A Frog So Small, It Could Not Frog

Most frogs can jump and land with the precision and grace of an Olympic gymnast. And then there’s the pumpkin toadlet.

Brachycephalus mirissimus, a miniaturized frog from the Atlantic Forest of southern Brazil.
Luiz F. Ribeiro

The leap of a frog is a quintessential evolutionary feat. The critter’s girthy gams thrust from behind to springboard the body up and out; a pair of acrobatic arms stretch forward to seamlessly break the fall. The landing is “very precise, very controlled,” says Richard Essner, a biologist at Southern Illinois University Edwardsville. One might expect that any frog worth its salt should be able to stick it. “And most frogs,” says Marcio Pie, a biologist at Edge Hill University, in the United Kingdom, “do.”

Then there is the poor pumpkin toadlet. Spanning roughly a centimeter from snout to bum—about the width of a Skittle—and outfitted with scrawny, toothpick limbs that just barely hold its bulbous body aloft, it is the dachshund of the amphibian world, about as aerodynamic as you might expect. When these little frogs jump, they leap spectacularly, their airborne bodies imbued with all hope. Then their bodies twist and invert, “tumbling, cartwheeling,” Essner told me, some somersaulting head over heels, others pirouetting in an almost rotisserie-esque spin. In their final descent, the toadlets sometimes reach for a handhold, but the effort is for naught. They crash to the ground, arms akimbo, landing not on their forelimbs with grace, but on their butt, their belly, their back, their head, in bouncing-beach-ball defeat. “They fall on their face all the time,” Pie told me, stifling a laugh. “It’s sad.” (He assured me that the fumbles don’t seem to hurt the frogs.)

Essner, Pie, and their colleagues recently cooked up a euphemism for this flippy faux pas: “uncontrolled landing.” And they caught it several times on film. What hamstrings the frogs’ flights doesn’t seem to be a lack of brawn or grit. “The muscles work just fine,” Pie said. Instead, their klutziness seems to be a sensory deficit, rooted in the frogs’ absurdly tiny inner ear—the seat of their vestibular system, which coordinates movement and balance. The vestibular structures in these frogs are so smol that they verge on nonfunctional, making it extraordinarily difficult for the amphibians to orient themselves in space while walking, much less maneuver mid-flight. Pie can’t know for sure, but he has a hunch that being a pumpkin toadlet is a deeply discombobulating experience—almost, he said, like being perpetually drunk.

That the pumpkin toadlet has shrunk some of its anatomy out of the realm of utility makes a weird sort of sense. It is, after all, one of the most diminutive frogs in the world, a full-on “miniaturized” animal that evolved to be wee even at maturity. (Pumpkin toadlets, by the way, hatch not as tadpoles, but as half-centimeter-ish-long frogs, already in adult form.) And these toadlets, which tromp about the mountainous cloud forests of Brazil, are found in leafy habitats where larger ones are not, Pie said, suggesting that their petiteness is in some way essential to their survival there. But having a body smaller than that of a honey bee “comes with some pretty clear functional consequences,” Martha Muñoz, a biologist at Yale University, told me. Pumpkin toadlets have shrimpy legs and arms, and fewer fingers and toes than other frogs; their high surface-area-to-volume ratio makes them easily dry out. The teeny toadlets also have all sorts of issues with their head, which can shrink only so much before it malfunctions. At least two pumpkin-toadlet species have inner-ear structures so underdeveloped that the frogs are deaf to their own mating calls.

Their ability to navigate is also quite pitiful. For the vestibular system to function properly, a fluid called endolymph has to traverse the canals of the inner ear, bopping sensory hairs that send electrical impulses to the brain. But when Essner, Pie, and their colleagues scanned the skulls of several species of frogs, they found that, in the pumpkin toadlets, those fluid-conducting tubes were extraordinarily narrow. They are among the smallest vestibular structures ever documented in a full-grown vertebrate. “We had to measure them in micrometers,” Pie told me. Which makes it quite difficult for liquid to flow at adequate speeds when the frogs are rotating and rocketing about—as they are wont to do mid-flight, while they’re spinning “really fast,” says Marguerite Matherne, a mechanical engineer at Northeastern University who has studied animal motion. Like syrup caught in a pinched straw, “it’s harder for the fluid to accelerate along with their head.”

Video of a frog jumping
Brachycephalus coloratus (left) and Brachycephalus pernix (right), two species of pumpkin toadlets, trying and failing to land their jumps. To compel the frogs to leap, researchers tapped behind them or directly nudged them with a straw. (Richard Essner / Southern Illinois University Edwardsville)

When the researchers filmed a fleet of frogs hopping about in the lab, they saw the consequences of that stymied flow play out. While species with adequately sized inner ears bounded happily back and forth, always landing firmly on their forelimbs, the pumpkin toadlets careened like kites in a hurricane. In the team’s trials, the little frogs landed on their back more than a third of the time, sometimes bouncing so vigorously upon touchdown that they rocketed back up and inverted yet again. “Oh my God, the derpiness,” Muñoz said.

The toadlets’ problem, Essner told me, isn’t really with takeoff; they seem to manage that part okay. Once aloft, though, they can’t get their bearings, and fail to course-correct. The situational awareness they lack is what divers and gymnasts use to subtly reposition themselves mid-jump. In its absence, though, the toadlets “have no idea what their bodies are doing,” Matherne told me. “It has no feedback to know the best way to stabilize itself.” Their aerodynamic mistakes add up until the toadlets crash at the end of their doomed forward vault.

Landings this catastrophic have been documented only a few times among amphibians before, in frogs that, sadly, had their vestibular systems deliberately disrupted. For Essner and Pie, that comparison speaks to just how disoriented pumpkin toadlets must be. So maybe it’s not entirely surprising that the frogs don’t seem to like to jump—looking “kind of grumpy,” Essner said, when they do—and deploy the tactic only as a last-ditch escape. It does work: The toadlets’ clumsy cavorts, coupled with their peewee size and yen for hiding among dead leaves, can make them infuriatingly difficult to snare. “Two people looking all day might just catch one frog,” Pie told me. The toadlets, though, prefer to simply walk to get around, and do so painfully slowly, “almost chameleon-like,” Essner said. It may be the only way they can avoid overtaxing their puny inner ears.

I asked Essner, who’s made a career out of studying jumping, if he thought the costs of miniaturization seemed worth it—if there was even a point to a terrestrial frog that couldn’t properly hop. “Boy, I have gotten that question a lot,” he said. But Essner isn’t so worried, especially because he’s recorded a couple of other species that are wildly incompetent at touching down, managing only belly flops galore. “And they’re fine,” he said.

The pumpkin toadlets, too, get by—even thrive. On the tropical forest floor, there are plenty of insects for them to nosh on; at least a few species are toxic, and colored in bright warning hues to keep predators at bay. Even their jumps aren’t that bad, Muñoz points out: “They still rapidly put distance between themselves” and whatever’s after them. It’s true that, in botching their landings, they’ve largely eschewed what most people might assume makes a frog fundamentally frog. But maybe the true marvel is that the toadlets have figured out a way to live without the signature hoppity-hop, all while embodying one of life’s extremes. The frogs may represent some of the tiniest amphibians that nature has ever, and will ever, produce; any littler, and there’d basically be no vestibular system at all. “It’s very possible,” Pie said, “that you can’t get a functioning frog smaller than that.”