SCIENTIFIC EDUCATIONAL CENTER science idea

The bright orange amphibian, which shines green under UV light, is different from other pumpkin toadlets due to its appearance and call

Pumpkin toadlets look exactly like what their name suggests. Less than half an inch-long, these tiny, orange frogs hop around the sweltering forests along Brazil’s Atlantic coast. But how many species of these frogs are there? The question isn’t just important to biology, but for conservationists seeking to preserve unique rainforest amphibians.

To researchers, pumpkin toadlets belong to the genus Brachycephalus. Determining how many Brachycephalus species exist, however, isn’t easy. As many as 36 have been named, but researchers sometimes disagree on which species are valid or which species a particular population of frogs should be assigned to. Different populations of these frogs look very similar to each other, not to mention that their genetic makeup only varies slightly.

No one line of evidence can distinguish different pumpkin toadlet species from each other. Instead, researchers have to use an integrated approach that involves genes, gross anatomy and natural history, looking at everything from frog skeletons to their songs. That’s what led Universidade Estadual Paulista herpetologist Ivan Nunes and colleagues to name the tiny Brachycephalus rotenbergae as a new species in PLOS ONE today.

Some pumpkin toadlets live in the Brazilian state of São Paulo, and, Nunes says, biologists working there suspected that the local Brachycephalus species was new. The orange frog has a rounded snout, dark spots on parts of the skull and a specific acoustic profile to its chirps that are different from already-named pumpkin toadlets. Last year, Instituto Nacional da Mata Atlântica herpetologist Thais Helena Condez highlighted the frog as a new, then-unnamed species in a paper on pumpkin toadlets that used genetic sampling. “The new study follows our first evidence,” Condez says, “and shows an integrative approach considering distinct information based on genetics, morphology and bioacoustics.”

The new pumpkin toadlet species isn’t quite like a leopard frog that you might see swimming through a pond. Its body is short and squat, or what experts refer to as “bufoniform” or toad-like. The amphibian also has bony plates attached to its skeleton along the skull and back, capped with what anatomists call roofing bones that sit beneath that skin and have a roughened texture to them. Rather than living at the water’s edge, the new pumpkin toadlet is mostly active during the day on the forest floor. And its bright colors might be even more important than its song for communicating with other frogs.

The new species even glows under ultraviolet light. Just like other pumpkin toadlet species, parts of the frog seem to shine a neon green when flashed with UV light. Exactly why these frogs have evolved this ability is unclear. “There’s an idea that fluorescence acts as signals for potential mates, to signal to rival males or some other biological role,” Nunes says, but more studies are needed to understand why.

Even without UV light, though, the new pumpkin toadlet is strikingly colored. That might be a telltale sign of toxins. While not yet studied in the new species, other frogs in the same genus carry poisons in their skins called tetrodotoxins. This is a potent defense—shared by other animals like pufferfish and blue-ringed octopus—that triggers a range of symptoms from a pins-and-needles feeling to convulsions, heart attack and even death. Given that the presence of the toxin seems to be associated with bright, “don’t eat me” coloration, the new species likely carries the same defense.

Brachycephalus rotenbergae is far from the last pumpkin toadlet, or previously unknown amphibian species, to be found. “We have a lot of cryptic species to be discovered,” Nunes says, many of them likely dwelling in the forests of Brazil. Identifying them will likely rely on an integrated approach like the one used to name the new pumpkin toadlet, Nunes notes, especially because two species can look the same while having different genetics.

Twining these different lines of evidence will be critical to untangling the identity of these frogs. New species may be found, Condez says, but frogs thought to be two species might also be the same. Genetic clues are often used to cut through these conundrums, detecting relationships that might otherwise be difficult to detect.

Now that Brachycephalus rotenbergae is recognized as a new species, researchers will have to keep an eye on this frog’s future. The forests the frog calls home are within the São Francisco Xavier Government Protected Area and, for the moment, the frogs don’t seem especially rare or prone to extinction. But that might change. While the forests are protected from human development, feral boars have become a major problem in the area. Wild boars rooting around and tearing up the soil might disturb or otherwise damage the toadlet’s habitat, not to mention the understory homes of as-yet-undescribed species in the area. That’s just one subject Nunes and colleagues hope to study now that the new toadlet has been recognized, giving researchers all the more reason to hop back into the field.

Image - The new species of toadlet, Brachycephalus rotenbergae, opens its mouth in a defensive posture. (Edelcio Muscat)

smithsonianmag.com