Island-Hopping Frogs: How Geology, Ocean Currents and Evolution Shaped Biodiversity in the Pacific

Amphibians are among the last animals one might expect to find on remote oceanic islands. Their permeable skin makes them highly sensitive to salt, and mostly rely on freshwater to reproduce. Yet across the Pacific, from Southeast Asia to Fiji, a group of frogs has achieved what seems almost impossible: colonising island chains separated by thousands of kilometres of open ocean. How did they get there, and what can their journey tell us about how biodiversity evolves on islands? 

Our frogs belong to the family Ceratobatrachidae (Figure 1), a surprisingly diverse group of 104 currently recognised species. Even more surprising is where they are found, as these frogs appear on both sides of the Wallace’s Line (as shown in Figure 2), the invisible biogeographical boundary slicing through Southeast Asia that separates the fauna of Asia from that of Australia and Oceania. For animals so closely tied to freshwater, this is an impressive accomplishment. 

Interestingly, most species in this family can be grouped as terrestrial, semi-aquatic, arboreal or scansorial, suggesting a remarkable ability to exploit different ecological niches.

Adding another twist to the story is the frogs’ striking sexual size dimorphism (SSD), where males and females consistently differ in body size. In fact, the most extremely sexually dimorphic frog species known belongs to this very family, with females growing up to forty times heavier, and three times longer than their male counterparts. In the animal world, such differences are often linked to sexual selection, with larger females favoured for reproduction. Also, sexual selection is a powerful driver of species diversification. Could it, too, have helped shape the spread and diversity of Ceratobatrachidae across the Pacific?

Crossing ocean barriers is usually the preserve of much smaller organisms, think snails hitching an accidental lift on birds across the sea. Frogs, on the other hand, are not exactly known for their seafaring abilities. So how did they manage it? Until now, no study had directly tested the evolutionary hypothesis that might explain this trans-marine dispersal success.

One tempting explanation is that Ceratobatrachidae represent an adaptive radiation, the process where a single ancestral species rapidly diversifies into many, each shaped by different environments. At first glance, this makes sense. Case closed? Not quite.

To untangle this evolutionary mystery, the authors of this paper—published in the Zoological Journal of the Linnean Society—took a wide-angle view, combining genetic data, geography and the geological history of the Pacific.

They built a “family tree” of these frogs using DNA from multiple genes, which allowed them to see how species are related and estimate when they first appeared. Then, they looked at traits like body size, difference between males and females, and lifestyles to map how these traits changed over time. By combining this with information on where the frogs live and how islands formed and shifted, they could explore how movement between islands and ecological changes shaped the incredible diversity we see today.

The study found that, across their history, Ceratobatrachidae frogs showed constant rates of evolution and retained many of their ancestral traits, with only a few changes in habitat preference, body size and SSD. Surprisingly, these findings  go against the adaptive radiation hypothesis. Instead the most plausible explanation is a pattern of episodic dispersal: frogs hopped across islands when they were close together (millions of years ago!), followed by slow diversification as the islands became more isolated. In other words, these frogs are patient, slow-and-steady explorers of the Pacific.

Take the genus Cornufer, for example. These frogs show remarkable variation in body size because some colonised large islands very early, when open space and minimal competition allowed them to grow impressively large. On the other hand, on islands where other frogs were already present, species stayed smaller, with less difference between males and females. Habitat also played a role: terrestrial frogs could become bigger and more dimorphic, while tree-dwelling species remained small and agile, perfectly adapted to life among the branches. In short, size and romance, in case of sexual dimorphism, were shaped by both lifestyle and location. 

But what about crossing the ocean? Ceratobatrachidae may tolerate a little salt, but their real secret was geography. Over millions of years, chains of ancient islands stretching from the Philippines to Fiji acted as stepping stones. Frogs likely hitched rides on vegetation rafts during floods, aided by shifting currents and warmer climates. Ultimately, at the crossroads of geology and evolution these small amphibians turned into true Pacific explorers.

To wrap things up, these frogs show that evolution doesn’t always need to be fast; sometimes slow, steady, and adventurous works just as well. By rafts, currents, and a bit of island luck, they’ve spread across the Pacific, diversified, and thrived. Ultimately, Ceratobatrachidae remind us that even tiny travellers can make epic journeys.