Six New Miniature Frog Species Discovered in Mexico

The amphibians are found under leaf litter and hatch fully grown

An image of a mini tan frog sitting on top of a Mexican coin.
Genetically, each species were different, but the team also found differences in the skull shapes, the levels of bone formation in the skeleton, and the number of warts on the hands and feet. (Pictured: C. cueyatl) Jeffrey W. Streicher

Hidden on forest floors in Mexico, scientists have discovered six new tiny frog species about the size of a thumbnail. At only a half inch long, one species (Craugastor candelariensis) is now considered Mexico's smallest frog. 

The miniature amphibians have gone unnoticed by researchers because of their small stature, neutral brown coloring, and similarity to existing species, a statement explains. Details about the frogs were published last month in Herpetological Monographs.

The six amphibians were categorized in the Craugastor genus. They are known as direct-developing frogs, meaning they skip the tadpole stage and hatch as fully-formed adults, reports Liam James for the Independent. The frogs have not been witnessed hatching yet, but researchers suspect they are less than 10 millimeters, or 0.4 inches long, Live Science's Mindy Weisberger reports.

"Their lifestyle is utterly fascinating," says study author Tom Jameson, a conservation biologist at the University of Cambridge, in a statement. "These frogs live in the dark, humid leaf litter of the forests, which is like a secret world—we don't really know anything about what goes on there. We don't understand their behavior, how they socialize, or how they breed."

Researchers studied 500 frog specimens, originally found in Mexico, from museum collections, per the Independent. The frogs were marked as undefined species in the Craugastor genus. When one of the scientists compared the frogs' genetic data, they found a pattern of multiple undescribed species. The discovery inspired the team to revisit the frogs for updated classification, Live Science reports.

The team conducted additional genetic analysis on several species and built 3-D digital models with CT scans. These techniques allowed researchers to pinpoint tiny differences between the similar-looking frogs, per Live Science. After the analysis, the team confirmed the new species as C. bitoniumC. candelariensisC. cueyatlC. polaclavusC. portilloensis, and C. rubinus

The name C. cueyatl honors the rich human history in the Valley of Mexico because cueyatl means frog in Nahuatl, the language spoken by indigenous people from the area where it was found, a statement explains.

The team also found differences in skull shape, skeletal bone formation, and warts on their feet, Live Science reports.

The species' tiny size places them at the bottom of the food chain, serving as a delicious meal for birds, lizards, small mammals, and other frogs. Chytridiomycosis, a deadly fungal infection, is annihilating frog populations worldwide. Human-caused factors also post the greatest threat to the frogs' survival.

"The real threat to these frogs comes from habitat loss, climate change, and disease," Jameson told Live Science. 

Many mini frogs are only found in small areas, like a single hilltop in Mexico, making them highly vulnerable. For example, C. rubinus was named after garnet mines near the hillside where the amphibian resides. However, mine expansion may endanger and wipe out the species, Jameson explains to the Independent.

Still, the team is hopeful that the unique amphibians will survive. They identified critical areas in Mexico where the species live and plan to work with NGOs and the Mexican government to protect the six new species, reports the Independent.