Frogs are able to scream - but humans can't hear them.

Frogs scream at the top of their lungs

Frogs scream at the top of their lungs

Experts have discovered that the amphibians shriek at an ear-piercing volume as a defensive mechanism to fend off other animals.

Researchers in Brazil's Amazonian jungle noticed that small leaf litter frogs were throwing back their heads and opening their mouths wide.

It appeared as if they were screaming but the scientists were unable to hear any noise.

However, the team was able to hear the sounds of the frogs using high-frequency audio recorders and captured the first documented case of "defensive ultrasound" being used by the animals.

The researchers have also theorised that screams can be used to notify other frogs to help out in an attack on another animal.

Ubirata Ferreira Souza, from Brazil's State University of Campinas, said: "Some potential predators of amphibians, such as bats, rodents and small primates, are able to emit and hear sounds at this frequency, which humans can't.

"One of our hypotheses is that the distress call is addressed to some of these, but it could also be the case that the broad frequency band is generalist in the sense that it's supposed to scare as many predators as possible."