A little less than 12,000 years ago, rising sea levels split mainland Australia from the island of New Guinea to the north. Though New Guinea is now covered in rainforest and northern Australia is primarily savannah, the millions of years they spent together mean that certain plants and animals still live in both regions.
But for the most part, common northern Australian species spotted in New Guinea haven’t been spotted all over New Guinea—except for green tree frogs (Litoria caerulea). Over the years, multiple isolated populations of green tree frogs have been discovered both north and south of New Guinea's Central Cordillera, the mountain ranges that run from one end of the island to the other.
One theory to explain how the frogs spread so far and wide is that humans unwittingly transported them to various areas—say, in shipments of fruit, which has been known to happen. To try to pin down how and when green tree frogs arrived in New Guinea, Australian researchers went to study the frogs themselves. And in the course of that study, as Axios reports, they discovered that some of New Guinea’s green tree frogs might not actually be green tree frogs. For one thing, they weren’t green: They were brown.
The study, published in the May 2021 issue of the Australian Journal of Zoology, details the many minute morphological distinctions between green tree frogs and this previously unrealized species, which the researchers first spotted in 2016 and nicknamed the “chocolate frog.” Its official name is Litoria mira, from the Latin word mirum, meaning surprise. Surprise is, unsurprisingly, what they felt when they first saw this unexpectedly brown frog.
While the researchers believe that they’re the first to identify the chocolate frog as a new species, they don’t think they’re the first to find it. A 1968 study identified several New Guinea frog specimens as Litoria caerulea, but didn’t describe them, and these researchers think those frogs were probably Litoria mira.
As for whether any chocolate frogs hitched a ride on a fruit crate, the researchers no longer think so. Small genetic differences between populations north and south of the mountains suggest that they were already there before humans started interfering. Moreover, humans don’t muck around those areas very often anyway.
“Because the frog lives in very hot, swampy areas with lots of crocodiles, all these things discourage exploration,” Steve Richards, a researcher at the South Australian Museum and co-author of the study, told CNN.