Is The Flea Toad The Smallest Vertebrate In The World?

Is there a lower limit on vertebrate body size? How small can a vertebrate possibly get?

A team of herpetologists at the Universidade Estadual de Santa Cruz just reported that a tiny frog found only in a hilltop forest in southern Bahia, Brazil, may be the smallest vertebrate in the world. The scientists came to this extraordinary conclusion after capturing, measuring and releasing 46 individual wild Brazilian flea toads, Brachycephalus pulex, and finding that the adult males averaged just 7.1 millimeters (0.27 inches) in length — almost one millimeter smaller than the world’s next smallest frog species.

Adult female flea toads, which grow somewhat larger so they can produce eggs, have a slightly longer body size, averaging just 8.15 mm. This is slightly longer than the previous tiny record-holder: Paedophryne amauensis of eastern Papua New Guinea.

The Brazilian flea toads are 30% smaller than any adult frog previously seen; they are so tiny that one individual is smaller than a pea, two individuals can comfortably perch atop a pinky fingernail, and more than a dozen adult flea toads can fit on the face of a Brazilian One Real coin.

This tiny frog species was previously known to science, having been discovered and scientifically described in 2011 (ref). However, at the time, its discoverers mistakenly thought the frog was a toad, hence its name.

In their study, the researchers captured, measured and released a bunch of the wild frogs to determine how small they are — a seemingly easy task, right? But of course, it’s not that simple.

First, the species seems to live only on two forested submontane hilltops in the southern Bahia region of Brazil. This is the Serra Bonita Reserve, a privately-owned conservation area, so the researchers had to obtain permission before poking around in the forest in search of these frogs.

“Identifying the smallest frog in the world has been no easy challenge,” the authors noted in their paper.

Being so diminutive, these frogs are difficult to find, and their camouflage of mottled brown colors on their back makes them even more difficult to see in the leaf litter, even if you know they’re there.

Further, the researchers had to make sure they were actually measuring adult frogs, so they checked both their gonads and the vocal slits in their throats — which are present only in adult males.

Even in these tiny pea-sized tiny frogs, the males are smaller than the females — a form of ‘sexual dimorphism’ that is seen in 90% of all frog and toad species.

And yet, even though the Brazilian flea toad is the smallest vertebrate yet systematically measured, it may not be the smallest of the small. Another research group previously proposed that the smallest body size that is physiologically possible for vertebrates is just above 6 millimeters in length (ref).

But such a miniature size presents problems. The authors pointed out in their paper that B. pulex has no hard ‘bony elements’ aside from a very hard and thick (hyperossified) skull and spine. Further, they note, most tiny frog species have peculiar traits such as fewer toes than other frogs, or poorly formed ears, suggesting that they may have trouble surviving at all.

“Has the lower limit of vertebrates really been reached?” the authors asked in their paper.

Source:

Wendy H. Bolaños, Iuri Ribeiro Dias, and Mirco Solé (2024). Zooming in on amphibians: Which is the smallest vertebrate in the world? Zoologica Scripta | doi:10.1111/zsc.12654