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The world’s largest frog builds nests in streams, which seems to entail shifting rocks that weigh up to two kilograms. This heavy lifting may explain why goliath frogs have evolved such large bodies.

Goliath frogs () can be 34 centimetres long and weigh over 3.2 kilograms. They are only found in Cameroon and Equatorial Guinea, where they live near streams.

Despite their fame as the world’s largest frogs, almost nothing is known about them, says Mark-Oliver Rödel at the Berlin Museum for Natural History in Germany. “The entire African continent, and in particular central Africa, is extremely understudied,” he says, and goliath frogs are hard to study because they are “skittish”.

Goliath frogs are , partly because they are hunted for their meat, so Rödel’s team considered whether to start a captive breeding programme. Realising they needed to know more about how the frogs reproduce, they talked to Cameroonian frog hunters, who told them that the adults build and guard ponds in rivers. “It sounded very fascinating and completely unexpected,” says Rödel, so the team began exploring the Mpoula River in western Cameroon for nest sites.

One, two, three, lift

They found that goliath frog nests are small pools in streams. Some are natural, but others are built by the frogs by clearing rocks from the stream bed. Some stones were visibly moved from day to day, as they had been turned upside-down. The nests were always cleared of debris like leaves. A typical nest had several hundred eggs, which developed into tadpoles and eventually small frogs. Adult frogs guarded the nests throughout the night.

It is not clear why the frogs clear the nests of debris, much of which could be food. Rödel suspects it gives the adults a clear view of predators like shrimp and fish, which are a threat to the tadpoles.

The team did not manage to see the frogs moving heavy rocks, but several local people gave statements that they had seen it, and many rocks had clearly been moved. Rödel plans to return with more cameras to catch the frogs in the act. He suspects they do it with their hind legs, which are much stronger than their front legs and power their extreme leaps – goliath frogs can jump 5 metres in one bound.

This heavy lifting may explain the size of goliath frogs, argues Rödel. He points out that some other hefty frogs also do significant digging. In South America, male Rosenberg’s gladiator frogs construct nests for their tadpoles, while in southern Africa several metres long to guide their tadpoles into new pools, if the original pond is drying out.

“All these species which are doing this laborious work are big,” says Rödel. “If you’re doing something like that, you cannot be 2 centimetres in size.”