Frogs use a myriad ways to defend themselves against predators. Some are poisonous. Some are brightly coloured. Some even inflate their bodies with air to appear bigger.
Another way frogs have been known to defend themselves, or alert others nearby, about a predator is to make loud sounds.
Recently, scientists have found that one tiny species of frog endemic to the Brazilian Atlantic rainforest emits ultrasonic sounds that are inaudible to humans but can scare off predators.
In their work, published in the journal Acta Ethologica earlier this year, Brazilian scientists from the Institute of Biology, São Paulo, and the Project Dacnis preserve recorded the ultrasonic ‘screams’ of the tiny leaf litter frog (Haddadus binotatus) while in the rainforest of Brazil.
The researchers found that the frog – which is smaller than an inch – while emitting the scream would raise the upper portion of its body, open its mouth wide, and throw its head back. This behaviour was, they said, was a typical defensive movement against predators.
The frog would then close its mouth just a little and appear to emit a call.
However, the researchers did not hear any sounds.
They subsequently used specialised equipment to check for the presence of a call, and found that the frog was screaming at a frequency much higher than humans could hear.
After analysing the call, they ascertained that the call’s frequency spanned the 7 kHZ to 20 kHZ range, which is within humans’ hearing range, but also reached up to 20 kHZ to 44 kHZ, which is beyond what the human ear can discern.
Researchers think leaf litter frogs emit these ultrasonic calls to stave off predators or, possibly, attract other animals that might attack the predator and protect the frogs.
One of the scientists involved in this study, Mariana Retuci Pontes, had previously suspected the frogs used ultrasonic screams as a defence mechanism after she had come across what looked like a Hensel’s big-headed frog (Ischnocnema henselii) in Brazil’s rainforests.
When she picked up this frog to photograph it, it made defensive movements that resembled those of the leaf litter frog. However, she did not have the equipment to record its calls on hand at the time and could not tell if they were ultrasonic.
The finding that the leaf litter frog emits ultrasonic sounds has kicked up questions for the researchers about which predators are affected by the screams, how they react to it and what the scream’s ultimate purpose is.
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