Clever Tree Frogs Shout For Sex By Using Storm Drains As Megaphones

Human males try all kinds of things to seduce females. Some show off their fancy cars, some learn to play the guitar, some drive big trucks. The male Mientien tree frog scoffs at our puny human tricks. All it needs is a nice storm drain.

Like many frogs, the male Mientien tree frog (Kurixalus idiootocus), a native of Taiwan, advertises his availability for breeding by emitting a mating call. The females listen to the calls, evaluate them, choose their preferred mating partner, and then it's game on – froggy style.

The problem is that in a noisy forest, it's sometimes hard to pick out a meaningful sound from all the racket. As a result, many frogs have learned to take advantage of features in their environment that can artificially boost their calls, perhaps making them sound louder or deeper-pitched. Tree-hole frogs, for example, use their tree holes as a sort of microphone, taking advantage of the resonance effects of the hollowed out chamber to project their calls over greater distances.

While frogs evolved to capitalize on things like tree holes, human-made structures are relatively new. Concrete walls, asphalt roads, telephone poles, and all the other trappings of human material culture have changed the acoustic environment in which animals now find themselves. Those animals have a choice: they can either adapt to their new sonic environment, or they can abandon their older methods for communication and find new ones.

In Taiwan, the Mientien tree frog has had to contend with open concrete storm drains, or what National Taiwan University researcher Yu-Teh Kirk Lin and colleagues have called "miniature urban canyons." They're commonly found in suburan and rural areas alongside paved roads and foot trails. Lots of frogs live in the drains, sometimes reproducing there as well, and some animals use them as movement corridors. "The impervious parallel concrete walls allow sounds to ricochet and linger, and can alter sound characteristics due to reverberation," they say.

That can mean one of two things for animal calls: they can become degraded, due to the effect of the echo, since receivers would hear not just the initial signal but also the reflected ones, or they can become strengthened. In particular, narrow low frequency sounds can benefit from an echo, since the reflection can increase the amplitude and length of the sound waves.

The researchers discovered that calls from within the drains sounded louder and longer than calls made outside of drains. The researchers suspected, but could not verify, that storm drain-enhanced calls were more attractive to females. Previous studies have indicated that female frogs preferred longer calls, so it's a reasonable assumption to make.

What's unique about this particular frog species is that the males' mating calls have a relatively high frequency (2-3 kHz). High frequency calls are especially vulnerable to degradation because of obstacles in the environment. So high frequency callers, like the Mienetien tree frogs, suffer a trade-off: they can either call towards a particular direction for a distant receiver, or they can call in all directions but only reach receivers who are close.

The frogs have learned to overcome that limitation by calling from within storm drains, since the drains enhance both volume and duration, allowing the frogs' calls to reach receivers both nearby and farther away.

"Concrete drains are miniature canyons, but are not analogous to anything in Mientien tree frog natural habitats," the researchers say. "Therefore, it is interesting to find those frogs preferentially calling in the drains." The frogs have taken the human built environment and turned it into a tool, rather than an obstacle to overcome, allowing their species to survive an environment dominated by our species.

[Journal of Zoology]

Header image: LiChieh Pan/Flickr