A frog’s lifecycle is likely one of the earliest bits of science that many of us remember learning. They start as eggs, hatch into tadpoles, and soon grow into the recognizable adult amphibians. Pretty simple and straightforward, right?
While that remains true for the vast majority of the planet’s nearly 8,000 known frog species, a handful of the amphibians have evolved a more streamlined reproductive process. Members of the Nectophrynoides genus (more commonly known as tree toads) don’t bother with the egg and tadpole stages. Instead, they give birth to tiny, live “toadlets.”
Nectophrynoides comprise only about one percent of all frogs and toads, but that number is officially increasing. In a study published in the journal Vertebrate Zoology, biologists describe an additional three tree toad species discovered living in the jungles of Tanzania that have this tadpole-less life cycle.
“It’s common knowledge that frogs grow from tadpoles—it’s one of the classic metamorphosis paradigms in biology,” Mark Scherz, study co-author and herpetology curator at the Natural History Museum Denmark, said in a statement. “But [frogs] actually have a wide variety of reproductive modes, many of which don’t closely resemble that famous story.”
To confirm these three newest tree toad species, Scherz and colleagues needed to consult some of the very first Nectophrynoides specimens ever collected. In 1905, German researcher Gustav Tornier announced the discovery of a Tanzanian toad whose live births contradicted common knowledge of the amphibians. Those initial specimens have since spent 120 years in the archives of Berlin’s Museum für Naturkunde. The team visited the museum and collected DNA samples from the original toads using a technique known as museuomics. As its name implies, museuomics is the process of obtaining genomic data from biological material housed in museums.
“Phylogenetic work from a few years ago had already let us know there was previously unrecognized diversity among these toads,” explained University of Copenhagen biologist and study co-author Christian Thrane. “But by travelling to different natural history museums and examining hundreds of preserved toads, I was able to get a better idea of their morphological diversity, so we could describe these new species.”
“Our museomics work was able to reveal exactly which populations those old specimens belonged to, giving us a lot more confidence for future work on these toads,” added University of Potsdam museuomics expert and study co-author Alice Petzold.
The prevailing theory is that tree toads evolved their unique birthing strategy due to their habitats. Simply put, it’s easier to forgo laying eggs for live birth if your home is far from a habitable body of water. Unfortunately, the remarkable evolutionary adaptation is exacerbating the threat of extinction. The Eastern Arc Mountains of Tanzania are one of the most biodiverse regions on Earth, but they’re also disappearing due to deforestation and climate change.
The study’s authors noted that most known tree toads are already on the verge of eradication. One species, Nectophrynoides asperginis, is already extinct in the wild, while Nectophrynoides poyntoni hasn’t been seen since 2003. As wonderful as it is to find new examples of the impressive toads, conservation efforts will be the only way to ensure they can continue breeding new generations of toadlets.
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