How Do Frogs Colonize Oceanic Islands?

The São Tomé and Príncipe Reedfrog (Hyperolius molleri) is the only amphibian species found on both islands. It occurs from sea level up to over 1200 meters elevation and will breed just about anywhere (even a water-filled pig trough!) Photo by Andrew Stanbridge.

With the recent discovery of offshore oil, São Toméans will soon face the challenge of reconciling rapid economic development with preserving their natural heritage.  Young Explorer Rayna Bell will return to the island with a team of expert scientists to discover just how many species occupy the habitat and how rare and irreplaceable they might be.


Unlike birds and reptiles, amphibians are usually absent from oceanic islands because their moist skin makes them sensitive to salt water and they can’t survive a long journey at sea. One reason the oceanic islands of São Tomé and Príncipe are so remarkable is that they host not just one but seven endemic amphibian species (six frogs and one caecilian). Although seven species may not sound very impressive, there are no endemic amphibians in either the Hawaiian or Galapagos Islands, but those archipelagos have only been around for a few million years while the island of Príncipe is over 30 million years old!

The mystery of how and when frogs got to São Tomé and Príncipe is what initially inspired my interest in these unique islands. For the past four years I’ve been trying to figure out which mainland African species successfully crossed the Gulf of Guinea and evolved into the São Tomé giant reedfrog and its sister species, the São Tomé and Príncipe reedfrog.

Biologists Rayna Bell and Bob Drewes searching for the São Tomé and Príncipe Reedfrog in “pristine habitat”. Photo by Andrew Stanbridge
Biologists Rayna Bell and Bob Drewes searching for the São Tomé and Príncipe Reedfrog in “pristine habitat”. Photo by Andrew Stanbridge

There are over 120 species of reedfrogs in Africa, so choosing among candidates is a bit like searching for a needle in a haystack, but I’ve been using genetic methods to guide my search. The basic idea is to compare the DNA of candidate “ancestor species” with DNA of the island species, because the true ancestral species should be the closest genetic match.

So far, the best genetic match is a species called the cinnamon-belly reedfrog that is found throughout Central Africa. Now I’m trying to narrow down my search even further to pinpoint the exact location of the population that dispersed to the islands sometime in the past 30 million years. It’s still too early to say for sure, but it looks like the source population is somewhere along a major river drainage in Gabon, an equatorial country located only a few hundred kilometers away from the islands.

When I first started this project, I figured my chances of actually finding the ancestral species, let alone the specific source population, were slim to none. But the presence of seven endemic amphibians on two tiny oceanic islands serves as a constant reminder that with enough time, anything is possible!

Watch Rayna, Bob, Velma and Andrew attempt to catch frogs in Príncipe:


NEXT: The Quest for Giant Treefrog Tadpoles

Changing Planet


Meet the Author
Rayna is a graduate student in ecology and evolutionary biology at Cornell University studying diversification and conservation of African reedfrogs. When she's not in the field or planning her next expedition, Rayna spends her time in the genetics lab and teaching biology to undergrads at Cornell.