Why are frogs diverse in some parts of Africa’s rainforests and less so in others? The patterns of cooling and glaciation during the last ice age would probably not have been your first answer or even your last-ditch guess, but it is, nonetheless, correct.
“When the glaciers were at their maximum global extent, the earth’s climate was cooler and drier, and forests that are continuous today contracted to what were essentially islands in a sea of savannah,” said Gregory Jongsma, the acting curator of Zoology at the New Brunswick Museum.
Jongsma is the lead author of a new study , conducted by researchers at the Florida Museum of Natural History and published in the journal Ecology and Evolution, which shows that even though it’s been 12,000 years since the last ice age, tropical African frogs still haven’t forgotten about it.
The study specifically focuses on the Lower Guinean Forests of Central Africa, part of what’s better known as the Guineo-Congolian rainforest or, more simply, the Congo. Centered around 0 degrees latitude, these forests are a visual representation of the planet’s obliquely unequal heating by the sun, which wrings water from the atmosphere like a damp rag onto the equator, creating a humid belt of jungle prominently bookended by deserts.
“I became obsessed with this area of the world, and Gabon specifically, around the age of 10,” Jongsma said. “I read a series of National Geographic articles about this individual named Mike Fay doing what was called a mega transect, where he walked from the Republic of Congo west across Gabon.”
Fay was a conservationist who made the 2,000-mile, cross-jungle trek wearing sandals and shorts, his blistered feet regularly coated in iodine and laminated in duct tape to stave off infection. He and his crew bushwacked their way through trailless forest until they reached the Atlantic Ocean, 456 days after they’d started. He undertook the trip to record biological data and stopped frequently to do things like identify the seeds embedded in elephant dung, count gorillas and chimpanzees, and take video recordings of anything that moved, slapping away the maddeningly insistent mosquitos and tsetse flies all the while.
“So Gabon became enlarged in my brain at a young age, which worked out really well, because it’s an incredibly interesting part of the world from the standpoint of an evolutionary biologist,” Jongsma said.
Documenting biodiversity is the first, most essential step toward understanding how species in a given area have evolved and why their ecologies look and function in a particular way. Scientists have learned a lot about the diversity of Africa’s rainforests since the time of Mike Fay, to the extent that they can now answer some of these higher-order questions.
Jongsma’s specialty is frogs. For several years, while working on a doctoral degree at the Florida Museum of Natural History, he’d frequently travel to Central Africa to tick off the miles on his own Fay-esque expeditions in Cameroon, Uganda, Angola, the Republic of Congo and Gabon, collecting frogs along the way.
Slowly, he began to notice a pattern. Some lowland forests that seemed perfectly suitable for frogs strangely supported less diversity than others. Additionally, endemic species — those that live in a restricted area defined by geographic or cultural borders — were mostly clumped together in what appeared to be random spots in Cameroon and Equatorial Guinea.
Back in Florida, Jongsma looked to see whether environmental conditions in central sub-Saharan Africa could explain the distributions he’d seen.
He narrowed his focus to frogs in the clade Afrobatrachia, a group that study co-author David Blackburn, curator of herpetology at the Florida Museum, has studied since his own days as a graduate student doing fieldwork in Central Africa.
Afrobatrachia was ideal for answering this sort of question for several reasons, not least of which because it accounts for more than half of all frog diversity on the entire African continent. And although species in this group were once thought to be distantly related to each other, with geographic origins in multiple parts of the world, work done by Blackburn and others has shown that it’s actually a tight-knit clade that evolved in and is endemic to Africa.
It includes the rainfrogs, known for their visual and auditory similarity to an angrily deflating balloon. It also includes the hairy frog, which has long, hair-like structures protruding from its abdomen and internal claws that it can only deploy against adversaries by breaking its fingers and pushing the claws out through its skin.
Most importantly for this study, there are species in Afrobatrachia that are specialized to live in all sorts of forest environments. Some live in the canopy, others in burrows. Some prefer the more typical pond or stream habitat, while others have evolved a degree of independence from water by skipping the tadpole stage of their lifecycle and laying eggs that hatch into fully assembled frogs.
The environmental conditions most closely associated with frog diversity were sure to be discernible in this ecologically varied group.
The next thing to consider was whether the distribution of frogs aligned most closely with current or historical conditions.
