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Zoology

While the rains don’t arrive

Tree frog exclusive to the Caatinga lives in holes in the trees that it closes with its hard and spiny head

On feeling the first signs that the earth is getting warmer and water is getting scarcer, the tree frog fits itself legs first into a tight hole in the tree and closes the hole using its flat and boney head in the form of a shield as a lid. This smooth and humid skinned tree frog, whose length measures between 10 and 15 centimeters, can remain lodged there for months or years, depending on the intensity of the drought, practically without losing any water, until the rains return. It spends its days immobile, half asleep, and only awakens at night, should it detect some insect near at hand. In this case, the prey is rapidly swallowed and, satisfied, the frog returns to its state of dormancy, with the organism functioning slowly. The Corythomantis greeningi is a notable example of the notable adaptation of amphibians to the chronic lack of water in the Northeast semi-arid region.

For decades it was thought that its ability to save water was due only to its dry and hard head like a stone, which closes the entrance to the hole or the rocky niches in which it hides itself. However, a team from the Butantan Institute coordinated by the biologist Carlos Jared has demonstrated that the head on its own, even while functioning as a lid, collaborates little towards the economy of water. “The very act of hiding itself and of creating a barrier with part of its body, allows for a brutal economy of water”, he explains. In an article published in the English magazine Journal of Zoology, Jared and other researchers from the Butantan Institute, the University of São Paulo (USP) and of the Federal University of São Paulo (UNIFESP) demonstrated that this rare head plays a much more important role: to protect the tree frog against predators. As well as being a helmet, it is covered with spines and poisonous glands, liberated by way of lumps similar to dark warts, much larger on the head than on the rest of the body.

Even knowing of the venom, biologist Jared, with the purpose of demonstrating how hard and thin is the head of this tree frog, secured with his hand one of the sixteen examples brought back from Angicos, in the State of Rio Grande do Norte, and maintained at the Butantan animal breeding unit. Feeling itself imprisoned, the animal immediately began to spin and rubbed its head between Jared’s fingers, liberating a whitish and viscous liquid, whose lethal level approaches that of the venom of the pit viper, a fact to which a Butantan team testified. “It hurt a little but it was superficial, it didn’t get into my bloodstream”, says the biologist after having washed his hands in a hurry. Jared suggested in a study published in 1999 that the skin secretion of this species would also have an antibiotic action, since the animal remains for a long time closed in a humid environment, probably populated with  fungi and bacteria. As another Butantan team has proven, there is in fact an antibiotic in this skin secretion.

Natural Camouflage
In the fight against its predators, the tree frog is also able to count upon the spines that form a bony layer on the skin and cover its entire head, even up until the eyelids. “With these spines”, says Jared, “it becomes very difficult for the predators to swallow the tree frog or to even remove it from its hiding place”. He believes that the spines and the poisonous glands even work against small animals such as bloodsucking insects that have discovered its camouflage – the head has the same texture and color of the bark of the tree – and they want to remove some blood from it.
Identified in 1896 by the Belgian biologist George Albert Boulanger from examples kept at the British Museum, in London, this species, exclusive to the Caatinga, gained the popular name of ‘jia-de-parede’ (wall frog) – because at times it appeared stuck to the walls of houses in the north of the state of Minas Gerais up to the  the state of Maranhão. And as well this, it was a way of differentiating it from the true ‘jia’, also known as the Labyrinth frog [local name: rã-pimenta] (Leptodactylus labyrinthicus), of an impressive size, ten times heavier than the tree frog and capable of eating two entire mice with one complete swallow.

Other amphibians make use of tricks even in the opposite direction that allow them to resist the stunning heat of the semi-arid climate. This is the case of the ‘sapo-cururu’ (Bufo jimi), a very big show off: who can be seen hunting insects even under an intense sun. He resists because, according to Jared, his skin is made up of a thick calcified dermal layer that bar the exit of water. This means of defense appears to be absent only in the region of the groin, intensely vascular, from where water penetrates into the body of amphibians. “A frog seated upon a humid area can be drinking water in this manner”, says the Butantan biologist.

On the other hand, the Proceratophrys cristiceps frogs, another species exclusive to the Caatinga, open up the road of their search for humidity with their rear feet implanted in the sand of the beds of temporary rivers, whose surface has already dried up. They can remain buried in a line of up to one meter of sand and reappear asleep when the local inhabitants dig out a well in the dry riverbeds in search of water. The state of stupor that the Proceratophrys shows at these times is the equivalent in the tropics to hibernation – it is called estivation, brought about by drought rather than cold, when the metabolism of the animals practically stops.

When the rains return, in the first months of the year, the plants are reborn from one day to the next, the earth covers itself with greenery and the tree frogs, the toads and the bull frogs come out of their state of stupor: then the croak, croak, crock of the males begins in the search for females to mate. The male Proceratophrys croak in unison and create a strong sound that even the most distant females manage to hear. Time cannot be lost: they need to reproduce and to feed themselves quickly, before the era of the rains stops and the drought returns to lay waste to the backlands.

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