Have you ever come across a mass of foam clinging to a branch or rock over hanging a body of water and wondered what an earth it was? I remember first seeing this strange thing in Hwange during the rains, and asking the guides at the camp what it was.
We often used to get the Chriomantis frogs lurking in the bathrooms, or behind jars in the storeroom, but I had never put two and two together and realised that these little guys were responsible for the weird lumps of foam dotted around the landscape in the rains.
This little guy is found throughout southern Africa, and is mainly an arboreal frog who can survive in arid regions. They have tiny sticky disks on their toes allowing them to cling to most things. They also come in various shades of grey and brown, depending on the air temperature. They have water-loss resistant skin which, with the change in colour can reduce the amount of water that is loss from their little bodies. The darker they are, they are more in tune with the air temperature around them, but the lighter they go the more they are regulating their body temperature by reflecting the heat away. You often see them sitting there with their arms and legs tucked closely into their body, and this is because this ‘waterproof skin’ only appears on their backs and not on the underside of their body. So by tucking everything in tightly they are only exposing the waterproof bits to the elements.
The Foam-Nest Frog is an extreme example of polyandry behaviour. The female will select her chosen branch, and begins releasing eggs onto the branch along with a secretion that she whips and churns, with her back legs, into a foam. She will then be joined by up to 12 males who will all come and release their sperm onto the eggs thus fertilising them. It is not uncommon for more than one female to share nests as well. Females return the next night to add another layer of foam to the nest to secure the eggs and as it drys forms an outer crust. Records have shown that an average size foam nest can hold up to 1200 eggs. When the tadpoles hatch they rely on the air in the bubbles for oxygen, all the while squirming and wiggling about, making their way to the bottom of the nest. Eventually the crust on the nest breaks and the tadpoles fall into the water below and develop into adult frogs.
