This frog is the world’s smallest known vertebrate

The tiniest adult was just 6.5 millimeters (under 3/10 of an inch) long

A tiny brown frog sits just off center on a Brazilian real coin.

Can you spot the Brazilian flea toad? At just 7 millimeters long on average, this frog has been named the world’s smallest known amphibian and smallest known vertebrate. Here, one is dwarfed by a 27-millimeter (1-inch) wide $1 Brazilian real coin.

W.H. Bolaños, I.R. Dias and M. Solé/Zoologica Scripta 2024

A Brazilian flea toad’s head is too tiny to bear its many crowns.

The critter (Brachycephalus pulex) is neither a flea nor a toad. Instead, it’s a wee frog with two big new titles. It’s just been named the world’s smallest known amphibian and the smallest known vertebrate.

From snout to rump, one adult measured just under 6.5 millimeters. That’s about a quarter of an inch. It’s small enough to sit comfortably on a pinkie fingernail. And it sneaks under the previous record by about half a millimeter.

A team reported this mini male February 7 in Zoologica Scripta.

Mirco Solé is a herpetologist at the Universidade Estadual de Santa Cruz in Ilhéus, Brazil. He’s part of a team that studied 46 adult Brazilian flea toads — 24 males and 22 females. On average, males measure about 7 millimeters (0.27 inch) long. Females measure about 8 millimeters (0.31 inch). Before, the smallest known vertebrate was the Paedophryne amanuensis frog from Papua New Guinea. Males of that species average about 8 millimeters (0.31 inch) long.

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Frogs this teeny have some big changes to their bodies compared with larger species. For instance, the foot of a Brazilian flea toad has just two toes. Most frog feet have five, Solé notes. And some other small Brachycephalus frogs have oddly shaped inner ear tubes. That feature makes them clumsy jumpers. Brazilian flea toad ears haven’t been tested, Solé says. But, he notes, these animals do seem to be decent hoppers.

There are likely even tinier vertebrates not yet discovered, Solé says. After all, Earth has lots of unexplored nooks and crannies. If found, such creatures might reveal new biology related to body size.

Anything smaller than about six millimeters long “would really challenge morphology and physics,” he says. “But who knows.”

Erin I. Garcia de Jesús is a staff writer at Science News. She holds a Ph.D. in microbiology from the University of Washington and a master’s in science communication from the University of California, Santa Cruz.

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