In April 2018 I was driving across the flat expanse of Tsavo East National Park. A huge storm was hanging over the Voi River to the east; dark curtains of rain trailing below the cloud. As I approached the storm blew away and the sun came out, but the road was soaked, puddles everywhere. It was like driving down a shallow river. And, to my astonishment, running off the road in every direction was an explosion of bullfrogs. The rain had brought them out from underground in their thousands. Here was the explanation why, in some African legends, frogs are believed to have fallen from the sky. Life in Africa, especially in the dryer areas, is tough for amphibians. In the gentle climates of Europe, it rains a lot; ponds and rivers are ever-present and happy frogs can always find somewhere to keep wet. But in much of Africa, for most of the year the bush country is bone dry; the seasonal rivers contain only sand, the pans are hard crusts, with not a drop of water to be found.
And then, suddenly, down comes the rain, creating temporary pools. Africa’s savannah frogs use these in an astonishing high-speed reproduction strategy. In the dry season they aestivate, waiting out the hard times in a state of suspended animation, buried deep in the soil, often surrounded by a waterproof cocoon that they secrete themselves. But when the rains come, they are galvanised into action. Stimulated by changes in air pressure, the boom of thunder and the patter of raindrops, the frogs wake up, dig their way to the surface and rush to the pools. As darkness falls, the males begin to call, filling the night with melody. The females arrive and frantic mating occurs. The female releases a stream of eggs, the male on her back fertilises them, the primed eggs settle in the water. They hatch into tadpoles in a few days. The tadpoles have gills, they eat, they metamorphose into froglets, and they keep eating the insects that have also appeared. Their weight may increase fifty-fold in just forty days or less.
As the rainy season ends and the pools dry out, the frogs burrow back down, often to depths of a metre or more, secrete their cocoons, slow down and await the next big rainy season. It is a mad lifestyle; evolved to suit a hostile environment. I once arrived at a lodge in northern Botswana in the teeth of a huge rainstorm. Bullfrogs were everywhere. I asked the lodge owner when he had last seen bullfrogs. Seven years ago, he told me.