The study, published in the journal acta ethologica, was led by researchers from the Institute of Biology at the State University of Campinas in São Paulo.
Ubiratã Ferreira Souza
"Some potential predators of amphibians, such as bats, rodents and small primates, are able to emit and hear sounds at this frequency, which humans cannot," said Ubiratã Ferreira Souza, lead author of the study and a master's student at IB-UNICAMP.
"One of our hypotheses is that the distress call is directed at some of them, but it could also be that the broad frequency band is generalist, in the sense that it should scare off as many predators as possible."
According to earth.com, the research also considers the possibility that the frog's alarm call could attract another predator to the site to attack the initial threat. This defensive behavior has been observed in the leaf frog, which is endemic to the Brazilian Atlantic Forest.
By recording the frog's distress call at two different times and analyzing the sound, the researchers discovered that it ranged from 7 to 44 kHz, entering the ultrasound range at frequencies beyond human hearing, which is limited to 20 kHz.
When emitting this call, the frog adapts a defensive posture, indicative of its strategy to ward off predators.
This posture includes raising the front of the body, opening the mouth wide and moving the head back, followed by a vocalization that begins within the range of human hearing and intensifies in ultrasonic territory.
"Considering that the diversity of amphibians in Brazil is the greatest in the world, with more than 2,000 described species, it would not be surprising to discover that other frogs also emit sounds at these frequencies," said Mariana Retuci Pontes, co-author of the study and a doctoral student at IB-UNICAMP.
An accidental discovery by Pontes may indicate that another species uses this ultrasonic defense mechanism.
During a visit to the Alto Ribeira Tourist State Park, in São Paulo, the researcher came across a frog - probably an Ischnocnema henselii - with defensive behavior and songs similar to those of H. binotatus and with a Bothrops jararaca nearby, which suggests a predator-prey interaction.
According to Luís Felipe Toledo, senior author and professor at IB-UNICAMP, due to the similar habit, size and predators that threaten the two species, it is possible that Ischnocnema henselii also uses ultrasonic distress calls.
Toledo's initial suspicion of H. binotatus 's ultrasonic ability dates back to 2005, but the limitations of the equipment at the time meant that it could not be confirmed.
Although there are recordings of three species of Asian amphibians that emit ultrasonic sounds, these sounds seem to serve the purpose of intraspecific communication.
Among mammals, the use of ultrasound is predominant in species such as whales, bats, rodents and some small primates.
However, the use of ultrasound by amphibians as a defense mechanism to deter predators had not been documented before this study.
Following this groundbreaking discovery, the research team is ready to explore several intriguing questions that have arisen.
The aim is to identify the predators that can detect the distress call, understand their response and determine whether the purpose of the call is to scare off potential threats or to summon the predators of those threats.
"Is the call intended to attract an owl that is going to attack a snake that is about to eat the frog?".
Leaf frogs are crucial to the ecosystem, as they help control insect populations that serve as prey for larger animals. They have a unique life cycle, with many species laying their eggs in the foliage, 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 frogs helps scientists understand the health of forest ecosystems and the impacts of environmental change.
