Grey Foam-nest Tree Frogs

Grey foam-nest Tree Frogs (Chiromantis xerampelina) are also called ‘Southern Foam-nest Tree Frogs’ or just ‘Foam-nest Frogs’. Their native distribution range is Southern- and Eastern Africa, in a variety of habitats. Minimum habitat requirements are temporary pools of water having trees with overhanging branches. Reason for including this frog species into the ‘Wonders of Nature’ section is their coloration adaption and unique reproduction cycle.

Brief description of Foam-nest Tree Frogs

Foam nest frogs can be easily distinguished from other frogs by the ‘kink in the spine’. In fact, it is not a kink per se, but there are two protrusions on the backside which give this impression. These frogs got large eyes with horizontal pupils and their feet are heavily webbed. The fingers are arranged in opposable pairs and got expanded discs.

Coloration adaption

Normal coloration of Chiromantis xerampelina during the daytime depends on the color of the surface they dwell on and the ambient temperature. Skin color can vary from chalky white to darkish brown, or any mottled color in between.

Once, we were catching a Foam-nest Tree Frog at Guernsey PNR near Hoedspruit and released him on a tree. Within 5 minutes time he changed color from his original brownish appearance to a replication of the tree bark color we sat him on. See photos above. It was like a ‘Wonder of Nature’ how fast he completely blended into his new habitat.

Changing the skin color of frogs under hotter temperatures is scientifically proven since 2006. Tattersall et.al. showed in this article that there is a complex interaction between thermoregulation and water balance in frogs. Simply expressed, the lighter the skin color the higher the reduction of water loss.

Besides of thermoregulation by change of coloration, Foam Tree Frogs also save water by rectal water reabsorption. And their relatively impermeable skin is resistant to desiccation. This allows them to survive dry months under tree barks and detritus. Instead of liquid urine they produce urea and uric acid, which they excrete in solid form, further saving water. They are therefore very seldom seen swimming or sitting in water like other frogs do. During dry periods they are able to reduce their body shape to a minimum and cover themselves with mucus as a kind of watertight cover.

Reproduction procedure

Foam Tree Frogs are spawning both on trees and in stagnant, oxidized water. But the majority of them spawns on trees. At the onset of the rainy season, which is in Southern Africa around December, females expose themselves to branches over water. Immediately they will be joined by up to 12 males. One of the males will take an amplexus by an axillary clasp. There is no rivalry amongst these males, and all are accepted to attend. The female begins producing a mucus-like fluid from her cloaca, which she churns with her hindlegs into an elastic, white froth. Into this froth she will lay 500 – 1250 eggs. Both the male in amplexus as well as all other males will add their sperm fluids into this egg-froth mixture.

This nest construction takes around six hours, and the female will leave temporarily about 2 – 4 times to get into water and rehydrate herself. After every return another male will be in amplexus. That polyandrous behavior assures that only the strongest sperms will fertilize the eggs and the superfluous sperms will serve as additional nourishment for the tadpoles.

Tadpole development

Next day, the female will seal the outside surface of the foam nest by another type of frothy material. It waterproofs the foam nest, avoids drying out the eggs and reinforces it against predation. About six days after hatching, the weight of the tadpoles will be so heavy, that they can wiggle out and fall through the solid foam into the water body below.

After only five days the young froglets – still with a long tail attached – leave the dangerous watery environment and climb the next available tree, where they finalize their metamorphosis into a young frog.

Lessons learned about Grey Foam-nest Tree Frogs:

• Foam-nest Tree Frogs are uniquely adapted to dry environments.
• They will change skin color for camouflage, thermoregulation, and against water loss.
• These frogs show an extreme polyandrous reproduction behavior, which assures genetically strong offspring.
• White foam masses on branches over open water bodies are the nests of this frog species, in which the tadpoles incubate.