Mientien Tree Frog 2014 06 05
Scientists have discovered that the tiny mientien tree frogs love singing … in the drain. Image: Brandon Po-Han Chou

When humans build things, animals usually die. Highways, strip malls, parking lots ... when we pave over paradise, natural habitats – and the animals that depend on them – are destroyed. But one group of amphibians in Taiwan is reaping the benefits of unstoppable human sprawl. Taiwanese road workers have inadvertently constructed an amplification system crisscrossing the island that helps singing male tree frogs find a mate. 

The diminutive mientien tree frog (Kurixalus diootocushas a high-frequency call that sounds like a cricket chirp. When mating season arrives, males usually congregate in the Taiwanese underbrush and sing a chorus of calls. Females gather to listen in the branches overhead and choose the male with the loudest call to be the father of their offspring.

Unfortunately for the little males, high-frequency sounds do not travel as far or as wide as low-frequency ones, so it's an uphill battle to make their voices heard. But scientists from the National Taiwan University recently discovered that the amphibians have hit upon an ingenious solution to overcome their lacklustre vocal performances. By placing themselves in the drainage tunnels found along the edges of most roads in Taiwan, the tiny vocalists are able to amplify their calls.

Using naturally occurring objects to alter the quality of vocalisations is an old trick used by a handful of frog species. Bornean tree-hole frogs, for example, use hollowed-out trees as a way to amplify their mating calls, changing their frequency to match the resonant frequency of the space they're in. But the mientien tree frogs' inventive solution is the first known example of animals using human-made structures to enhance their vocalisations.

Mientien Tree Frog Drain 2014 06 05
A male mientien tree frog belts out a tune in a storm drain (while keeping a wary eye out for snakes). Image: Wen-Hao Bob Tan

Researchers were first alerted to the tree frogs' amplifying activities after noticing that males tended to congregate in storm drains when mating season rolled around. Field tests revealed that a frog vocalising from inside a drain produced a call that was louder and longer than normal. The storm drains act as a reverberation chamber, providing the perfect means of broadcasting calls to the females listening in the trees above. And since female tree frogs are attracted to the loudest and longest calls, drain-singers have an advantage over their non-urbanised brethren, who prefer to sing in the bush.

But setting up camp inside a storm drain is not without its risks. Snakes also enjoy spending time in storm drains. The concrete walls of the drain retain heat, which helps keep snakes warm and strike-ready. And since many small animals use the drains as both shelter and a transportation corridor, frog-eating snakes, like the Chinese green tree viper, are often observed slithering along these miniature urban canyons.

Snake attack is just one reason drain-singing is a risky endeavour. Perhaps just as problematic – and far more embarrassing – is what can occur after a female chooses her favourite singer. Like many frog species, after a female picks her potential mate, he hops onto her back and she carries him off into the sunset to commence love-making and egg-laying. But with the slippery vertical walls of the storm drain to contend with, a female can struggle to make her exit with the added weight of an eager male on her back.

It's likely for these reasons that the researchers discovered that drain-singing males tended to perch on branches and other objects while inside the drain. A slightly elevated position might allow them to keep an eye out for snakes, while also making it easier for the females to fish them out when it comes time to mate.  

These storm drains are unlike anything that the mientien tree frog typically encounters in its natural environment, making this a fine example of the animal kingdom's ability to (sometimes) adapt to the topographical Armageddon that humans regularly unleash. Some animals, like rats or pigeons, thrive in the concrete jungles we have erected all across the planet ... but the male mientien tree frogs are probably especially thankful for human engineering. Thanks to the Taiwanese Department of Transportation's network of drainage canals, these squeaky-voiced male frogs have been transformed into the Barry Whites of the amphibian world. 

Top header image: Matthew Matheson, Flickr