“There are two competing hypotheses when predicting diversity,” he said. “The ecological hypothesis says species are essentially in equilibrium with current conditions. Therefore, if there’s high rainfall and temperature or productivity, you’re going to have high diversity.”
This was true in some of the areas he’d been to, but the seemingly suitable lowland forests that maintained less frog diversity were just as hot and wet as the others. Current environmental conditions similarly did nothing to explain why endemics showed up where they did.
So Jongsma moved on to the alternative hypothesis.
“In the evolutionary camp, you’d say it’s the past conditions that have the largest impact on current-day diversity.”
Scientists infer the climatic conditions of past environments using multiple lines of evidence, including the composition of greenhouse gasses trapped in the air bubbles of ancient glaciers, the types of plants that grew in a given time period based on spores and pollen preserved in lake sediments, and the ratio of oxygen isotopes in the fossils of marine protists. This type of data has been collected and compiled for several decades and is easily accessible to anyone attempting to reconstruct general patterns of temperature and rainfall on Earth over the past few million years.
The authors created a 2.58 million-year historical climate map for Central Africa, but the resolution wasn’t fine enough to track the distribution of frogs. Earth’s complicated topography creates atmospheric currents and eddies as air flows over it, and these local patterns become increasingly hard to measure and predict as climate changes drastically on a global scale, as notably occurred during the last ice age.
The authors needed another variable in the mix to determine whether historical conditions in sub-Saharan Africa were responsible for the modern diversity of frogs.
So, naturally, they added more information about frogs! This time Jongsma chose ten abundant species that were not in Afrobatrachia, for which he created niche models.
“They’re common, widespread, and largely co-distributed, so there’s enough data there to build a really robust model of their distribution,” he said.
The extra step of modeling their distributions meant they could tie modern rainfall and temperature to the presence or absence of a frog species in a particular area, then hindcast back through time to see whether the general diversity of Afrobatrachian frogs would have been high or low in an environment based on the historical climate data.
Having done this, the authors used their analyses to peer into the past and compare what they observed to what exists now.
For the most part, the Congo rainforest remained stable throughout the Pleistocene ice ages. But as Earth’s climate cooled and dried, the Congo’s forest borders receded in some places, creating pockets of more or less stable forested areas called refugia. The name comes from the idea that when the global climate rapidly warms or cools, environments that remain stable offer a refuge to species that are otherwise displaced in areas that undergo significant change.
According to the study’s results, the ancient forest refugia in Central Africa aligned with the modern diversity of frogs. They are also closely aligned with the small distributions of endemic species.
There are two main reasons why refugia may harbor more diversity than surrounding areas, even after they’ve ceased to be a refuge.
The first is a sort of lag effect. As suitable environments contract during climate change, species crowd into refugia, and when environments expand again, they fan out. But this doesn’t happen automatically. Though the 12,000 years since the last ice age seems like a long time to humans, who live on the order of decades, it’s hardly even worth mentioning on the vast scale of geological processes on which evolution and migration operate. It’s possible, therefore, that frogs simply haven’t had enough time to even out their distributions since the last cold snap.
Secondly, the isolation that’s inherent to living in a refugia does funny things to species. Most notably, it makes more of them.
“Refugia have been proposed as species pumps,” Jongsma said
In this case, explained Blackburn, that might look like a frog that once had a wide distribution throughout much of the historic Congo. As the forest was partitioned by climate change, this one big population of frogs would have become several smaller populations, all isolated from each other. If this went on long enough, these populations would eventually evolve into new species. Refugia tend to have higher rates of diversity as a result.
Additionally, while some species expand back out into their former ranges when the climate reverses course, others stay put for one reason or another, which explains the higher rate of endemism in refugia.
The results have implications far beyond the interests of frog enthusiasts.
“All of the countries where this study was conducted have signed on to the 30X30 goal, which is an initiative to conserve 30% of their country’s land area by 2030,” Jongsma said. “If you’re going to expand a protected area or create a new one, considering where forests have been most stable might be an important consideration, depending on what you’re trying to conserve.”
Additional co-authors of the study are Narayana Barve of the Florida Museum of Natural History; Julie Allen of Virginia Tech; and Hannah Owens of the University of Florida.
Ecology and Evolution
Pleistocene Forest Stability Predicts Patterns of Frog Diversity in Central Africa
11-Mar-2026