Men trying to get a woman’s attention at a noisy party or crowded bar might be surprised to learn that they have something in common with a teeny, tiny Brazilian frog. An international team of researchers has discovered that female pumpkin toadlets are deaf to the mating calls of males. Sandra Goutte of the Universidade Estadual de Campinas in Brazil and now a postdoc at New York University Abu Dhabi led the team that made this surprising find. Sandra researches the evolution of anuran (that’s frogs and toads) communications systems. In most anuran species, males use their call to attract mates. The mating calls differ from species to species and have evolved over time in response to factors like the environment, females’ preference, and predators. A pumpkin toadlet’s call is very soft and difficult to hear, and it seems the species has evolved not to hear it at all.
The discovery that the frogs were deaf to their own calls was even more surprising since it wasn’t what Sandra and her colleagues set out to prove. They first tested the behavioural response of the frogs in isolation to the sound of their own calls played over a speaker. This experiment was intended to confirm that the frogs could hear the calls, yet the frogs didn’t react when they were played. This meant that either they couldn’t perceive the calls or they could hear them but couldn’t react. At this point, the researchers had no reason to suspect that the frogs couldn’t hear their own calls, but they decided to test the frogs’ hearing anyways.
“We only really considered that the frogs couldn’t hear after we had tested their hearing with auditory brainstem response tests,” explain Sandra. “From this experiment which was only supposed to verify our hypothesis arose the real discovery!” The auditory brainstem response tests they performed next examined the frogs’ hearing at a physiological level. It monitored the electrical impulses transmitted from the inner ear to the brain in response to sounds of different frequencies and amplitude. Sandra and her colleagues determined that the frogs hearing sensitivity threshold depended on frequency, showing that the toadlets weren’t sensitive to high-frequency sounds (like their own calls). To investigate whether the insensitivity was due to poor sound transmission from the air to the frogs’ bodies, the researchers then used a laser Doppler vibrometer to monitor how the skin vibrated in response to different frequencies. The experiment showed that the skin over the lungs had a resonant frequency close to that of the frogs’ calls, meaning that the sound of the calls can be transmitted via the body rather than the ears.
The final step was to look at the anatomy of the frogs’ inner ear in 4µm-thick sections that were put together digitally to create a 3D model of the pumpkin toadlet ear. The model showed that the basilar papilla, the organ responsible for perceiving high-frequency sounds, was underdeveloped and non-functional. Taken together, the listening test, auditory brainstem response test, and anatomical test indicated that the pumpkin toadlets are unable to perceive their own calls. “This experiment taught me that even when something seems obvious or logical (like ‘pumpkin toadlets call, so they must be able to hear their own calls’), we need to check it out thoroughly because nature is not always logical,” says Sandra.
Sandra says that the pumpkin toadlet and its close relative the Brachycephalus pitanga were likely always deaf to their own calls based on the fact that they share similar anatomical hearing loss. It’s likely that one of their common ancestors lost hearing and it was passed on even when the offspring split into two separate species. “The loss of high-frequency hearing in these frogs is still quite recent on a broad evolutionary scale since other species of the same genus which share recent common ancestors with the other two frogs have retained fully developed ears,” adds Sandra.
Although she’s working with a different group of frogs now, Sandra hopes to be able to work on pumpkin toadlets again soon to establish just when in their evolutionary history they lost their hearing. “This study focused on the receiving side of communication. Previously, I have worked on the evolution of signals in frogs, which is often studied separately from the auditory system. Signals and sensory systems are two parts of communication systems, and we need to integrate both sides if we want to really understand how communication systems evolve.”