Newly identified species of Tanzanian tree toad leapfrog the tadpole stage and give birth to toadlets

By Mindy Weisberger, CNN

(CNN) — Scientists have newly described three extraordinary species of tree toad that leapfrog over the egg-to-tadpole stage. The females give birth on land to dozens of toadlets, each measuring just a few millimeters long.

Live birth, or skipping the egg-laying and larva stage, is extremely rare in amphibians. Among more than 4,000 species of frogs and toads, fewer than 1% are viviparous, or bear live young. This strategy may have evolved as an adaptation in habitats lacking easy access to water where frogs and toads typically lay their eggs, the study authors reported.

“Describing these new species that give birth to live young is fascinating and helps us understand the evolutionary flexibility of amphibians, one of the most diverse and ecologically sensitive groups of vertebrates,” said Dr. Diego José Santana, curator of amphibians and a conservation ecologist at Chicago’s Field Museum of Natural History. He was not involved in the new research.

The three varieties of tree toad from Tanzania were previously classified as one species: Nectophrynoides viviparus. However, when scientists analyzed the physical traits and genetic data of hundreds of museum specimens, along with vocal recordings of toads in their habitats, they determined that the three types of toad were separate species in the Nectophrynoides genus.

These newly described species provide researchers with a clearer picture of the diversity in tree toads that bear live young and could help to conserve them. Some wild populations in several Nectophrynoides species are declining, and species with smaller ranges are especially vulnerable to disruptions from deforestation and the climate crisis, according to the study.

“Further surveys are needed to fully understand the distribution, ecology, and possible population trends in these areas to inform future conservation strategies,” the authors wrote.

Voices, toes and glands of tree toads

Toads that reproduce in water lay an impressive number of eggs — in some cases about 20,000 in a single clutch.

“They can do that every couple of weeks,” said Dr. Mark D. Scherz, lead author of the study published November 6 in the journal Vertebrate Zoology.

“They lay those eggs and they develop into teeny, tiny tadpoles, of which only a fraction ever make it out of the pond,” Scherz, curator of herpetology at the Natural History Museum of Denmark and an associate professor of vertebrate zoology at the University of Copenhagen, told CNN.

Live birth, in which the eggs are fertilized internally, by comparison, yields toad broods that are significantly smaller: around 40 to 60 toadlets. “The maximum we’ve ever recorded was, I believe, over 160 fully developed toadlets inside one individual,” he added.

It’s the larger and more developed young that may have an advantage when they enter the world, whether hatched from eggs or born alive, Scherz noted.

In 1905, German herpetologist Gustav Tornier found and described the first viviparous toad species, N. viviparus, in Tanzania. They measure no more than 1.5 inches (37 millimeters) long, have slender fingers with rounded discs at the tips, and glandular masses (sometimes referred to as “warts”) on their limbs. The toads come in a variety of hues, including white, pale gray, buttery yellow, caramel brown, earthy red or black.

Scientists have since identified more than a dozen Nectophrynoides species that bear live young. N. viviparus is the most widespread species in the genus and the only one found in the Southern Highlands of Tanzania as well as the country’s Eastern Arc Mountains, where all other Nectophrynoides species live.

But when animal populations are divided by great distances or geographic barriers — which is the case in many of the forest habitats where Nectophrynoides toads live — new species can emerge. Scientists previously found genetic variations between Tanzanian N. viviparus populations in the Southern Highlands and Eastern Arc Mountains. By 2016, researchers suggested that the population of N. viviparus toads outside of the Southern Highlands could, in fact, include different species.

For the new study, the scientists reviewed 257 specimens — some more than 100 years old — from five museum collections in Europe, examining toads’ physical traits and extracting genetic data. The research team also listened to recordings of the toads’ calls, which can vary between species in audio frequency, duration, and the number and timing of acoustic “pulses.” (One tool for identifying such vocalizations, developed by another team of scientists in 2021, is called the repeat interval‐based bioacoustic identification tool, or RIBBIT.)

Based on the toads’ vocalizations, their DNA, and subtle variations in body sizes and shapes of their toes and leg bumps, researchers identified three previously unknown Nectophrynoides species, naming them N. luhomeroensis, N. uhehe and N. saliensis. Some of the female specimens contained “large yolky eggs” that would nourish their young internally, the study authors wrote. Partially developed embryos inside other females confirmed that these species bore living toadlets.

In addition to separating the outliers that were formerly classified as N. viviparus, the researchers also revisited the oldest N. viviparus specimens to confirm their identity and clarify what defines the species.

“There’s no question that this is now correctly assigned,” Scherz said. “It’s really a game changer.”

With nearly all the data coming from museum specimens, this work “underscores the importance of continued investment in taxonomy and natural history collections,” Santana told CNN in an email. “Working at the Field Museum, which holds specimens from all over the world, I’m constantly reminded that many new species are still waiting to be recognized.”

Tree toads’ family tree is a work in progress

More work needs to be done untangling questionable specimens from existing species of Nectophrynoides, according to Scherz.

Identifying the three species that are new to science “is a small part of the big puzzle of this genus,” he said. For example, the species N. tornieri, described in 1906, is now known to be a species complex, a group containing individuals that are genetically very similar but potentially different enough to be considered separate species.

But Tanzanian tree toads face more pressing concerns than which goes where in the Nectophrynoides family tree. Their forest homes are increasingly threatened by human activities, political instability and the climate crisis. One species — N. asperginis, or the Kihansi spray toad — is already extinct in the wild. N. poyntoni, or Poynton’s forest toad, though not considered extinct, has not been spotted in the wild since 2003 when the species was discovered.

What’s more, the toads’ habitats have yet to be thoroughly explored, and scientists can only guess at how many unknown species in the region may exist, Scherz said.

“Even if you go to these places a few times, often you’ll come back later, maybe with a different team, and you find things that you’ve never seen before,” he said. “Our picture of the distribution of these species is far from complete.”

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