How to whip up the perfect frothy frog 'meringue' nest

By Rebecca Morelle Science reporter, BBC News

Scientists have revealed how frogs perform the architectural feat of building floating foam nests.

These meringue-like structures, which help the amphibians protect their young, are renowned for their stability under the harshest of conditions.

Now, by filming Tungara frogs, researchers have found that they are built using a meticulously timed, three-stage construction process.

The research is published in the Royal Society's journal Biology Letters.

The team says that knowing more about how the foam is created could help scientists create "bio-foams" for use in medical applications, such as treating injuries at the scenes of accidents.

Floating fortresses

Tungara frogs, like many frogs species, create foam nests to protect their young as they mature from eggs to tadpoles.

But while these floating refuges look delicate, as if they could collapse into the pond they sit upon at any moment, they are in fact remarkably sturdy.

The nests are surprisingly tough despite their delicate appearance

Malcolm Kennedy, an author of the paper, from the University of Glasgow, said: "These are exposed to full sunlight, high temperatures, all kinds of infections, including parasitic ones, and yet they survive for four days without any damage, until the tadpoles leave - or if there aren't any eggs, they'll last for two weeks.

"And unlike other foams, they do not damage the membranes of eggs and sperm. They are a remarkable biological material.

"But until now, we did not now quite how the frogs used these material and made the foams."

To find out more, the research team went to Trinidad in the West Indies to train their cameras on amorous pairs of Tungara frogs (Engystomops pustulous).

The Tungara frogs were caught on camera in Trinidad

By studying the footage, frame by frame, the researchers found that the small brown amphibians whipped up their nests in several phases.

Professor Kennedy explained: "In order to begin, the male sits on the back of the female, and puts his legs underneath her legs, to collect a foam-precursor fluid."

The male frog then begins to whip this up, mixing in air bubbles by vigorously kicking his legs. He does this in short bursts, gradually increasing this "mixing" duration each time.

"This overcomes some of the biophysical problems; if he mixes for too long in the beginning, then this would disperse the fluid and it wouldn't make a foam at all," said Professor Kennedy.

Like clockwork

In this first phase, this frothy bubble raft contains no eggs. But as the male moves on to stage two of construction, he gradually begins to blend in eggs, provided by the female, who is all the while sitting beneath him. He carefully manoeuvres the eggs into the centre of the foam.

This material is resistant to bacterial and microbial damage Professor Kennedy

As the male does this, the length of time that he spends mixing and resting remains exactly the same.

Professor Kennedy says: "They do this about 200 times - they are a bit like clockwork at this stage.

"Eventually they build this 'meringue'."

Finally, in the "termination stage", the frog starts to slow down; the period between each mixing session gradually increases until finally the nest is complete.

The team believes that understanding this nest building process could help us to create a similar foam in the laboratory.

Professor Kennedy said: "This material is resistant to bacterial and microbial damage - and if you could make a spray can that could produce this, it could potentially be used on burn victims, for example, because it would prevent them from infection, but it doesn't damage cells."

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