Meet the frogs that build ‘cloudy’ nests in South Africa’s trees

For most frogs, life begins as a game of odds. They lay their eggs in the water, where fish and insects gobble up about 98% of them before they even have a chance to hatch. The African grey foam-nest tree frog (Chiromantis xerampelina) figured a way out: it took its nest into the air.

In the rainy season between October and February, a female frog finds a branch overhanging a pool and begins secreting a special fluid. As the male and the female mate, the pair uses their hind legs like whisks to churn the secretions into a thick, bubbly froth. The result is a foamy nest.

A few days later, the eggs hatch inside this nest. When the tadpoles are ready, they simply break through the bottom of the foam and drop into the pool below, beginning the next stage of their lives.

While a single pair of frogs can build a nest, these frogs prefer a ‘foam party’ approach. Researchers from three Australian universities recently studied these frogs in South Africa and found that when it comes to nesting, there is real strength in numbers.

The findings were published in the journal Evolution on June 2.

Better homes

Sometimes, a single female is joined by a dozen or more males. This results in a better home rather than chaos. More males meant more legs churning, leading to nests up to three-times larger than those built by a single pair.

While smaller nests dry out quickly, killing the eggs inside, larger nests hold onto moisture for longer, ensuring more tadpoles survive from the nest.

You might wonder: why would a male frog help build a nest if he is competing with 12 other frogs?

Using DNA testing, the researchers found that in these crowded nests, paternity is often split. By cooperating to build a large, moist fortress, the males ensure at least some of their offspring survive, rather than losing all of them to a dried-out nest.

The scientists also suspected that the males’ skin and fluids contain surfactants — compounds similar to laundry detergent — that keep the bubbles from popping.

A neighbourhood effort

Occasionally, the frogs go even bigger, forming communal spawns where dozens of females and hundreds of males create mega-nests. These giant structures are virtually immune to drying out.

So why doesn’t every frog join a mega-nest? Because evolution is a balancing act. In a large crowd, it is easier for predators to spot the foam. Diseases can also spread faster. There is also a risk of cheaters — females who show up to lay eggs but don’t contribute any of the expensive fluids needed to make the foam.

The African grey foam-nest tree frog reminds us that survival is not always about being the strongest or the fastest. Sometimes, it is about having the best teammates, the best chemistry, and the most impressive bubble-making skills in the forest.

D.P. Kasbekar is a retired scientist.