How these foot-long, 7-pound frogs got so jacked

When leg day is a matter of life or death.

Goliath frogs are, somehow, even bigger than you think they’re going to be. They’re more than a foot long from snout to butt (or “vent,” if you’re a herpetologist), but that doesn’t capture the bulk of their thick, muscular legs, which can propel them 10 feet in a single jump. All of this makes them the largest frogs on Earth—and honestly, thank goodness there isn’t a bigger one. This is as much frog as the world can handle.

Biologists know very little about these chunky amphibians because they live in a fairly small region ranging from southwestern Cameroon into Equatorial Guinea, where they spend their lives in and around rivers in the dense rainforest. Local Goliath frog hunters have long said the critters were building nests, or little side pools, branching off from the main waterways to lay their eggs in, but scientists had never gone in to confirm that behavior or study it.

Strangely, though, that’s not why the team that ended up publishing a recent study in the Journal of Natural History began their project. A paper from the 1980s had mentioned that the Goliath tadpoles eat only one particular plant species, but according to Mark-Oliver Rödel, a herpetologist at the Leibniz Institute for Evolutionary and Biodiversity Research, he and his colleagues thought that was nonsense. The plant in question didn’t fully overlap with the frogs’ territory, and it would be extremely unusual for tadpoles to only eat one plant. But Goliath frogs are endangered, in part because they’re hunted for food and to keep as pets, so it was time to find out for sure just how precarious their food supply might be. “We needed to know more about the biology of the species just to make sure we know what to do,” Rödel explains, “in case a captive breeding program might be the last chance for the Goliaths’ survival in the future.”

The team searched a single quarter-mile stretch of the Mpoula River for four months, checking everywhere for signs of frog nests. Human activity has shrunk the Goliath frogs’ habitat, but in the end Rödel and his colleagues found 19 nest sites of three different types. Some were just pre-existing rock pools off the main river, which the frogs had simply cleaned out for their own purposes. Others were natural side pools that the frogs had clearly enlarged. Most intriguing of all were the shallow depressions the researchers found mid-river, with each encircled by a protective ring of stones.

goliath froglet
A Goliath froglet Schäfer et al (2019)

Because these amphibious behemoths are surprisingly skittish for their size, the team couldn’t directly observe the frogs building these nests. They could only make observations of what had changed in the hours they’d been gone. Some of the stones had clearly been recently moved (which you can tell when, for example, what used to be the moist bottom is now facing upwards)—a few of these weighed up to 4.4 pounds, which is more than half a Goliath frog’s maximum weight. It’s the equivalent of an average American adult man moving a 121-pound rock.

No one can be entirely sure how the frogs manage this feat, since they haven’t been directly observed doing it yet. But Rödel says that pushing stones with “their huge and very muscular hind legs” seems like the only possible method.

By rolling rocks and clearing pools of leaves and pebbles, the Goliath frogs are making ideal nest sites for their eggs. Rushing rivers can sweep away frog spawn, so having solid materials in an offshoot of the main artery seems to give tadpoles (which, by the way, start out just as tiny as every species) a better shot at life. These enclaves can even provide some protection from potential predators, like shrimp, that would otherwise eat young frogs.

This nest-building behavior might actually have driven the hefty amphibians to grow so big. Shifting rocks and clearing leaves or pebbles from a pool is a serious physical task for a frog—you might need to be a foot long to do it. It stands to reason that, in a species that uses this method to protect its young, individuals capable of moving bigger stones and creating higher barriers around their nests would have more offspring.

What’s not exactly clear yet is who does the building. Local frog hunters say they think the males build the nest, then emit a kind of whistling sound to call a female over for mating (Goliaths don’t have normal frog throat architecture, and are otherwise completely mute).

Rödel says he’s curious to find out more. Does each nest only belong to one breeding pair? And how do the frogs decide which type of nest to use? It’s almost certainly a trade-off between the energy you invest in creating a small pond and the chances that your offspring will survive, he explains. If there’s already a useful structure, like a rock pool, you can just clear out debris—you don’t need to dig a whole new pond. But those existing areas can also flood in heavy rain, which gives predators an opportunity to enter your nest. Industrious pond-diggers give their eggs more protection, but their shallow depressions are more likely to dry up if it doesn’t rain enough—and it’s a lot of energy to invest in the first place. “Thus each of the three nest types has advantages and disadvantages, and the frogs need to choose what is best at a certain time and under certain circumstance,” Rödel says. “But so far it is unclear if the frogs are following this logic, and if so, what clues they use to pick one or other.”

The team hopes to get back next year and set up camera traps, finally capturing proof of the building process and discovering how it works. They’re also curious about whether the male or female frogs guard the nest once it’s built. Preliminary photos taken this year suggest one parent sticks around all night, moving position every 30 to 90 minutes. But there are still lots of questions. Hopefully soon we’ll get video footage of these enormous froggos pushing boulders and clearing leaves like the talented amphibious architects they are.

Sara Chodosh

Sara Chodoshis an associate editor at PopSci where she writes about everything from vaccine hesitancy to extreme animal sex. She got her master's degree in science journalism at NYU's Science Health and Environmental Reporting Program, and is getting a second master's in data visualization from the University of Girona. Contact the author here.