A pair of blue bulges puffs outward as a male Indian bullfrog (Hoplobatrachus tigerinus) calls. Side puffs are just one of 20 patterns that male vocal sacs evolved for amplifying the calls of frogs and toads.
Dr. Raju Kasambe/Wikimedia Commons (CC BY-SA 4.0)
Here’s a great case of real life turning out to be stranger than fiction.
From baby’s first storybook to sly adult graphic novels, the story we’re told is the same: Male frogs croak with the bottom of their mouths ballooning out in one fat, rounded bubble. Yet “that’s actually only half the species of frogs,” says herpetologist Agustín Elías-Costa of the Bernardino Rivadavia Natural Science Museum in Buenos Aires. The diversity of body parts for ribbitting is astounding.
Some males serenade with a pair of separate puff-out disks like padded headphones that slipped down the frog’s neck, throbbing in brilliant blue. Some have sacs that look like balloon Mickey Mouse ears in khaki. Others ribbit with a single upright like a fat horn stub on some inflatable swimming pool toy rhino.
All together, 20 basic forms for vocal sacs have evolved among frogs and toads, Elías-Costa and herpetologist Julián Faivovich report in March in the Bulletin of the American Museum of Natural History. Still, about 18 percent of the 4,358 species examined didn’t have vocal sacs at all.
The team studied 777 specimens over 10 years of visiting museums around the world, including the Smithsonian’s National Museum of Natural History in Washington, D.C. “Libraries of nature,” Faivovich calls them. Just drawing a picture of something doesn’t authenticate details the way a preserved specimen does. These collections for biodiversity studies are “what makes them a science,” he says.
The survey showed that vocal sacs disappeared between 146 and 196 times across the very twiggy evolutionary branchings of the frog and toad family tree. That’s “an astounding number considering their biological importance,” Elías-Costa says. Even without sacs, the animals still emit sounds because, like human speech, frog and toad ribbits originate from the larynx. Vocal sacs amplify the sound and could convey nuances of male quality and sexiness, but can also tip off eavesdropping predators. Females in a few species vocalize too, but it’s mostly a male endeavor.
Now that the researchers have worked out the patterns of which species have kept or lost vocal sacs, “we can ask the ecological questions,” Elías-Costa says. Frogs and toads shop for mates in a wide range of environments, from lake edges to cozy tree holes, or even the itsy pools in the sunken center of the spiky whorls of bromeliad leaves. Each has different risks — and acoustic issues — necessitating different types of vocal sacs.
