An international team of researchers have identified three new species of enchanting, pustular, tree-dwelling toads from Africa. Their solution for having offspring away from water? Skipping the tadpole phase altogether, and giving birth to live toadlets. The study is published today in the open access scientific journal Vertebrate Zoology.
Most textbooks will tell you only one story of frog reproduction: Eggs to tadpoles to froglets to adults. But for three newly discovered species found in Tanzania this is not the case. The three new species of frogs belong to an unusual group of African toads in the genus Nectophrynoides - commonly called "Tree Toads."
Instead of laying eggs that hatch into tadpoles, the female Tree Toads carry their offspring inside their bodies and give birth to fully formed, tiny toads. This makes them among the very few amphibians in the world capable of internal fertilization and true live birth.
"It's common knowledge that frogs grow from tadpoles-it's one of the classic metamorphosis paradigms in biology. But the nearly 8000 frog species actually have a wide variety of reproductive modes, many of which don't closely resemble that famous story" says Assoc. Prof. Mark D. Scherz, Curator for Herpetology at the Natural History Museum Denmark, a coauthor on the study.
Only a handful of frog species from South America and southeast Asia have developed similar strategies making these toads a rare case in the animal kingdom.
"Live-bearing is exceptionally rare among frogs and toads, practiced by less than 1% of frogs species, making these new species exceptionally interesting," says H. Christoph Liedtke a co-author from the Spanish National Research Council, who has specialized in the evolution of amphibian reproductive modes.
120-year-old frog DNA
Back in 1905, a German researcher, Gustav Tornier, presented to the Royal Prussian Academy of Sciences in Berlin, the discovery of a toad from Tanzania that, astonishingly, gives birth to live young. At the time, it was the only known species of frog in the world to do so.
The frogs originally found by Tornier are today housed at the Museum für Naturkunde in Berlin, and the research team were able to secure DNA from the original frogs using methods collectively known as 'museomics'.
"Some of these specimens were collected over 120 years ago. 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," says Dr Alice Petzold of the University of Potsdam, who carried out the museomics portion of the study,
"Phylogenetic work from a few years ago had already let us know there was previously unrecognised diversity among these toads. 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," says Christian Thrane from University of Copenhagen and first author on the study.
Protecting endangered species
Beyond its evolutionary interest, the discovery could have important implications for conservation. Many of these live-bearing toads inhabit small, fragmented habitats and are under threat from deforestation, mining, and climate change.
The new species are from the Eastern Arc Mountains (EAM) of Tanzania, an imperiled biodiversity hotspot famed for the many species that are found nowhere else on Earth. These mountains that rise from the plains are cloaked in lush forests, but Dr Michele Menegon, another coauthor on the study who works for a conservation organisation whose work focuses on forest protection, notes how highly fragmented these habitats are and that this is impacting the biodiversity, including the toads that dwell there.
Her colleague from the University of Dar es Salaam in Tanzia raises the same concern.
"The forests where these toads are known to occur are disappearing quickly," says John V. Lyarkurwa, a researcher from the University of Dar es Salaam, who has been studying amphibians in the EAM, including these toads, and a coauthor on the study.
Most of the Tree toads are already on the brink of extinction, with one species in this genus, Nectophrynoides asperginis, already extinct in the wild, and another Nectophrynoides poyntoni not observed since its discovery in 2003. The future of these beautiful toads is very uncertain.
The full paper is available at https://doi.org/10.3897/vz.75.e167008
