Endangered Lantau toads show genetic and call differences that could signal an emerging species

Lingnan University finds endangered Lantau Short-legged Horned Toad

The short-legged horned toad, an endangered species in Hong Kong, is a small frog measuring around 40 mm in length. It is characterised by short hind limbs, small horn-like projections above the eyes, and skin coloration and patterns that closely resemble dead leaves. (The photo shows the Short-legged Horned Toad population on Lantau Island.) Credit: Lingnan University

The short-legged horned toad (Boulenophrys brachykolos), listed as endangered on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, is the only known small frog species in Hong Kong belonging to the genus Boulenophrys of the family Megophryidae.

A study led by the Division of Science at Lingnan University has found that the Lantau Island population of the short-legged horned toad shows clear genetic divergence from other local populations, along with several notable differences in the mating calls of males.

The research team suggests that the Lantau population represents an early stage of speciation. Geographic isolation of Lantau Island from other landmasses likely contributed to these unique evolutionary traits. The team recommends treating the Lantau population as a separate conservation unit in the future.

These findings have been published in the journal Current Herpetology.

Distinct genes and faster mating calls

The short-legged horned toad is found in mountain streams on Lantau Island, Hong Kong Island, Kowloon, the New Territories and adjacent Shenzhen. The research team, led by Lingnan University, collected 19 samples of the short-legged horned toad (nine from Lantau Island, six from Hong Kong Island and four from Kowloon) to examine their genetic and morphological characteristics.

Lingnan University finds endangered Lantau Short-legged Horned Toad

The study shows that Short-legged Horned Toads are visually almost identical, making evolutionary differences difficult to detect through traditional field surveys alone. (The photo shows the Short-legged Horned Toad population on Hong Kong Island.). Credit: Lingnan University

The results show that the short-legged horned toads on Lantau form a distinct genetic group separate from those on Hong Kong Island, Kowloon and Shenzhen. This suggests that the Lantau population has been separated from other populations for a considerable period.

In addition, the team recorded and analyzed the mating calls of male short-legged horned toads from Lantau Island, Hong Kong Island and Kowloon. They found significant differences between the calls of the Lantau Island population and those of other local populations. The Lantau population produces calls with a faster rhythm and shorter individual notes.

Similar appearance masks evolutionary differences

In terms of physical appearance, the team found no marked differences between frogs from Lantau Island and other regions, indicating that physical characteristics alone may be insufficient to distinguish the Lantau population from other populations.

Jonathan Fong, the corresponding author of the study, associate dean (administration and education) of the Wu Jieh Yee School of Interdisciplinary Studies and associate professor of the Division of Science at Lingnan University, said the study provides a more comprehensive approach to evaluating evolutionary divergence among populations of the same species.

He said, “Many amphibians look very similar, making evolutionary differences difficult to detect through traditional methods that examine morphology alone. By integrating genetic and acoustic data, we found that frogs on Lantau Island have a distinct genetic makeup and male toads have different mating calls. This suggests that the Lantau population is following a distinct evolutionary trajectory.”

Isolation strengthens the case for separate conservation

Sung Yik-hei, the first author of the study and adjunct assistant professor of the Division of Science at Lingnan University, explained that Lantau Island became geographically isolated from the mainland approximately 10,000 years ago. The geographic isolation likely reduced gene flow between populations, allowing the Lantau short-legged horned toad population to develop unique genetic traits over time. The study also highlights that Hong Kong may harbor undiscovered biodiversity.

Sung said, "The results suggest that the short-legged horned toad population on Lantau Island may represent an early stage of speciation, although current evidence is insufficient to determine whether the Lantau population constitutes a distinct species. The short-legged horned toad is an endangered species, and if the Lantau population declines or becomes extinct, its unique genetic traits could be lost and could not be replaced by populations from other areas.

“We therefore propose that conservation management recognize the Lantau population as an independent conservation unit, giving due consideration to its specific conservation needs to ensure the survival of this unique native population.”