In the lush landscape of Bahia, Brazil, biologists made a discovery that challenges the limits of the animal kingdom. The Brazilian flea toad, a creature so tiny it can perch comfortably on a coin, has been crowned the world’s smallest vertebrate. It is also the world’s smallest known amphibian.

The tiniest of the tiny

It all started over a decade ago when Brachycephalus pulex, commonly known as the Brazilian flea toad, was first identified in 2011. Despite its name, this is a frog, not a toad. Toads generally have drier, bumpier skin and prefer drier environments, while frogs have smoother, moister skin and are more commonly found near water.

Since the flea toad was first discovered, more specimens were gathered over the years. Researchers meticulously measured both male and female specimens of the frog endemic to Brazil’s Atlantic coast. Males, on average, measure a mere 7.10 millimeters in length, surpassing the previous record holder, the Paedophryne amauensis frog, by a fraction of a millimeter.

Both frogs live in the hot tropics, and this is no coincidence.

Astonishingly, the smallest Brazilian flea toad specimen measures only 6.45 millimeters long, really pushing the limits of what’s considered physically possible for vertebrate animals. For comparison, the blue whale, the largest vertebrate in the world, measures about 25.8 meters — at least 3,600 times larger than the flea toad.

Adaptation with some challenges

According to Simon Loader, a zoologist at the Natural History Museum London, living in the tropics is to be expected. This distribution is linked to the challenges smaller animals face in regulating their body temperature in colder environments. Smaller animals tend to lose heat faster and thus struggle to maintain their body temperature more than animals with larger volumes. Brookesia nana, a nano-chameleon, (20 millimeters) and the bumblebee bat (wingspan of 145 millimeters), along with other tiny vertebrates, all live in warm habitats.

Moreover, the study highlights how extreme miniaturization can lead to unique physical adaptations. The Brazilian flea toad lost its digits and developed extra bony structures on their skulls and vertebrae. A V-shaped pattern on its chest with dark brown stripes makes this frog easily recognizable if you can spot it, of course.

However, these adaptations come with their own set of challenges. For instance, some frogs in the Brachycephalus genus struggle with mobility and hearing due to their unusual inner ear structures. These evolutionary compromises underscore the delicate balance of nature and the incredible diversity of life forms. Brazilian flea toads haven’t been formally tested for balance, although the researchers anecdotally report they’re decent hoppers.

There are still many nooks and crannies to explore, especially in dense rainforests. Anything smaller than about six millimeters long “would really challenge morphology and physics,” said lead author and herpetologist Mirco Solé of the Universidade Estadual de Santa Cruz in Brazil. “But who knows.”

The findings appeared in the journal Zoologica Scripta.