Amphibians rely on water for a number of very important reasons. Firstly, although most modern amphibians possess lungs, many of them are extremely simple and are not completely sufficient for their needs. This means that oxygen intake is often supplemented by absorption through their thin, permeable skin – and this can only happen if it is suitably wet.
Secondly, just as moisture is absorbed through an amphibian’s skin, liquid from the body can also pass through it in the other direction and be lost. In a dry atmosphere, amphibians lose body liquids very quickly and can die from dehydration. Desiccation is a frog’s worst enemy.
And there’s another very crucial need that ties amphibians to water. Amphibian eggs, like fish eggs, are not waterproof; they usually need to be laid in water to stop them from drying out. When young amphibians hatch, they have no legs and lungs, and most possess feathery external gills that they use to extract oxygen from water. They are, of course, tadpoles and are unable to survive on dry land for very long.
Some amphibians that live in parts of the world where there is an extended dry season manage to avoid water loss through their skin by coating themselves in a waxy substance that is produced from glands on their bodies. The waxy monkey tree frog does this enthusiastically using all four of its limbs, rather like a human sunbather anointing him or herself with sun cream, though with the flexibility of a contortionist. When it has covered its entire body, the frog closes its eyes, which, if kept open, would also lose moisture through evaporation. It then goes into torpor until the cooler, wetter night arrives.
Although some anurans have minimised their need their water, their permeable skin still means that their surroundings must be relatively moist if they are not to dry out. Nonetheless, a few pioneering species live in places that you would never expect to find a water-loving creature: the desert. How can these amphibians possibly survive somewhere that might not receive rain for months on end?
The bushveld rain frog (seen in the image at the top of the page) lives in the arid, barren bush country of South Africa. Understandably, it avoids the sun as much as possible, spending the day in a cool, sandy burrow and only emerging at night to forage for its favourite food, termites. Due to its short, stumpy legs and delightfully spherical body, it cannot hop, so it comically waddles across the sand.
During the rainy season, there is a flurry of amphibian activity, for it is now – and only now – that these desert frogs breed. Males use their calls to attract females, which are swollen with eggs. The male is so small, relative to the female, and his limbs are so short, that he cannot clasp her during mating as most frogs do. The female, therefore, produces a glue-like substance that allows a male to literally stick himself to her rotund body – it’s the only way he can hold on to her as she waddles across the desert. Occasionally, though, males become stuck to females in the wrong position – back to front or even sideways.
If a male has managed to secure himself to a female in the correct position, the mating pair, while still coupled, now digs backwards into the sand or soil. It is important that the males are glued tight or they might otherwise become dislodged during the burrowing process. When the frogs find a suitably moist patch underground, the female lays her eggs and the male fertilises them. Unusually, these eggs hatch directly into miniature froglets, completely bypassing the tadpole stage.
In Australia, there is another anuran that has adapted to arid conditions. Though the water-holding frog can be found in a wide range of habitats, including forests and wetlands, it also lives in the hot, dry Australian outback. This amphibian takes life underground to the extreme. Unlike the bushveld rain frog, water-holding frogs need standing water in order to lay their eggs. And since that’s extremely rare in the desert, they must be prepared for a very long wait. Some individuals have been known to spend years underground, beyond the desiccating effects of the sun, waiting for the perfect conditions.
Only when the brief, infrequent rains arrive do these desert frogs appear on the surface. They are quick to find mates and reproduce, with the eggs being laid in small, shallow, temporary pools. Development of the tadpoles is extremely rapid, and so it must be if they are to grow into froglets before the puddles dry up again.
The adults, having now reproduced, prepare to return underground. They take in as much water as they can, absorbing it through their skin and storing it in their tissue and their specially adapted bladder. In fact, they can store so much water that they double in weight and become almost spherical in shape. These tightly bloated amphibians – whose name now becomes apparent – then dig new burrows. They secrete mucus from their skin, which hardens around them, creating a transparent, watertight cocoon that makes them look as though they have been plastic-wrapped. Their metabolic rate slows down considerably, allowing water-holding frogs to sit out the long drought for months, sometimes even years, on end.
The water held within these desert frogs’ bodies keeps them alive during long periods of inactivity, and that layer of mucus prevents water loss by evaporation through their skin. The Aboriginal people of Australia sometimes dig up these amphibians and give them a gentle squeeze to release their water. They suck the bottom of the frog and drink what is essentially very weak urine, which can be a life-saver in desperate times. The creatures are then returned to their underground chambers, usually unharmed but considerably lighter.
When the rains finally reappear, the frogs break out of their cocoons and make their way back to the surface for another short but frenzied breeding season. Though these amphibians may live for many years, their lives are, in essence, condensed down to the few brief periods when rain falls in the desert.
In our next (and penultimate) Freaky Frogs article, we’ll be taking a look at the completely aquatic African clawed frog, which was used as a pregnancy test until as late as the 1960s.