Life Can’t Get Much Hotter Than This
The world’s most heat-adapted creatures could be subverting their own evolution.
Anoles have always been happy in the heat. The svelte little lizards, a group some 400 species strong, thrive in the Americas’ warmest parts—from the balmy rainforests of South America up through the United States’ Sun Belt—where they spend their days basking on boulders and scurrying out to the sun-soaked tips of twigs, or even scampering over the blistering metal of exposed city pipes.
And when local temperatures get even hotter, as they now so often do, anoles take those changes in stride. Beneath the shady canopies of Caribbean rainforests, Martha Muñoz, an evolutionary biologist at Yale, and her students have found species that have rapidly evolved the ability to withstand temperatures verging on 110 degrees Fahrenheit; elsewhere, near the forest’s perimeter, the researchers have discovered species that have taught themselves to shelter beneath rocks until it’s cool enough to leave. On this fast-warming planet, animals have just three options: “Behave, adapt, or die,” Brian Cheng, an evolutionary biologist at the University of Massachusetts at Amherst, told me. So far, anoles are excelling at the first two.
But after years of observing the lizards, Muñoz worries that their survival strategies are reaching a limit. “I don’t think there is a happy ending,” she told me. Life has a physiological ceiling—a temperature that even eons of evolution cannot allow an animal to tolerate. And some of the heat-tempering tactics that are helping the creatures cope now could speed their demise in the long term. Scientists are finding that, especially among ectotherms—so-called cold-blooded creatures that lack an internal thermostat—behavior and evolution don’t always complement each other; sometimes, those two natural forces may even be at odds. Of the choices creatures have, death is the only outcome that has no caveats.
For most ectotherms—many of them small, fast-reproducing, and ultra-diverse—speedy evolution would seem an excellent choice to counteract the pressures of climate change. In the Caribbean, anoles in both natural forests and densely packed cities have reshaped their physique to better withstand the heat. In Cleveland, Sarah Diamond, of Case Western Reserve University, has documented acorn ants evolving higher heat tolerances in just a couple dozen generations. In laboratories, water fleas can be pushed to weather several more degrees of warmth in a mere two years. “We’ve all been a little bit surprised by the capacity of ectothermic species to evolve their way out,” Diamond told me.
But those examples might not be the norm. Even under extreme conditions, many gains in heat tolerance are minimal—less than a degree or so—or excruciatingly slow, and they don’t always pass on easily to future generations. Across a variety of species, heat tolerance seems to be a very finicky trait. In a recent analysis, Joanne Bennett, an ecologist at Australian National University, found that animals have been able to nudge their tolerance for high temperatures only about half as fast as they can alter their tolerance for cold ones.
That inertia probably has something to do with the limits of life’s tolerance for heat. Somewhere in the neighborhood of 110 or 115 degrees Fahrenheit, the chemical reactions that drive cellular life start to fail. With rare exceptions, ectotherms exposed to those sorts of temperatures for prolonged periods of time get sluggish and start to stagger about. Their limbs cease to move, “and then very rapidly, they begin to die,” Masakado Kawata, an evolutionary biologist at Tohoku University, in Japan, told me. Evolution, though powerful, cannot break biochemical rules, says Jhan Salazar, an evolutionary biologist at Washington University in St. Louis. Animals already near their maxima may simply be running out of adaptive steam: “Even if they want to keep going,” Salazar told me, “they cannot.”
But the threshold theory does not solve another puzzle. Some animals, most of them reptiles, appear to have halted the evolution of their heat tolerance well below the theoretical limit, despite living in habitats where weathering more warmth would seem to be a perk. Certain desert lizards are bizarrely bad at being superhot; some lowland anoles are just as wimpy about triple-digit temperatures as their cousins that live above 7,500 feet. Scientists have even found a handful of sun skinks in warmer regions of the rainforest that are somehow worse at tolerating heat than their relatives in cooler climes. In each case, evolution seems to have presented the animals with an obvious lever that they haven’t pulled.
These animals may be tumbling down a different escape hatch. On the rich landscapes of islands, freckled with all manner of rocks and vines and trees, an anole can avoid overheating by just skittering into the shade, Michael Logan, an evolutionary biologist at the University of Nevada at Reno, told me. Unlike evolution, this quick-fix change can work within a single generation, without the rigmarole of sex or the gamble of genetics; in the span of a single day, it can toggle on and off. Behavior, Logan said, is all it takes to “maintain the temperatures they want to be at.”
