Genetic analysis reveals new species of giant frog in the Philippines that is nearly identical to even larger species

Researchers from the University of Kansas published their findings in the journal Ichthyology and herpetology describing a new species of fanged frog, named Limnonectes cassiopeia, from the Philippine island of Luzon.

Surprisingly, specimens of the new species have been collected a dozen times over the past 20 years, but all have been mistaken for juvenile versions of the famous, nearly identical (but not too close, it turns out) giant fanged frog of Luzon.

“They’re called that because the males of some species have large bony protrusions, like fangs, on their lower jaws — it’s pretty unique,” said lead author Mark Herr, a doctoral student at the University of Kansas Biodiversity Institute and Museum of Natural History.

“It’s still a bit of a mystery, but it’s generally assumed that these fangs are used for fighting between males. The fangs of males are much larger than those of females. These males have large heads, and many of these frogs can be enormous, some as big as chickens elsewhere in Southeast Asia.”

Herr first identified Limnonectes cassiopeia while conducting a larger genetic study of all Philippine fanged frogs, which includes just over a dozen known species.

“I sequenced a lot of them to build a good genetic dataset, which allowed me to look at their patterns of kinship,” Herr said. “Many species were originally described, over the last century and a half, based solely on their physical appearance, before modern genetic analysis was available. In Luzon, there were two known species of fanged frogs: Limnonectes macrocephalus, the large one, and a medium-sized species called Limnonectes woodworthi. However, I discovered a different group next to macrocephalus that appeared in the genetic data as a separate branch.”

The discovery of this third, previously unknown species in the fanged frogs’ genetic data prompted Herr to begin physically examining the museum’s specimens, empowered and guided by this catchy genetics. He hoped to see if he could distinguish some of their morphology from that of the giant Luzon fanged frog, since genetic analysis suggested that they had evolved rather independently.

Fortunately, thanks to extensive work in the Philippines by Rafe Brown, Herr’s mentor at KU, and previous generations of KU researchers, there was an abundance of specimens at the KU Biodiversity Institute and Natural History Museum. Brown co-authored the new paper with Hannah Som, a KU master’s student.

“I checked my genetic work to make sure there were no errors, and there were none,” Herr said. “That led me to look at all the Limnonectes specimens from Luzon at the museum. We have hundreds of these frogs, and I carefully compared the 19 that had appeared in the new genetic group to those in the main group.”

By trying to distinguish the physical traits of the two species, Herr said it was easy to understand how they could be mistaken for the same species.

“I’m a herpetologist, I spend a lot of time looking at frogs. I watched these animals for a couple of weeks, staring at them, trying to see if I could see anything, and I couldn’t,” Herr said. “I couldn’t see any major differences. It’s not like one group was visibly colored or striped and the other wasn’t.”

Herr began to make detailed measurements of the fanged frog specimens.

“We have a set of standardized measures designed to find differences between populations, to see if those differences reflect speciation patterns,” Herr said. “I made comparisons between populations, began to suspect that they might represent different species, and then dug deeper, comparing different traits in detail against each other.”

Eventually, Herr arrived at his “big breakthrough” when he discovered that the genetically distinct species all had smaller toe disks and that they were white.

“The discs are important,” Herr said. “Statistically, relative to their body size, they can have smaller toe pads. So I went back to all the genetically identified specimens and found the key trait — a simple physical trait that anyone can assess in the wild. If they catch one of these frogs, they can turn it over and see it right away. Not only are the toe pads smaller, they’re white. They’re pale, completely depigmented, which distinguishes them from the main species, Limnonectes macrocephalus, which has dark gray pigmented toe pads.”

Herr said the overlooked detail was all the more remarkable because the frogs’ native island is more densely populated than Britain.

“People in Luzon have been catching these frogs – the whole group – for over 100 years,” he said, adding that they are part of the diet of several indigenous groups.

“They have these white disks… but I thought, who would look at that? Who would think about that? With these genetic resources, I now know that there is something different about these frogs. I knew I had to go see if that was true. Frogs are genetically divergent; so I thought, let’s see if this unique trait corresponds to a major difference in their overall physical makeup, their size and shape – the way frogs appear in physical space.”

Indeed, Herr remembered the five-star constellation Cassiopeia when he saw the new species’ five white pads, and named the new species Limnonectes cassiopeia.

Another major difference became apparent to Herr over time: the new species Limnonectes cassiopeia developed its characteristic large head earlier in its development, from juvenile to adult, than Limnonectes macrocephalus.

“Clearly the giant macrocephalic frogs overlap in size because they’re all very small to start with, and you wouldn’t be able to tell,” he said.

“You would think that these are young macrocephalic lizards, that’s what everybody who’s caught them has thought for the last 25 years. But I did some measurements to determine that once the males start to reach sexual maturity, they get these big, broader heads and they develop huge fangs. Sometimes when we look at their use of fangs for fighting, we see big male Limnonectes with scars on their heads, probably from fighting injuries. And my analysis showed that the big heads start to appear on this new genetic group starting from an overall smaller body size.”

Now that the new species has been recognized, Herr hopes to learn more about its behavior in the wild and how it differs from Limnonectes macrocephalus.

“Now that we know they’re different, we can go out and observe them, study their natural history and the details of it. Maybe they’re doing something totally different in the wild, you know?” he said. “That’s exciting to me.”