Even in one of the most densely populated places on Earth, nature is still capable of some big surprises. Biologists have described a new species of leopard frog discovered in New York City.

Only the second new frog species found in the continental United States in the past 30 years, it remained hidden in plain sight in a city of 8.4 million people.

"It's a pretty unique event," said Rutgers University ecologist Jeremy Feinberg, part of a group of researchers who made the discovery.

As reported by National Geographic, Feinberg and colleagues—including Louisiana State University geneticist Catherine Newman, fellow Rutgers ecologist Joanna Burger, University of Alabama biologist Leslie Rissler, and biologist Brad Shaffer of the University of California, Los Angeles—first revealed the existence of the new amphibian two years ago in the journal Molecular Phylogenetics and Evolution.

As the title of that journal suggests, however, they focused their initial work narrowly on the genetic uniqueness of the then-unnamed frog, which until then was considered a southern leopard frog.

Now, in a study published Wednesday in PLOS ONE, they describe what makes the native New Yorker so unique that it deserves a new species designation: Rana kauffeldi—named after the great herpetologist Carl Kauffeld, who in the mid-20th century speculated that an as-yet-unidentified leopard frog might reside in New York City.

Though its skin has subtly distinctive spots, R. kauffeldi's most revealing characteristic is the mating call of the males. The researchers describe it as a "single-note unpulsed chuck," unlike the pulsing and snore-like calls of the region's other leopard frog species.

Those calls are what led the researchers to the new frog, said Feinberg. While conducting southern leopard frog field studies, every so often they'd hear the unusual "chuck" sound above the pulses. Eventually they realized that the two calls rarely occurred in the same habitat.

Closer examination showed that R. kauffeldi predominated in open-canopied coastal marshes, "places where you can almost see and smell the ocean," said Feinberg, as well as bottomland floodplains within a few miles of river mouths.

That they heard mating calls at all was fortunate: R. kauffeldi breeds for just a few weeks each year. Within that brief time their chorus is often drowned—at least to our ears—by the sound of spring peepers.

"That helps keep them hidden," said Feinberg. "You have to win the jackpot to hear them."

Call of a Survivor

Since R. kauffeldi's first description two years ago, quite a few people have been listening closely enough to win that jackpot. Many reports of hearing the call have been filed, extending the species' range in a coastal ribbon from Connecticut to northeastern North Carolina.

Much of the newly found frog's habitat, however, has already been lost to development. That's especially true in New York City. Likely once found throughout the region, R. kauffeldi is now restricted to the borough of Staten Island, where Feinberg first discovered them and where wetland development is an ever-present threat.

"There's one population in Staten Island where all it would take is filling in one pond, and it would be gone," said Feinberg. What habitat does remain tends to be fragmented, producing isolated populations that may lack the genetic diversity necessary for long-term health.

Still, they've stuck around this long—and on a positive note, it appears that R. kauffeldi may be able to resist the chytrid fungal disease that elsewhere has caused an amphibian apocalypse. Even as other leopard frog populations in the region have declined or disappeared, said Feinberg, R. kauffeldi has persisted.