30 01 2014 Fringe Lipped Bat
The fringe-lipped bat spies on the frogs' courtship displays in the hope of scoring an easy meal. Image: Roger Le Guen, Flickr

When he’s feeling amorous, the male túngara frog settles himself into a shallow pond for the evening somewhere in Central or South America and woos the ladies with a sexy serenade. But there’s a dangerous sub-plot to this amphibian love story: the eager suitor is blissfully unaware that his courtship signals are being intercepted by a dangerous foe: the fringe-lipped bat.

New research published in the journal Science reveals that the courtship displays of male túngara frogs (Engystomops pustulosus) have some very unintended consequences for the frogs – ‘eavesdropping’ bats can use them to hunt down their next froggy meal.

Of course, the frogs are not complete suckers. They're well aware that potential predators could be lurking anywhere, and they'll quickly fall silent mid-serenade if they sense one in the vicinity. Unfortunately, it’s not their courtship song that gives them away … it’s the ripples they set in motion when they call from the water’s surface. 

In the video below, the male frog's movements as he advertises himself to potential mates, his vocal sacs inflating and deflating like a balloon, create waves or ripples on the water's surface.

The authors of the paper suggest female frogs may use these ripples to guide them to their crooning mate. The rippling waves may also help males to work out the location of any rivals who may be doing some wooing of their own in the same pond. “This probably improves the ability of males to make decisions about calling effort and risk relative to competition from nearby conspecifics [rivals],” the authors write.

But those tiny little ripples have a serious downside, too. It's these same signals that the frog-eating fringe-lipped bat (Trachops cirrhosus) uses to its advantage when hunting. Why doesn't the bat just rely on the frogs' much-more-conspicuous courtship calls, you ask? It comes down to the frogs' own anti-predator tactics. When a frog senses the shadow of a bat overhead, it immediately falls silent to avoid being detected. It's a good strategy ... but it's not enough. Those pesky ripples often betray its location. 

Travelling slowly on the water's surface, the ripples can be detected by hungry bats up to several seconds after the cautious frog has stopped making any sound – like a lingering, treacherous 'footprint' of the frog's presence. After setting up several experiments (never fear – plastic model frogs were used!), the study's researchers were able to demonstrate that the bats could pick up on the ripple signals (probably thanks to their echolocation skills) and use them to home in on their meal.  

And just like that, the amorous amphibian pays the ultimate price in his quest for a mate. Aah, love. It'll kill ya if you're not careful. 

Full study: Halfwerk, W. Jones, P.L.; Taylor, R.C.; Ryan, M.J.; Page, R.A. in press. Risky ripples allow bats and frogs to eavesdrop on a multisensory sexual display. Science.

Top header image: Brian Gratwicke, Flickr