In the animal world, defence against predators can be achieved in many ways. If you can’t run or fight back, you can hide or camouflage yourself to pretend you’re not there, perhaps mimicking a flower, leaf or rock. You can look like something that tastes horrible or resemble something that would be very bad to get too close to.
Now, a group of scientists from Unicamp University in Sao Paulo, Brazil, have discovered a new way for an animal to deter a predator: by screaming very loudly at it in ultrasound.
What's particularly surprisingly is that the animal involved isn’t a highly annoyed mini-mammal or an irritated insect, but a frog.
The species in question is the clay robber frog (Haddadus binotatus).
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Clay robber frog
The clay robber frog is common in Brazil’s Atlantic Coastal Forest, where it lives among the leaf litter of the forest floor.
Eggs from the 60mm-long females hatch directly into tiny froglets (they don't have a tadpole stage). This lack of dependence on standing water has helped make the clay robber frog one of the most common amphibians in the forests of Rio de Janeiro and Espirito Santo states.
Such abundance means the species has been well-studied by biologists, who thought they had pretty much closed the book on what was known about the biology of the frog.
That was until ecologist Ubiratã Ferreira Souza and colleagues decided to take a closer look at the calls of the species.
Ultrasonic scream
The scientists noticed that, along with the human-audible croaks and trills, there was a huge set of ultrasonic calls, too. Careful study revealed that these were only emitted under very particular circumstances.
Whenever the frogs were deeply startled they would arch their backs and open their mouths very wide. Oddly, audible sounds appeared as their mouths began to close. Before that, the team discovered these toff-coloured amphibians made calls (not audible to the human ear) that were not just high-pitched but also very loud. Although exactly how loud is still to be worked out.
The researchers think the calls either function as a sonic blast, causing shock and pain to a potential predator, or perhaps work to attract an even bigger predator, so that the current threat runs away.
Ultrasonic calls are rare in frogs; with only three tiny species in Asia known to make them, but there they serve the same sex-and-territory functions as familiar frog calls. The clay robber frog is the only known species to use them defensively, say Souza and colleagues.
The researchers now intend to determine exactly how the clay robber frog makes this call and how loud the call is.
Reader more about the study: Ultrasonic distress calls and associated defensive behaviors in Neotropical frogs. Ubiratã Ferreira Souza, Guilherme Augusto-Alves, Mariana Retuci Pontes, Lucas Machado Botelho, Edélcio Muscat & Luís Felipe Toledo.
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