The Brazilian flea toad may be the world’s smallest vertebrate

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

Scientists have bestowed the frog — which is native to Brazil but is neither a flea nor a toad — with two titles: The world’s smallest known amphibian and smallest known vertebrate. From snout to rump, one Brachycephalus pulex measures just under 6.5 millimeters, herpetologist Mirco Solé and colleagues report February 7 in Zoologica Scripta. That’s roughly half a millimeter shorter than the previous record holder and small enough to sit comfortably on a pinkie fingernail.

Solé, of the Universidade Estadual de Santa Cruz in Ilhéus, Brazil, and colleagues discovered the diminutive male amphibian among 46 adult Brazilian flea toads — 24 males and 22 females. On average, males measure about 7 millimeters long and females measure about 8 millimeters, the team reports. Their predecessor, male Paedophryne amauensis frogs from Papua New Guinea, averages about 8 millimeters long.

Frogs this minuscule have big changes to their bodies compared with their larger counterparts. For instance, Brazilian flea toad feet have just two toes instead of the typical five, Solé says. Some other small Brachycephalus frogs have oddly shaped inner ear tubes, making them clumsy jumpers (SN: 6/15/22). Brazilian flea toad ears haven’t been tested, Solé says, but the animals seem to be decent hoppers.

With many of Earth’s nooks and crannies still unexplored, there are likely even tinier vertebrates waiting to be discovered, Solé says. Anything smaller than about six millimeters long “would really challenge morphology and physics,” he says. “But who knows.”