DNA From 19th Century Toads Solve 115-Year-Old Mystery As 3 New Species Discovered

These species skip the tadpole stage entirely, an incredibly rare phenomenon among toads and frogs.

Most frogs lay eggs that hatch into tadpoles, but high in Tanzania’s misty mountain forests, a group of toads has abandoned this amphibian tradition entirely. Scientists have just described three new species of tree toads that give birth to fully formed baby toads. No tadpole stage required at all!

Live birth is vanishingly rare among the world’s 8,000-plus frog and toad species. Only 20 anurans worldwide have evolved this reproductive strategy, and 16 of them belong to a single genus called Nectophrynoides found exclusively in Tanzania. The three newly described species bring the total to 16, and each represents a separate evolutionary experiment happening on isolated mountaintops.

An international research team published their findings in the journal Vertebrate Zoology, revealing that what scientists had long considered a single widespread species was actually four genetically distinct species. The discovery came after researchers extracted DNA from museum specimens collected 120 years ago, some sitting in German collections since 1899.

Scientists Extract DNA from Century-Old Museum Specimens

The research team faced an unusual challenge: determining which populations belong to which species when the original specimens were collected before World War I. Museum labels from that era were often vague or incorrect, with some specimens simply marked “Southern Tanzania.”

Scientists from the Natural History Museum Denmark, University of Dar es Salaam, and several European institutions used museomics to extract and sequence DNA from 33 preserved specimens in Berlin’s Museum für Naturkunde. These included specimens collected by German naturalist Friedrich Fülleborn from Tanzania’s Southern Highlands between 1899 and 1900.

The team compared this historical genetic material with DNA from 257 modern specimens collected across Tanzania’s Eastern Arc Mountains and Southern Highlands. They also measured 27 different physical characteristics on each toad, from snout length to the shape of enlarged glands on their limbs.

The genetic analysis showed populations living in different mountain ranges had diverged by more than 3% in their mitochondrial DNA, a level of genetic difference that typically indicates separate species evolving independently for millions of years.

Nectophrynoides saliensis comes from Sali Forest Reserve in the Mahenge Mountains. Adults range from 20.8 to 34.3 millimeters. Their most distinctive feature is a spearhead-shaped parotoid gland that narrows to a thin point. The species has less distinct glandular masses on its limbs compared to the other two.

Tanzania’s Eastern Arc Mountains function like islands in the sky. Each mountain range is separated from its neighbors by lowland valleys that these forest-dwelling toads cannot cross. Populations became isolated as the mountains formed and forests fragmented, leading each group to evolve independently over millions of years.

The genetic analysis revealed that Nectophrynoides viviparus, the species originally described in 1905, is actually restricted to Tanzania’s Southern Highlands. Populations from the Udzungwa Mountains previously thought to be the same species are genetically distinct by 3.6 to 4.3%.

115-Year-Old Mystery: Specimens in the Wrong Place

Part of the research involved solving a geographic puzzle. Some specimens in the original 1905 type series were labeled as coming from Dar es Salaam and Amani, coastal locations where these highland toads don’t actually live.

When the team sequenced DNA from these specimens, they clustered genetically with modern populations from the Uluguru Mountains, hundreds of kilometers from the labeled sites. The location data was likely incorrect, a common problem with colonial-era collections where specimens were often obtained through intermediaries and actual collection sites were never properly recorded.

How These Toads Give Birth Without Water

Viviparity in amphibians requires several dramatic evolutionary changes. Unlike most frogs that release eggs to be fertilized externally in water, viviparous toads mate on land with internal fertilization. Females carry developing embryos in their oviducts for months, nourishing them through specialized maternal tissues similar to a placenta.

The eggs are large and yolky, providing nutrition for developing toadlets. Instead of hatching as aquatic tadpoles with gills, the young develop directly into miniature adults while still inside their mother. When born, they are fully formed terrestrial toads ready to hop away and hunt for food.

This reproductive strategy frees these toads from dependence on water bodies for breeding, an advantage in montane forest and grassland habitats where suitable ponds may be scarce. However, females can produce far fewer offspring than egg-laying species. The tradeoff is that each baby toad has a much higher chance of survival since it doesn’t face the aquatic tadpole stage.

Conservation Status Worsens With New Species Split

The taxonomic revision creates urgent conservation questions. Nectophrynoides viviparus was previously listed as “Least Concern” on the IUCN Red List based on an estimated extent of occurrence of 46,287 square kilometers across multiple mountain ranges.

Breaking this into four separate species drastically shrinks each species’ range. N. viviparus sensu stricto, now restricted to the Southern Highlands, occupies an estimated 304 square kilometers. N. luhomeroensis from the Luhomero Mountains occupies approximately 152 square kilometers. N. saliensis from Sali Forest Reserve has the smallest range at just 20 square kilometers.

Based on these revised distributions, the research team recommends that N. saliensis and N. luhomeroensis should be classified as Critically Endangered, N. viviparus as Endangered, and N. uhehe as Vulnerable. N. saliensis faces particularly serious threats because Sali Forest Reserve is under pressure from deforestation.

Some populations of N. uhehe have already gone extinct. The species was once found in Kiolela Forest Reserve, but that forest fragment no longer exists.

Of the 16 species now recognized in the genus Nectophrynoides, several are already in trouble. N. asperginis recently went extinct in the wild, though captive populations survive. N. poyntoni has had few confirmed sightings since its discovery.

Live birth in frogs represents one of evolution’s more peculiar experiments, a reproductive innovation that evolved independently only a handful of times across hundreds of millions of years of amphibian history. That 80% of the world’s live-bearing anurans belong to a single genus on Tanzanian mountaintops makes Nectophrynoides toads extraordinarily valuable for understanding how complex traits evolve. Losing any of these species means losing a unique solution to the problem of reproducing without water, refined over millions of years of mountain-top isolation.

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