Brazilian flea toad may be the world’s smallest vertebrate

A flea toad perches on 1 Brazilian real (the coin has a diameter of 27 millimeters)

Renato Gaïga

A tiny pea-sized Brazilian frog could threaten the current record holder for the world's smallest vertebrate.

The flea toad Brachycephalic pulex (actually a species of frog) was first described by scientists in 2011. Shortly after, Mirco Solé at Santa Cruz State University in Brazil, wondered whether the species could be the smallest amphibian ever discovered. But only a handful of specimens had been collected from the frogs' only known habitat, on two forested hills in southern Bahia, Brazil. And their gonads have not been examined, which is necessary to confirm whether they are adults.

Solé and his colleagues measured the body lengths of 46 flea toads, checking the maturity and sex of the frogs by examining their gonads and checking for vocal slits in their throats, which only males have.

Adult B. pulex males average over 7 millimeters long, slightly smaller than females. This makes them smaller than the males of Paedophryne amauensisa frog from Papua New Guinea that was until now considered both the smallest amphibian and smallest vertebrate.

“It’s absolutely clear,” says Marc Scherz at the Natural History Museum of Denmark in Copenhagen. “These are potentially the smallest extant frogs in the world, which is amazing.”

It's not just the average size that's shocking, but also the size of these flea toads compared to other mini frogs – exemplified by the smallest specimen in the study. “It’s 6.45 millimeters [long], which is 30% smaller than any adult male frog I have ever seen,” says Scherz. “It’s almost a millimeter smaller than the next smallest frog.”

At such small scales, frogs develop strange anatomical quirks, like losing toes or having ears so underdeveloped that they they cannot hear the songs of their own suitors. Some species can barely jump because their balance organs are too small.

Yet there might even be smaller vertebrates yet to be discovered, says Solé. Perhaps the next record holder will be another small frog, or even a parasitic male of a deep-sea anglerfish.

The subjects:

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