While it sounds like a cricket, it's not an insect: It's an extremely small frog.
The recently discovered animal is one of the smallest known vertebrates on Earth.
“We are talking about the limits of life size on Earth,” said LuÃs Felipe Toledo, a herpetologist at the University of Campinas in Brazil.
He and colleagues described the frog in a study published last week in the journal PeerJ, naming it Brachycephalus dacnis after the Dacnis conservation project through which it was spotted.
Dr. Toledo got his first hint about the very tiny frog when a colleague shared several audio recordings of miniature frog species he was collecting in the Atlantic forest of Brazil. As soon as he listened, he knew he was hearing something novel.
“Oh, that's actually two species that you have in your hands,” Dr. Toledo recalls saying.
Brazil's Atlantic forest has many frogs of the Brachycephalus genus, which are known also as saddleback toads. Their penchant for springing around and leaping distances about 30 times their body length has led to the nickname “flea toadlets.”
Because Brachycephalus frogs are so small, though, some different species look very similar and can't be distinguished by sight alone. Yet scientists have found that their mating calls are distinct enough so females will be attracted to the correct male of their species, and even an inexperienced human listener can hear the differences.
“You immediately know that they are different sounds,” said Dr. Toledo, who described the B. dacnis mating call as shorter, with fewer notes, soft in volume and chirpy in sound. “People usually think it's a cricket, not a frog.”
DNA tests provided confirmation that B. dacnis is genetically different from other species in its group.
This makes it the seventh flea-toad species known to science, and the second-smallest vertebrate species known on the planet. One specimen found from another Brachycephalus frog is slightly smaller, at 6.5 millimeters. These differences are almost imperceptible, according to Dr. Toledo, and just a matter of what specimens have been recorded so far — scientists could soon find a specimen from the new B. dacnis species that is just as small too.
“There are untold numbers of unknown tiny frogs out there,” says Mark Scherz , curator of herpetology at the Natural History Museum of Denmark, who was not involved in this study. He added that “these small species have been overlooked previously, by virtue of how hard they can be to find and collect.”
Before these discoveries, the record for the smallest vertebrate was held by the New Guinea Amau frog , at 7 millimeters long, and a freshwater dwarf goby fish, at 7.9 millimeters
Dr. Toledo's team also ran specialized high-resolution CT scans on the toadlet to uncover more about the inner workings of how a frog could still be a frog when it is this miniaturized.
While frogs usually have four fingers on their hands and five on their feet, B. dacnis, like other miniature frogs, has two fingers on its hands and three on its feet. Parts of its inner ears are missing too.
Several species of Brachycephalus also have fused and ossified bones in the cranium, said Célio Haddad from São Paulo State University, who was not involved in the study. But B. dacnis doesn't: Its head is made like a normal-size frog's. “We do not know exactly why this duality occurs,” Dr. Haddad said.
B. dacnis, like all Brachycephalus frogs, is also not subject to the kind of amphibian metamorphosis commonly taught in basic biology classes. It lays just two eggs each reproductive cycle, which hatch into fully formed frogs rather than tadpoles.
While miniaturization makes these frogs vulnerable to tiny predators, like ants and spiders, the trade-off seems worth it from an evolutionary perspective, said Jodi Rowley , curator of amphibians and reptiles at the Australian Museum, who was not involved in the study.
“Miniaturization allows frogs to live in a whole new world that larger frogs simply can't get into,” Dr. Rowley said, like thick leaf litter. In addition to providing shelter from predators, such spaces “are full of very tiny food that bigger frogs can't take advantage of.”
Given that the Brazilian Atlantic forest has been severely deforested, learning more about these tiny frogs aids their conservation.
“So much of our biodiversity, known and unknown, is small and camouflaged and, not surprisingly, often goes unnoticed,” Dr. Rowley said.
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