World's largest frogs can move rocks half their weight

The Goliath frog belongs to the largest known frog species in the world.(Credit: M. Schäfer/Frogs & Friends e.V.)

The world's largest frogs may also have the best pollywog daycare on the market. To protect its wee tadpoles, these enormous amphibians build their own "nursery ponds," sometimes moving rocks more than half their weight to do so, and then guarding the pond to ensure the next generation's survival, a new study details.

The finding marks the first time scientists have described the Goliath frog's(Conraua goliath) unique nest-building and parenting tactics. However, local frog hunters in Cameroon have known about it for years, and they were the first to tell the researchers about the frogs' parental dedication.

In fact, the researchers were studying something completely different (they were studying the diet of Goliath tadpoles) when "we heard about the breeding behavior of the Goliaths and decided to investigate if it [were] true or not," said study senior researcher Mark-Oliver Rödel, curator of herpetology at the Natural History Museum in Berlin. [15 of the Largest Animals of Their Kind on Earth]

The 7.3-lb. (3.3 kilograms) Goliath frog is native to Cameroon and Equatorial Guinea. To learn more about its nesting quirks, the scientists spent part of spring 2018 searching a 1,300-foot (400 meters) section of the Mpoula River in western Cameroon. They also interviewed four frog hunters and two villagers who lived near the river to learn more about C. goliath's habits.

In all, the scientists found 22 breeding sites, 14 of which had almost 3,000 eggs apiece. The team even set up a time-lapse video at one nest, which showed a Goliath guarding the nest at night.

These frogs are creative builders, constructing three different types of nests, the researchers found. One type, the rock-pool nest, was built on larger rocks within the river, meaning that "frogs were using pre-existing structures for breeding," the researchers wrote in the study.

For the second type, frogs used naturally existing shallow pools near the river as nests. It appeared that the frogs had enlarged these pools, the researchers noticed, in essence turning a cottage into a McMansion. For the third type, the frogs dug small ponds, surrounding them with large stones, some weighing up to 4.4 lbs. (2 kg).

Impressively, none of these nests had debris in them, suggesting that the frogs also acted as housekeepers, keeping the ponds clean for their tadpoles. "We have never observed them directly, but from indirect evidence, it is apparent that they push out material (e.g. leaves, pebbles) from natural ponds or push away larger and smaller stones to create their 'own' ponds," Rödel told Live Science in an email.

It's likely that the male frogs, which are more than 1.1 feet (34 centimeters) long, use "their huge and very muscular hind legs" to move the stones, he added.

While the researchers never directly witnessed a Goliath frog digging a nest, "the most detailed description we got (from one frog hunter) was that the male would construct the nest while the female waits in proximity," the scientists wrote in the study. "Once the nest is finished, the male whistles to attract the female, which then is grasped by the male and eggs are deposited. Afterwards, the female would guard the nest and subsequently open the nest towards the river."

Is daycare worth the cost?

The frogs invest a substantial amount of energy into nest-building, cleaning and guarding. But is it worth it? If their tadpoles survive, it absolutely is, but it appears each nest has benefits and challenges, the researchers found. Nests within a riverbed can flood from heavy rains, allowing predators such as shrimp and fish to get inside and devour the tadpoles, said Rödel, who is also the president of Frogs & Friends, the nongovernmental organization that co-funded the research. [So Tiny! Miniature Frog Species Are Among World's Smallest (Photos)]

Digging a pond alongside the river would sidestep these predators, but if it doesn't rain for a spell, the pond could dry up, killing the tadpoles. "Thus, each of the three nest types has advantages and disadvantages, and the frogs need to choose what is best at a certain time," Rödel said.

Goliath frogs aren't the only amphibian superparents out there. The gladiator frog (Hypsiboas rosenbergi) in South America builds nests for its young, while the male African bullfrog (Pyxicephalus adspersus) guards tadpoles and digs channels up to 40 feet (12 m) long to allow tadpoles to escape from drying pools, the researchers noted. However, Goliath is the only known African frog to build nesting ponds, the researchers said.

Unfortunately, the Goliath frog is endangered, according to the International Union for Conservation of Nature, largely because of habitat loss and fragmentation, pollution, disease and hunting. (The frogs are considered a luxury food and are often served at weddings, Rödel said.)

It would be a shame to lose these creatures without fully understanding them, he said. "The reason why we wanted (and actually did) study the tadpoles, was that we needed to know more about the biology of the species, just to make sure we know what to do in case a captive breeding program might be the last chance for the Goliaths' survival in the future."

The study was published online Friday (Aug. 9) in the Journal of Natural History.

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Originally published on Live Science.