The world's biggest frogs have been captured on camera lifting rocks - nearly two-thirds their body weight - as they build paddling ponds for their young.

They use the large pebbles and stones to erect dams and ponds to protect their offspring.

It is the first example of 'nest'-building in an African amphibian - and could explain how the Goliath frog grew into a giant.

Adults were observed in the wild moving rocks weighing up to 4.4lbs (two kilos) to construct the watery abodes - which they then guard.

Huge: Adults were observed in the wild moving rocks weighing up to 4.4lbs (two kilos) to construct the watery abodes - which they then guard

WHAT IS THE GOLIATH FROG? The Goliath frog itself weighs up to 7.25lbs (3.3 kilos) with its body reaching over 1ft Iin (34cm) long - not including the legs. It is listed as endangered with the population more than halving over the past 15 years. Native to Cameroon and Equatorial Guinea, it is so nutritious it is hunted as bushmeat. The frogs are also sold in the exotic pet market. Poachers take to the rivers and streams at night armed with guns and a torch. Advertisement

The males are believed to do the work while the females act as sentries.

The Goliath frog itself weighs up to 7.25lbs (3.3 kilos) with its body reaching over 1ft Iin (34cm) long - not including the legs. It is listed as endangered with the population more than halving over the past 15 years. Native to Cameroon and Equatorial Guinea, it is so nutritious it is hunted as bushmeat.

The frogs are also sold in the exotic pet market. Poachers take to the rivers and streams at night armed with guns and a torch.

The study published in the Journal of Natural History says their laborious pond-building sheds light on gigantism in Goliath frogs.

This is a biological phenomenon in which the size of an isolated animal increases dramatically in comparison to its relatives.

Lead author Marvin Schafer, a doctoral scholar at the Berlin Natural History Museum, said: 'Goliath frogs are not only huge but our discovery shows they seem to be attentive parents as well. The little ponds they make at the edges of fast-flowing rivers provide their eggs and tadpoles with a safe haven from sometimes torrential waters, as well as from the many predators living there.

'We think the heavy work they put into excavation and moving rocks may explain why gigantism evolved in these frogs in the first place.'

Despite their renown little is known about their biology - particularly their reproductive behaviour.

Numbers of the endangered species have declined by more than 50 per cent in just 10 years due to overhunting and deforestation.

Researchers first learned about the unusual level of parental care they provide from local frog hunters - who trap adults for bush meat. So two members of the team waded along opposite edges of the Mpoula River in the Cameroon looking for breeding sites.

At first, they could only distinguish them by finding eggs or tadpoles. However, they learned to spot material excavated and piled up in a way that defied the effects of the water current.

Offspring: Recently metamorphed juvenile frog, often found in vegetation near nesting sites, indicating they stay around their nursery for some time

Habitat: Nest type 2 with old eggs (grey mass in middle) leaf litter piled up in front

They identified 22 potential places - 14 of which contained nearly 3,000 eggs each, spread across the entire area. At the nest which showed the most recent signs of activity, they recorded a time-lapse video using a camera trap.

The scientists found that Goliath frogs create three different types of pond. For some, they simply clear naturally-occurring rock-pools of leaf-litter and debris.

The effect is still strikingly different to surrounding puddles, which have thick layers of leaves, debris and gravel.

For the second , frogs dig out leaf litter and gravel from shallow pools and push it to the edges, forming a dam. Dams are most obvious in the third, for which frogs clear depressions of shallow water of any large stones, moving them to edge and creating a circular pond.

This was the most reliable type, as eggs were least likely to spill out or become over-flooded after heavy rain. Infrared time-lapse revealed an adult spending all night guarding a nest from predators, only ending the vigil just before dawn.

The scientists were unable to determine which sex was responsible for building ponds or which for guarding them. But one hunter who described the behaviours in detail suggested males do the construction while females are the guards.

They didn't directly observe the cleaning or digging, but over five days followed progress of a nest, from the first digging attempts to the depositing of eggs.

Project leader Dr Mark-Oliver Rodel, president of berlin-based amphibian conservation charity Frogs & Friends, said: 'The fact we've only just discovered these behaviours shows how little we know about even some of the most spectacular creatures on our planet. We hope our findings, combined with further ongoing research, will improve our understanding of the needs of the Goliath frog so we can help support its continued survival.'

The finds were published in the Journal of Natural History.