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M. Schäfer/Frogs & Friends e.V.

The world’s biggest frogs build their own ponds

Some frogs are thumbnail-size, but Cameroon’s Goliath frog (above) is bigger than a whole human foot, measuring up to 34 centimeters and weighing up to 3.3 kilograms. Although the world’s largest frogs are well known for their girth, their jumping prowess, and their ability to provide a hearty meal for us humans, their reproductive behavior has long been a mystery. Now, scientists think they know how these forest-dwelling frogs care for their young: by building them their own swimming pools.

Most frogs lay their eggs on whatever happens to be nearby—leaves, twigs, even the ground itself. But researchers looking for Goliaths (Conraua goliath) along a 400-meter stretch of the Mpoula River in western Cameroon discovered a strange sight: empty dips in the shore rocks that had been cleared of leaves, gravel, and other debris. Soon, the researchers happened upon a few other cleared pools above the water line.

Some pools were empty. But others were full of tadpoles. Then, the researchers realized what they were looking at—Goliath frog nests. Altogether, they found 22 potential nests, 14 of which contained up to 3000 eggs each, the team reports today in the Journal of Natural History. A few nests held tadpoles of different ages, suggesting the frogs reuse the pools. When researchers filmed one nest with an infrared time-lapse camera overnight, they observed a parent watching over its young until dawn, keeping potential predators at bay.

The researchers say Goliath frogs, likely the bigger males, excavate the meter-wide ponds, moving sand and stones weighing up to two-thirds of their own body weight. It appears that some even use the excavated materials to build the walls of these pools. This impressive construction effort—plus the overnight watching—represents an unusually high degree of parental care among frogs, the researchers say. It also may explain just how the Goliath grew to be so big: The bigger the frog, the heftier the construction possible, and the more protection for vulnerable, newly hatched tadpoles.