The background noise

It is the beginning of the rainy season at the Montagne de Kaw, and it is raining non-stop. A uniformly grey sky is dropping vast quantities of water over the forest, seeping right through to the heart of the plant mass. The branches of the trees are bowed down with the weight and sometimes even snap. The long-awaited moment is finally here: the forest ponds, which had dried up, can now come back to life. The excitement is at fever pitch amongst the frogs, but they are not the only ones: their predators are also getting ready. The nights, which had been calm until now, have become a theatre of bustling activity once more. These sites only fill up with water periodically and boast the advantage of not containing any fish, which is already one predator less! The gathering – also called explosive breeding although frequently referred to as “frog orgies” – is of very brief duration: one night and part of either one evening or morning. This allows them to stay one step ahead of the predators, although not all of them will be fooled…

Noisy encounters

The first detectable sign of this phenomenon is the noise. Thousands of males sing in a tremendous din,  so loud that it can even damage the human ear – and Lord knows how many humans there are (sometimes too many for the good of the frogs). Amateur naturalists and expert herpetologists* delight in this outstanding performance: some for the pleasure of observing them, others to collect data.

To perform their concertos, the males use a vocal sac which is usually located beneath their throat. The females, however, do not have one of these. Before you see them, you can particularly hear thousands of male Dendropsophus minutus – tiny lemon-coloured tree frogs literally covering the lower branches, the edge of the pond and the pond itself. These little hylidae* make up the remarkable sonic ‘bass line’ of the encounter.

Trachycephalus coriaceus, with its impressive vocal sac, is also one of the species present in vast numbers. By their side, the delicate Allophryne ruthveni and imposing Ceratophrys cornuta also take part in the general cacophony. A few leptodactylus (Leptodactylus knudseni) are there too, overseen by the tiger-striped leaf frogs (Phyllomedusa tomopterna), who tend to sit in the branches of the trees and rarely descend to the water.

Mating and egg laying

Mating, or amplexus, takes place when the male has found the female and egg laying can occur. This frequently happens directly in the water. The female discharges her eggs and the male, firmly hanging onto his partner, fertilises them as they pass. There is no penetration and the act is consummated once the eggs have been fertilised.

Certain species lay their eggs on leaves above the water, as is the case for the tiger-striped leaf frog. Using her rear legs, the female uses a leaf to form an upside-down cone with a narrow orifice at its base and then, with the male, sticks it together using a first set of transparent eggs. The eggs which follow – the good ones – are fertilised in the same way as for other species. The sides of the leaf are folded back and stuck after each set of eggs is discharged, working upwards towards the top. A final set of transparent eggs is released to seal the top of the nest. When the tadpoles are born, they slide down to the orifice of the cone where they then fall directly into the water.

Calm returns

The pond has become peaceful once more and silence dominates its surroundings. After one intense night, the frogs, tree frogs and toads have disappeared back into the darkness. A few stragglers stay behind until mid-morning the following day, then each returns to his or her solitary ways in the undergrowth. Meanwhile, beneath the sleeping surface of the water and on the leaves of the branches above the pond, the next generation is getting ready to emerge.

There are dangers at this stage, too. Leptodeira annulata, for example, is a nocturnal tree snake which loves to devour the eggs laid on the leaves. Amongst other perils, a “false start” to the rainy season could lead to the ponds suddenly drying up. If this happens, the eggs are lost for good and the whole ritual must begin once again.

Baby boom

A few days after the eggs have been laid, most of the tadpoles are wriggling around in the pond, busy feeding themselves and trying to escape from the numerous predators – with varying degrees of success since predators are many! What with the dragonfly larvae, the water fleas and the water scorpions, it is a tough selection process for these would-be frogs. Even the tadpoles of certain species turn bad, such as the formidable Ceratophrys cornuta, the greedy offspring of the “horned frog”. Lurking at the bottom of the pond, they look out for prey swimming overhead, which they pounce on before dragging down to the depths to devour beneath the leaves. As soon as they have finished one meal, they move onto the next. And so it goes on!

This massacre continues until the tadpoles’ metamorphosis, but the sheer number of them acts as a form of protection for the species in itself. They are so many that, no matter how many predators there are and no matter how hard they try, swallowing them all is an impossible task!

New horizons

The survivors now have lungs instead of gills. Their legs have grown, first the rear ones and then the front ones, and their tail – the final energy reserve for their lives as tadpoles – gradually disappears. They leave the water once it has become a stump. These miniature frogs then set off to confront new perils and in turn reproduce following exactly the same cycle the year after – if they survive.