The World’s Smallest Vertebrate Is a Tiny Brazilian Frog, Study Finds

Adult male Brazilian flea toads are just over 7 millimeters long on average, and females measure about 8.15 millimeters

A frog sitting on a coin
A Brazilian flea toad sits on a Brazilian real. The coin is 27 millimeters across. Renato Gaiga

A tiny frog endemic to the state of Bahia on Brazil’s Atlantic coast may have just earned the title of the smallest vertebrate in the world.

The Brazilian flea toad, or Brachycephalus pulex, was first discovered in 2011—and despite its name, it is technically a frog, not a toad. Researchers immediately suspected the creature could break the record for the tiniest frog—and vertebrate—on Earth, but it wasn’t until a new study that scientists systematically measured the lengths of the species’ males and females.

The male frogs, which are smaller, measured 7.10 millimeters long on average, the team reported this month in the journal Zoologica Scripta. An earlier study of the frog Paedophryne amauensis, the previous record holder, found that its males averaged 7.70 millimeters in length.

“It’s absolutely clear,” Mark Scherz, an evolutionary biologist at the Natural History Museum of Denmark who did not contribute to the findings, tells New Scientist’s Jake Buehler. “These really are potentially the smallest extant frogs in the world, which is astonishing.”

The world’s smallest species tend to live in warmer areas, Simon Loader, a zoologist at the Natural History Museum London, told the Guardian’s Patrick Greenfield earlier this month. “This pattern is to do with surface area and volume: Smaller animals lose heat faster and struggle to maintain their body temperature when it is very cold.”

Ultra-small animals can also possess some unusual physical characteristics. Miniaturized frogs tend to have lost fingers and toes, grow extra bony material on their skulls and vertebrae and lack other bony elements, per the study.

Some Brachycephalus frogs, relatives of the flea toads, can have trouble jumping because of strangely shaped inner ear structures, according to Science News’ Erin Garcia de Jesús. Underdeveloped ears also make it hard for some frogs to hear the songs of potential mates, writes New Scientist.

brown frog on a dime
The formerly recognized smallest known frog, Paedophryne amauensis, on an American dime with a diameter of 18 millimeters. E. N. Rittmeyer et al., PLOS ONE (2012) under CC BY 2.5

For a long time, scientists thought the world’s smallest frog was the 8.6-millimeter-long Brazilian gold frog, discovered in the 1970s, per the Natural History Museum in London. But in 2012, researchers determined that P. amauensis males, native to Papua New Guinea, were not only the smallest frogs, but the smallest vertebrates—even littler than Paedocypris progenetica, a Southeast Asian fish species that had previously worn the crown, National Geographic’s Christine Dell’Amore wrote at the time.

The previous year, researchers had reported the discovery of the Brazilian flea toad in the Serra Bonita mountains in Bahia, noting it was missing fingers and toes and bore a v-shaped mark on its chest, with a dark brown stripe on each side.

But that 2011 study didn’t determine the maximum lengths of the male and female flea toads, and it didn’t compare their lengths to those of other tiny frogs, the authors of the new study write.

In their paper, the team measured 24 male and 22 female flea toads, determining their sex by looking at their reproductive organs, or gonads, and identifying whether they had vocal slits, which denote a male. Checking the gonads also helped confirm that the frogs were adults, and thus fully grown.

The female flea toads were about a millimeter longer than the males on average, measuring about 8.15 millimeters long, compared to the 7.10-millimeter males. Female frogs and toads are usually larger than their male counterparts, since larger sizes are tied to reproductive success, the study authors write.

Luci Kirkpatrick, an ecologist at Bangor University in Wales who was not involved in the study, tells the BBC’s Sarah Bregel that it’s important to track tiny animals, because they are “critical in making sure the food web functions properly.”

Though the tiny flea toads seem to have the record for now, smaller vertebrates could still be discovered, Mirco Solé, a co-author of the study and herpetologist at State University of Santa Cruz in Brazil, tells New Scientist.

The smallest individual male flea toad measured in the study was 6.45 millimeters long. An animal smaller than six millimeters “would really challenge morphology and physics,” Solé tells Science News. “But who knows.”

Of course, length is not the only measure of an animal’s size. “If we take it as a linear measure, the frogs are currently clear winners,” Scherz says to the BBC. “But if we go by mass or volume, then fish would likely claim the title, as they are very narrow-bodied and slender, whereas the frogs are rather round.”

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