Pumpkin toadlet frogs are only known case of an animal that continues to make a communication signal even after the target audience has lost the ability to hear it.
Humans trying to chat each other up in a noisy nightclub may find verbal communication futile. But it appears even more pointless for pumpkin toadlets after scientists discovered that females have lost the ability to hear the sound of male mating calls.
An international team from Brazil, Denmark and the UK has discovered that the males of two species of tiny orange frogs continue to make high-pitched calls despite neither females nor males being able to hear them.
It is believed to be the first case in the animal kingdom of a communication signal enduring even after its target audience has lost the ability to detect it.
Field studies began in Brazil’s Atlantic forest by playing frog calls to determine how these species, which possess a middle ear, could hear their own calls.
Lead researcher Dr Sandra Goutte at the Universidade Estadual de Campinas, São Paulo, was surprised to find the frogs refused to respond to her playback communication, didn’t change their calling behaviour and didn’t even orient themselves towards the sounds.
“I thought I would find the sound transmission pathway from the outside to the middle ear,” she said. “We didn’t think it would be possible that they would not be able to hear their own calls.”
Laboratory tests of the frogs’ hearing abilities were then undertaken by Associate Professor Jakob Christensen-Dalsgaard at University of Southern Denmark.
Tiny electrodes were attached to the frog’s nostril and under skin by its ear to measure its hearing sensitivity, in much the same way as human hearing is tested.
Christensen-Dalsgaard’s findings were confirmed by anatomical studies at Cambridge University, which revealed that the part of the ear responsible for high-frequency hearing is vestigial in these species.
Scientists would expect that if a communication signal is not perceived then it would rapidly be lost through evolution.
Goutte, who is now based in Paris and studies the evolution of communication systems in frogs, said: “I guess they have lost the ability to hear the calls quite recently. The call is quite soft so maybe it has already decreased in function.”
Such calls to attract the opposite sex could not only be pointless but costly if they attracted predators or parasites. According to Goutte, though, the brightly-coloured pumpkin toadlets are highly toxic and have no known predators.
But although their futile calling poses no apparent predation risk, it still uses energy and time. Goutte believes that the calling continues because it may actually be a visual signal.
“The predation risk is low but they are expending energy so we think maybe the behaviour is kept because of a visual signal, with the movement of the throat.”
The frogs are diurnal and are known to use other visual signals: when threatened, they wave their arms and gape with their mouths. If the movement of the throat is a mating signal then the call itself might represent a byproduct of the true signalling behaviour.