But hiding from the sun can also mean hiding from the environmental pressures that might otherwise coerce a population to evolve. Saúl Domínguez-Guerrero, a postdoctoral fellow in Muñoz’s lab, thinks that this phenomenon, known as the Bogert effect, is what’s stymieing adaptation among the anoles he’s studying along the outskirts of forests in Puerto Rico and the Dominican Republic. At the edge of the tree line, where sunlight filters down in shards, temperatures have climbed, but many anole species’ heat-tolerance thresholds haven’t really budged in kind. Instead, Domínguez-Guerrero has found, the lizards are coping with the heat by prolonging their stints beneath boulders or leaves, or tucked into the nooks of tree trunks, especially during the hottest hours of the day.
The anoles’ tactic is accomplishing its primary objective: maintaining lizardly chill. But Muñoz worries that the strategy is now cooling the species’ evolutionary engines, too, which will bode poorly if temperatures eventually get too hot for too much of the day. All behaviors have costs. “If an animal is seeking thermal refuge, they’re not doing something else”—foraging for food, wooing a potential mate, Cheng, the Amherst biologist, told me. Once those trade-offs become unsustainable, evolution could kick in. But the lizards will have missed out on years, potentially decades, of adaptation in the meantime. In 2015, a team led by Lauren Buckley, an evolutionary biologist at the University of Washington, modeled that exact effect in eastern fence lizards in the southern United States, whose evolution has already been impeded by a behavioral brake. The researchers found that the animals that had been most reliant on the shade would likely struggle to make up for lost time—and, within a few decades, suffer serious declines. If the goal is to keep up with warming, Buckley told me, evolution can’t afford to be bogged down.
But the other extreme—evolution run amok—exacts its own tax. Closer to the heart of the forests where Muñoz’s team is collecting data, the researchers have found another group of anoles that have increased their heat tolerance a good five degrees, to about 108 degrees Fahrenheit, in just 11 years’ time. They’ve had little other choice: Beneath a canopy that thick, shade is ubiquitous—rendering just about every habitat thermally the same, and forcing the lizards’ evolutionary hand. “They have to conform to whatever the mean temperature is going to be,” Logan told me.
The pace of the lizards’ thermal leap alarms Domínguez-Guerrero. “To be evolving that fast means they are very thermally stressed,” he told me, and their rush to adapt potentially claimed a lot of lives, which may have left the surviving population less genetically diverse. The next time the anoles are forced to evolve at such a blistering clip, they might not have the raw evolutionary material to make it through. In a study published earlier this year, Logan’s lab ran a simulation that supports that notion. The model showed that animals that too rapidly tweak their thermal tolerance end up “so well adapted to current conditions that they lose all the variants that could deal with more variable conditions later on,” Logan told me. That sort of overpruning can sometimes play a major role, he said, in making species go extinct.
No matter what, an animal will eventually reach the end of its evolutionary potential. Some of the fast-evolving anoles in the Caribbean are now knocking right up against the highest thermal tolerances ever recorded among their kind, Muñoz told me. And they probably aren’t alone. At this point, across many ectotherms, in all sorts of global ecosystems, “I think most organisms are actually already pretty close” to the maximum tolerances they can evolve, Buckley told me.
If that’s the case, scientists may struggle to tell. That’s the trick when compensation works well: Animals are often “fine, fine, fine, and then boom, not fine,” Diamond told me, leading to “a really strong crash.” Even at an individual level, animals teetering on that precipice don’t always appear to be in distress. In a cruel biochemical twist, cellular machinery tends to work best just a few degrees shy of the temperatures that incapacitate it—which means that many animals operating at their peak could be steps away from peril. “There is a really small margin of error,” Brooke Bodensteiner, an evolutionary biologist studying with Muñoz at Yale, told me. Among anoles, a single degree Fahrenheit can mean the difference between an animal bolting about at full bore and keeling over. In the end, some of the world’s most heat-adapted animals may be among the first to be lost to climate change, Bennett told me. Already pushed to life’s evolutionary edge, on a planet of ever-greater extremes, they will have nowhere left to go.
