A frog so small it can’t frog

A frog so small it can’t frog

A frog so small it can’t frog

A frog’s jump is a typical evolutionary achievement. The beast’s miserly gams pushed from behind to make the body jump up and down; a pair of acrobatic arms stretch forward to break the fall seamlessly. The landing is “very accurate, very controlled,” said Richard Essner, a biologist at Southern Illinois University Edwardsville. You’d expect any frog worth its salt to be able to hold it. “And most frogs,” says Marcio Pie, a biologist at Edge Hill University in the United Kingdom, “to do

Then there’s the poor pumpkin toad. At about an inch long from snout to butt — about the width of a cone — and fitted with skinny toothpicks that barely hold up its bulbous body, it’s the dachshund of the amphibian world, about as aerodynamic as you’d expect. When these little frogs jump, they jump spectacularly, their bodies in the air drenched with all the hope. Then their bodies twist and turn, “tumbling, cartwheeling,” Essner told me, some headlong flips, others pirouettes in an almost rotisserie-like twist. In their last descent the toads sometimes grab something to hold on to, but the effort is for nothing. They fall to the ground, arms folded and land gracefully not on their front legs, but on their rears, their stomachs, their backs, their heads, in a bouncing beach ball defeat. “They fall on their faces all the time,” Pie told me, suppressing a laugh. “It is pathetic.” (He assured me the fumbles don’t seem to work) hurt the frogs.)

Essner, Pie and their colleagues recently came up with a euphemism for this crazy faux pas: “uncontrolled landing.” And they captured it on film several times. What paralyzes the frogs’ flights doesn’t seem to be a lack of muscle or grit. “The muscles are working fine,” Pie said. Instead, their clumsiness seems like a sensory deficiency, rooted in the frogs’ absurdly small inner ear—the seat of their vestibular system, which coordinates movement and balance. The vestibular structures in these frogs are: so smol that they are nearly non-functional, making it extremely difficult for the amphibians to orientate themselves in space while walking, much less maneuver mid-flight. Pie isn’t sure, but he feels that being a pumpkin toad is a very confusing experience—almost, he said, like being intoxicated all the time.

That the pumpkin toad has shrunk some of its anatomy from the realm of utility makes some weird sort of sense. After all, it is one of the tiniest frogs in the world, a completely “miniaturized” animal that evolved to become tiny even in adulthood. (By the way, pumpkin toads don’t hatch as tadpoles, but as half a centimeter long frogsalready in adult formAnd these toads, which forage through Brazil’s mountainous cloud forests, are found in leafy habitats where larger ones aren’t, Pie said, suggesting their smallness is somehow essential to their survival there. But having a body smaller than a honeybee’s “has some pretty obvious functional ramifications,” Martha Muñoz, a biologist at Yale University, told me. Pumpkin toads have shrimp legs and arms and fewer fingers and toes than other frogs; due to their high surface-to-volume ratio, they dry out easily. The tiny toads also have all sorts of problems with their heads, which can only shrink so much before they malfunction. At least two species of pumpkin toads have inner ear structures that are so underdeveloped that the frogs are deaf to their own mating sounds.

Their ability to navigate is also quite pathetic. For the vestibular system to function properly, a fluid called endolymph must cross the canals of the inner ear, shedding sensory hairs that send electrical impulses to the brain. But when Essner, Pie and their colleagues scanned the skulls of several frog species, they found that those liquid-conducting tubes in the pumpkin toads were extraordinarily narrow. They are among the smallest vestibular structures ever documented in a fully grown vertebrate. “We had to measure them in micrometers,” Pie told me. That makes it quite difficult for fluid to flow at sufficient rates when the frogs are spinning and darting around — as they are used to in flight, while spinning “really fast,” said Marguerite Matherne, a mechanical engineer at Northeastern University who studies the motion of studied animals. Like syrup caught in a squeezed straw, “it’s harder for the liquid to accelerate along with their heads.”

Video of a frog jumping
Brachycephalus coloratus (left) and Brachycephalus pernix (right), two species of pumpkin toads, trying not to land their jumps. To force the frogs to jump, researchers tapped after them or pushed them directly with a straw. Credit: Richard Essner, Southern Illinois University Edwardsville

When the researchers filmed a fleet of frogs hopping around the lab, they saw the effects of that obstructed flow play out. While species with sufficiently large inner ears jumped happily and always landed firmly on their front legs, the pumpkin toads flew like kites in a hurricane. In the team’s trials, the little frogs landed on their backs more than a third of the time, sometimes bouncing so vigorously on landing that they shot up again and vice versa. “Oh my god, the folly,” said Muñoz.

The toads’ problem, Essner told me, isn’t really in the takeoff; they seem to manage that part well. Once up, however, they are unable to orient and fail to correct course. The situational awareness they lack is what divers and gymnasts use to subtly reposition themselves mid-jump. But in their absence, the toads 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 Toads crash at the end of their doomed forward leap.

Such catastrophic landings have been documented only a few times before in amphibians, in frogs whose vestibular systems were, unfortunately, deliberately disrupted. For Essner and Pie, that comparison speaks to how disoriented pumpkin toads must be. So perhaps it’s not entirely surprising that the frogs don’t like to jump — they look “a little cranky,” Essner said, when they do — and use the tactic only as a last-ditch escape. It works: The toads’ clumsy cavorts, combined with their peewee size and yen to hide among dead leaves, can make them insanely difficult to snare. “Two people watching all day might only catch one frog,” Pie told me. The toads, however, prefer to just walk to get around, doing so painfully slowly, “almost chameleon-like,” Essner said. It may be the only way they can avoid overloading their puny inner ears.

I asked Essner, who has made a career out of studying jumping, if he thought the cost of miniaturization seemed worth it — if there was even a point for a terrestrial frog that couldn’t jump well. “Boy, I’ve been asked that question many times,” he said. But Essner isn’t too concerned, especially since he’s captured a few other strains that suck a bit to the touch, managing only belly flops in abundance. “And they’re all right,” he said.

The pumpkin toads also survive – even thrive. On the tropical forest floor there are plenty of insects for them to sniff; at least a few species are poisonous and colored in bright warning shades to keep predators at bay. Even their jumps aren’t that bad, Muñoz notes, “They still quickly put distance between themselves” and whatever is chasing them. It is true that by ruining their landings they have largely shunned what most people would think makes a frog fundamental frog† But perhaps the real miracle is that the toads have found a way to live without the signature hoppity hops, while embodying one of life’s extremes. The frogs may represent some of the tiniest amphibians nature has ever and ever will produce; each smaller, and there really wouldn’t be a vestibular system at all. “It’s entirely possible,” Pie said, “you can’t get a functioning frog smaller than that.”