Inaudible screams: Amphibians use ultrasound to deter predators

A team of scientists has identified the first documented case of amphibians using ultrasound for defense against predators. This distress call is of ear-piercing intensity for various animals, but inaudible to humans. 

The study was led by researchers at State University of Campinas’s Institute of Biology (IB-UNICAMP) in São Paulo.

Scaring as many predators as possible 

“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 study lead author Ubiratã Ferreira Souza, a master student at IB-UNICAMP.

“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.”

The research also entertains the possibility that the frog’s alarm call might draw another predator to the scene to target the initial threat. This defensive behavior was observed in the leaf litter frog (Haddadus binotatus), endemic to Brazil’s Atlantic Rainforest.

Ultrasound beyond human hearing 

Upon recording the frog’s distress call during two separate instances and analyzing the sound, researchers discovered it spanned from 7 kilohertz (kHz) to 44 kHz, entering the range of ultrasound at frequencies beyond human hearing, which caps at 20 kHz.

While issuing this call, the frog adopts a defensive posture, indicative of its strategy to ward off predators. This includes lifting its body front, opening its mouth wide, and retreating its head, followed by a vocalization that starts within human hearing range and escalates into ultrasonic territory.

“In light of the fact that amphibian diversity in Brazil is the highest in the world, with more than 2,000 species described, it wouldn’t be surprising to find that other frogs also emit sounds at these frequencies,” said co-author Mariana Retuci Pontes, a PhD student at IB-UNICAMP.

Additional amphibians may use ultrasound calls 

An accidental discovery by Pontes might indicate another species employing this ultrasonic defense mechanism. During a visit to São Paulo’s Upper Ribeira State Tourism Park, she encountered a frog, likely a Hensel’s big-headed frog (Ischnocnema henselii), exhibiting similar defensive behavior and calls as the H. binotatus, with a lancehead pit viper (Bothrops jararaca) nearby, hinting at a predator-prey interaction.

According to senior author Luís Felipe Toledo, a professor at IB-UNICAMP,  due to the habit, size, and similar predators threatening both species, it is possible for Ischnocnema henselii to also use ultrasonic distress calls. Toledo’s initial suspicion of H. binotatus‘s ultrasonic capabilities dates back to 2005, though equipment limitations at the time precluded confirmation.

Ultrasound as a defense mechanism 

Although recordings exist of three Asian amphibian species emitting ultrasound calls, these sounds are seemingly serving the purpose of intra-species communication. 

Among mammals, the use of ultrasound is prevalent in species such as whales, bats, rodents, and certain small primates. However, the use of ultrasound by amphibians as a defense mechanism to deter predators was not documented before this study.

Intriguing questions remain

Following this groundbreaking discovery, the research team is set to explore several intriguing questions that have emerged. 

They aim to identify the predators that can detect the distress call, understand their response to it, and determine whether the purpose of the call is to frighten potential threats or to summon the predators of those threats. 

“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?” said Souza. 

More about leaf litter frogs

Leaf litter frogs are a fascinating group of amphibians that inhabit forest floors around the world, thriving in the dense, damp layers of fallen leaves. 

These small, often camouflaged frogs have adapted to blend into their environment, making them elusive and sometimes difficult to spot. Their coloration mimics the hues of the decomposing leaves, twigs, and soil, providing excellent camouflage against predators. 

Leaf litter frogs are crucial for the ecosystem as they help control insect populations and serve as prey for larger animals. They have a unique lifecycle, with many species laying their eggs in the leaf litter where the moist environment supports the development of the eggs and tadpoles. 

This habitat specialization makes them particularly sensitive to changes in their environment, such as deforestation, pollution, and climate change. Studying leaf litter frogs helps scientists understand the health of forest ecosystems and the impacts of environmental changes.

The study is published in the journal acta ethologica.

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