Evolutionary Science

Miniature frogs set record as first vertebrates to lose the ability to balance

Researchers have shown that a group of frogs have become so small they’re no longer able to maintain balance during quick maneuvers, like jumping.

June 22, 2022
The Scitech

Amphibians are exceptionally good at being small. There are salamanders the size of your thumb nail, pygmy newts that live in moss patches and feast on microscopic insects, and inch-long African frogs that spend their entire lives in and around the banks of small puddles. In fact, the title for the world’s smallest vertebrate is currently held by a species of frog that could perch comfortably on the head of a pencil eraser.

A new study published in the journal Scientific Advances shows that all this downscaling doesn’t come without serious drawbacks. In an evolutionary first, researchers have shown that a group of frogs alternately called pumpkin toadlets and flea toads have become so small they’re no longer able to maintain balance during quick maneuvers, like jumping.

The researchers studied four species in the genus Brachycephalus, showing that these miniature frogs are able to jump when gently prodded, but invariably lose control of their trajectory mid-leap, ungracefully pirouetting through the air before coming in for a crash-landing.

“They’re not great jumpers, and they’re not particularly good walkers either. They sort of stomp around in a stilted, peg-like version of walking.” said study co-author Edward Stanley, director of the Florida Museum of Natural History’s Digital Discovery and Dissemination Laboratory.

Researchers were able to pinpoint the likely cause of this impractical mode of transportation through oVert, a four-year initiative among 18 institutions to create to create 3-D models from CT scans for more than 20,000 museum specimens across the U.S.

Co-author Amber Singh stitched together hundreds of frog scans while working with the oVert project as an undergraduate student at the University of Florida. Pumpkin toadlets stand out in these scans as having the smallest vestibular system yet measured in any vertebrate organism.

The vestibular system is a fluid-filled network of spiraling chambers inside the inner ears of animals that looks a bit like a nautilus. When an animal moves its head, the liquid in these chambers moves along with it, brushing against tiny hairs that send electrical impulses to the brain that allow it to maintain balance, tell up from down and detect acceleration.

Natural selection has placed strict constraints on the vestibular system, however. Baleen whales, a group that includes the largest mammals on Earth, have chambers that are only slightly larger than those of humans. This doesn’t seem to be much of an issue for large vertebrates, but for organisms on the smaller end of the size spectrum, the vestibular system takes up proportionally more and more space inside the skull. The chambers within the skulls of pumpkin toadlets look like overinflated balloons.

“Even though the canals are as big as they can possibly be relative to their heads, they’re still not big enough for the liquid to move at a rate that would allow them to maintain balance,” Stanley said.

Source: University of Florida news release. Author: Jerald Pinson