The rainforest can be a noisy place to be, so how do you make yourself heard if you end up in trouble? For the clay robber frog, the answer is to give off an almighty scream – but it’s one that we humans can’t naturally hear. However, a team of scientists have now successfully recorded it for the first time.

The use of ultrasound is common in the animal world for communication and echolocation – bats, dolphins, and whales are all known to use it. Frogs can use ultrasound to chatter too, though some researchers suspected that they might employ it to make distress calls as well. 

Now, the screams of the clay robber frog (Haddadus binotatus), a species endemic to the Brazilian Atlantic Rainforest, have helped to confirm this theory.

To do so, the team had to get the frogs into defense mode. This involved holding the frogs by their back legs, a tried and tested method for simulating an attack by a predator. In response, the frogs raise up the front of their bodies, jerk their heads back and open their mouths wide almost as if preparing themselves, and then partially close their mouths.

As recordings revealed on two occasions, this slight closing of the mouth coincided with a high-frequency distress call. Though parts of the calls were between 7 to 20 kilohertz, a frequency that humans can hear, there were also components that went above 20kHz and up to 44 kHz – that’s ultrasound territory, which humans can’t hear.

The defensive positioning of the frogs suggests that the distress call also has a defensive purpose, but exactly how the call is meant to ward off predators is unclear. One possibility is that it scares a whole host of different predatory animals away.

“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,” said first author Ubiratã Ferreira Souza in a statement. 

“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.”

But screaming their heads off to scare away predators is just one theory. It might also be a call out to the predators of their predators, the researchers suggest.

“Could it be the case that the call is meant to attract an owl that will attack a snake that’s about to eat the frog?” Souza hypothesized.

It’s a question that the team hopes to answer with future research, which they also hope will determine whether there are other species of frogs silently screaming too.

The study is published in the journal acta ethologica.