A Brazilian torrent frog stakes his small claim by the side of the stream, where he spends the day uttering a high-pitched chirp. When a rival male gets too close, he lets out a series of peeps and squeals and waves his arm at the intruder. The offender retreats. Message received.
Torrent frogs (Hylodes japi) have some of the most sophisticated communication methods yet observed in frogs, using a diverse repertoire of movements and sounds to ward off competitors frogs and attract mates, according to a new study published in PLOS ONE.
To get their point across to their fellow frogs, torrent frogs may do more than chirp and wave their arms: They also tap their toes, shake their heads, inflate their vocal sacs or use a number of squeaks and squeals.
The study reveals previously unknown visual displays performed by the frogs and an elaborate courtship ritual, suggesting H. japi and other species in the genus Hylodes have evolved more advanced modes of communication than scientists previously thought.
H. japi, only recently identified as a distinct species, live in the forest of Serra do Japi in southeastern Brazil. The territorial males are most active in the rainy season between October and March. They spend their time on the banks of fast-flowing streams, where they sit, chirp and wait.
"They are calling all the time during the day. ... It's beautiful," said Fábio Perin de Sá, a herpetologist at the University Estadual Paulista in Sao Paolo, Brazil, and the study's lead author. "You can go near the forest, and you can hear it from [inside] the car. And you can hear it all the time."
In the study, de Sá, who first identified and described H. japi in 2015, wanted to document and characterize the many ways in which the frogs communicate. But the secretive amphibians can be challenging to observe, he said.
Over 15 months of fieldwork, de Sá and his colleagues conducted forest stakeouts, sometimes camouflaging themselves or leaving their camera out to record. Eventually, the frogs got used to the researchers' presence.
And the scientists began to notice patterns.
The frogs were busiest during the day, but sometimes the males continued their calls all night. The purpose of all that chirping is to advertise the male's presence. It's a warning to other males to stay away and an invitation to females to come on over.
When a female finds her way to the male's area, the male changes his tune to a pattern of peeps and squeals and begins a series of various toe-trembling, arm-shaking, head-bobbing and vocal sac-inflating displays.
If the female likes what she sees and hears, she'll lift her arm and touch one of his feet — to which the male responds with a five-note courtship call.
The researchers found the male was three times more likely to produce courtship calls if he could see the female touch his foot. A courtship call signals the end of the 53-minute ritual, which ends in the couple jumping into the stream together.
The researchers believe the combination of movements and sounds, as well as touches, work together to more efficiently improve the message the frog is sending. The frogs choose which vocal sac to inflate or which arm to raise in their displays, which would increase the efficiency of their communications, the authors wrote.
The frogs live in a darkened environment with noisy streams, and it's possible these conditions facilitated the evolution of more visual communication, de Sá said.
On the dark forest floor, the frogs' markings may aid their visual displays. When a frog waves a reddish brown arm in front of its cream-colored belly, the contrast between the two body parts causes a flashing effect that can be seen more easily by potential mates and rivals alike.
Male frogs also use both acoustic and visual signals to avoid getting into fights with each other. When a male spots another frog in his territory, he'll start peeping and squealing and waving his arms. The display resembles the male's reaction to spotting a female.
"Other males see (arm waving) as an aggressive signal, but females see it as a courtship signal," de Sá said. "It has a double function. Different signals in different contexts have different functions."
The study notes that other Hylodes species use similar calls and forms of visual communication, but none more complex than H. japi. The study acknowledged, however, that the more a species is studied, the more diverse its repertoire seems to be.
In complex tropical forest ecosystems with many species, the organisms themselves tend to exhibit more complex behavior, the authors wrote. Visual communication in frogs tends to occur in day-dwelling species and seems to be more complex in species that live and feed near noisy streams, such as the Brazilian torrent frog.
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