antelope are sexual ornaments built to attract females – and to repel rival males. But they also help dissipate heat. So in a warmer future, they could help antelope survive. Perhaps the horns grow larger to help regulate temperature as well as attract mates.
Some scientists have predicted that organisms as a whole may become smaller because it is easier to survive with less body mass to cool down. But in some species, larger members are more sexually successful, such as bulky elephant seals that can fight smaller ones. While doing this, they generate excess heat, but their bodies have adapted to cooling by increasing cuticular blood flow during fights.
Even if organisms currently tolerate heat to some degree, they may find it more difficult in a warmer world. After sex, male Japanese beetles will “hang out on the female for a long time,” Leith says, as she stands guard to prevent rival males from interfering with fertilization. But this exposes them to heat, preventing the chance of seeking out watery plants to eat and cool off. “So it’s a bit at odds with the adaptations that are beneficial to climate change,” Leith says. In a warmer future, rising temperatures could harm reproduction for many in the animal kingdom, jeopardizing their survival. (Or, as Leith suggests, they could develop other reproductive adaptations, such as the “mating plug,” produced by many males from kangaroos to scorpions, which promotes its own fertilization success more quickly in promiscuous mating conditions.)
If the climate crisis becomes this severe, less reproductively adaptive organisms may succumb to higher temperatures, eventually causing them to become extinct. But those with an evolving heat tolerance could be strong suitors and produce more offspring. And if their offspring have the same adaptability, “heat tolerance can evolve and follow climate change more quickly in populations where competition for mates is more intense,” Leith says.