Stand in awe of the small but mighty pumpkin toadlet. He might only be an inch long, but his skin is packed with some of the most potent toxins on Earth. Strutting proudly through the mulch, he lets out a series of high-pitched buzzes to let nearby females know that in this patch of damp, decomposing leaves, he is king — and ready for a queen. There's only one problem. As scientists explain in a new study published in Scientific Reports, those boastful calls fall on deaf ears. Literally.
These tiny frogs can't hear their lovers' calls (but don't worry — they find them anyhow). Image: Renato Augusto Martins via Wikimedia Commons
Despite the name, it's not because pumpkin toadlets are "earless" frogs. Plenty of their earless relatives — species that lack parts of their outer and middle ears — have no trouble with sonic communication. Gardiner's Seychelle frogs hear just fine through their mouths. Golden frogs hear through their skin. It's just these carrot-coloured mini frogs that can't hear their own lustful language.
And it's not entirely clear how these tiny orange frogs ended up this way. They like to spend their days crawling about the damp leaf litter of Brazilian montane Atlantic coastal forests. Even with their cheeto colouring, they're tough to spot amongst the muck of the forest floor — so it would make sense to communicate with sounds.
Stellar communication is particularly important when mating season rolls around. Males defend small territories in the hopes of finding eager mates, and to advertise their availability, they stand tall on their front legs, showing their chests in all their tangerine glory, and calling out. The calls they make — two to six minute pulsing buzzes — were presumed to help direct female frogs to them.
But early observations of pumpkin toadlet behaviour suggested something was a bit off with that idea. For one, the calls are very quiet in relation to the background noise of the forest. And the frogs are also consistently visual, but not consistently vocal. In the rare cases where wild males were observed encountering one another, the resident males would vocalize and display, wrestling if necessary to protect their turf. But when the frogs were presented with a mirror, males leapt silently into action, furiously waving their arms in front of their eyes without making noise.
Now, scientists have discovered that there's a reason the calls seemed somewhat inconsistent: the frogs can't hear them.
"The high-frequency, low-amplitude calls of pumpkin toadlets should… require a sensitive hearing apparatus to be detected," the authors explain in the paper. Given their earless-ness, it was unclear how the frogs pulled that off. So an international collaboration consisting of scientists from Brazil, Denmark and the U.K. set out to determine exactly what the frogs can hear.
Calls recorded in the field in Brazil were first played for wild frogs. To the scientists' surprise, the frogs didn't seem to notice them. Then the researchers looked a little closer at the frogs' hearing, playing them sounds while monitoring their auditory brainstem responses. While the frogs did respond to low-frequency noises, they completely ignored any sounds above 1 kHz in frequency, including their own 3.7 — 5.7 kHz calls.
To fully understand this apparent deafness, the team studied the frogs' inner ear tissues and created three-dimensional models. What they found was a mess — the animals lacked key structures that help their earless relatives hear. The cells that normally sense sound were disorganized, and likely non-functional.
It's kind of amazing frogs this teeny can find each other without sound. Photo by Sandra Goutte
"We have never seen this before," said Jakob Christensen-Dalsgaard, an associate professor at the University of Southern Denmark and co-author of the new paper, in a press release. The lines of evidence were pretty conclusive: "These frogs make sounds that they cannot hear themselves."
Since other members of the genus have fully functional inner ears, the authors suggest that high-frequency calls are a remnant behaviour — the first example of vestigial sound communication.
"One would think that if a signal is not perceived by its target audience, it would be lost through evolution," said lead author Sandra Goutte. That's because making noise, while helpful for directing potential mates to your location, has the side-effect of letting everyone know where you are — including those that consider you a tasty meal.
But the tiny frogs probably aren't too concerned about being found. It's thought that their bright orange colours act as a neon sign signalling danger. Any predator stupid enough to ignore that warning soon discovers that their meal's skin contains tetrodotoxin and 11 other potent paralytic poisons. Because of these toxic chemical defences, the pressure to quickly become silent may be lessened, especially if the act of calling is still functioning as a visual signal.
So, if you're feeling a little down about your love life and are worried you're doomed to crawl this dreary planet alone, let the tale of the toadlet provide you with some comfort. After all, most of their attempts to attract a mate are an unintelligible waste of time for everyone involved, and yet, somehow, they still manage to figure it all out in the end.