Frog tadpoles 'scream' underwater discover scientists

Tadpoles of one frog species let out an audible "scream" when they come under attack, scientists have discovered. They only make the noise, described as a brief, clear metallic sound made up of a series of notes, when in distress.

It is the first time any vertebrate larva has been found to use sound to communicate underwater. The discovery that frog tadpoles can make sounds also raises the possibility that a host of aquatic larvae communicate in a similar way. The distress calls are made by tadpoles of the horned frog Ceratophrys ornata which lives in Argentina, Uruguay and Brazil, researchers report in the journal Acta Zoologica. Scientist Dr Guillermo Natale of the National University of La Plata in Buenos Aires, Argentina, and his colleagues, were studying the mating calls of adult frogs.

Many adult amphibians use loud sounds such as croaks to advertise their presence, and often to attract sexual partners. Until now though, researchers did not realise that amphibian larva might also produce sounds underwater. That changed when Dr Natale caught a horned frog tadpole in a pond using a hand-held net.

A tadpole "screams" when poked underwater
"We heard a brief, clear and very audible metallic-like sound," he told the BBC. C. ornata tadpoles are difficult to find in the wild, so the researchers caught a wild pair of breeding adults, and began a programme to rear the young amphibians in captivity. This enabled the scientists to better study the noise they had heard in the field.
The team discovered that C. ornata tadpoles are naturally aggressive and carnivorous, often eating the tadpoles of other frog species that they encounter. However, "much to our astonishment, they do not eat each other," says Dr Natale, who is also an assistant researcher Argentinean Research Council (Conicet). That may be because of the "screams" emitted by the tadpoles. The researchers found that when C. ornata tadpoles come into contact with, or are prodded by, an external object such as a metal spatula, they let out a brief, metallic sound consisting of a short series of higher frequency pulses. Each "scream" lasts for just 0.05 seconds.

Producing distress calls is likely to help prevent the tadpoles cannibalising each other.

Underwater call
The tadpole produce the sound by pushing air out of their lungs. The lungs develop very early in this species; tadpoles that are just three days old are capable of emitting loud distress signals. They continue to emit distress calls underwater both as tadpoles and after they have begun metamorphosis (when they become froglets).
The tadpoles also produce the sounds when they are removed from the water.

In fact, when out of the water, they make the distress call more frequently. This could be because the tadpoles can more easily access air, which they then expel.

"That tadpoles communicate somehow is simply amazing," says Dr Natale. "They possess the structures to do so within 3 days of life." He and his colleagues now want to study how and why the ability develops so rapidly, and how the sound is perceived by other tadpoles. "[We want to know] what information specifically is communicated," he says.

Few larvae of any animal species are known to produce sounds. Those that do tend to be insect larvae, which live on land, making their sounds into the air rather than underwater. For example, juveniles of one species of common silkmoth caterpillar (Antheraea polyphemus) make "clicking" sounds with their mandibles to warn off predators such as ants.

But as far as the researchers can tell, horned frog tadpoles are the first underwater larvae, and first larvae of any vertebrate, to make sounds. The discovery could have far-reaching implications for our understanding of the behaviour and ecology of amphibians, many of which are threatened by disease, habitat destruction and illegal trade.

"We have definitely underestimated their abilities," says Dr Natale. "In more than 200 years of [amphibian research] this has never been reported."

http://news.bbc.co.uk/earth/hi/earth_news/newsid_8609000/8609235.stm