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. This allows it to absorb more heat from the Sun and gives the crater a head start in producing sperm in cooler temperatures than its competitors, most of whom have to wait until the weather warms up. Fortunately lizards with black testicles fertilize more eggs, giving them an advantage in sexual selection, and for their better genes to be passed on.
Estimating several examples from the animal kingdom, a New study reveals that the sexual traits and behaviors of many organisms, from courtship rituals to self-defense weapons, require high thermal expenditure—often to dangerous levels. But, many, like the rose-bellied lizard, have evolved mechanisms to tolerate heat, allowing them to reproduce in a competitive environment, and still survive. In a potentially hotter future, species with such thermal tolerance for reproduction have a good shot at survival; Whereas for others, global warming may be their downfall.
“When a suite of traits develops to satisfy one demand, that may be beneficial to meet other demands—or it may trade off with satisfying other demands,” says Washington University in St. Noah Leith, one of the co-authors of the paper.
Reproduction is forcing organisms to “express these sexual traits that make their body temperatures very hot,” Leith says. In the animal kingdom, and at various stages of mating, from attraction to fertilization, researchers found countless examples of reproduction affecting body temperature. In some cases, sexual preference comes at a thermal cost to the animal. For example, lionesses are more attracted to lions with darker manes. But, like that lizard’s melanized testes, the darker material absorbs more heat, exposing lions with those attractive manes to greater heat stress, making them less likely to adapt to warmer climates. . And those who bring in black-maned animals tend to produce fewer offspring than light-bodied lions.
But at other times, animals have evolved to have thermal defenses to suit those preferences. Cicadas, insects whose males are known for their loud buzz during mating, can generate metabolic heat to generate buzz and raise their body temperature by as much as 20 degrees. While it may seem like “they like to cook on the inside,” Leith says, it actually helps them engage in their “corsing” during cooler times of the day when rivals aren’t as active, And most hunters are not around.
As climate change brings warmer temperatures around the world, animals that have adapted to tolerate the heat could certainly have an advantage. “There are really great ways that sexual selection can enhance adaptation to climate change,” Leith says. The sharp and elegant horns of the bongo antelope are sexual ornaments created to attract females – and to fight off rival males. But they also help dissipate heat. Therefore, in a warmer future, they may help the antelope to survive. The horns will probably grow larger to help regulate the temperature and further attract mates.
Some scientists have predicted that organisms as a whole may be smaller because it is easier to survive with less body mass to cool off. But, in some species, larger members are more sexually successful, such as heavy elephant seals that can fight off smaller ones. When they do this, they generate additional heat, but their bodies have adapted to cooling by increasing cuticle blood flow during fights.
Even though organisms are currently tolerating heat in some capacity, they may still find it difficult in a warm world. After sex, male Japanese beetles “will hang onto the female for a long time,” Leith says, standing on guard to prevent rival males from interfering with fertilization. But by doing so they are exposed to heat, which does not give water plants a chance to eat and cool off. “So it’s at odds with adaptations that are beneficial to climate change,” Leith says. In a warmer future, rising temperatures could be detrimental to reproduction for many in the animal kingdom, compromising their survival. (Or, as Leith suggests, they may have evolved other reproductive adaptations, such as “mating plug” Produced by many male animals, from kangaroos to scorpions, which promotes the success of their own fertilization by more rapid mating conditions.)
If the climate crisis becomes so severe, less reproductively-adaptive organisms could succumb to warmer temperatures, eventually making them extinct. But those with developed heat tolerance can be stronger suitors and produce more offspring. And, if their offspring have similar adaptation abilities, “then heat tolerance can develop and track climate change more quickly in populations where competition for mates is more intense,” Leith says.