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A frog's love song is a bat's dinner bell

Fatal call One male frog's loud, low mating call in search of a love connection has a deadly unintended effect - attracting frog-eating bats, researchers have found.

While the male tungara frog's love song entices the female of the species, it also creates small ripples on the ponds and puddles where the frogs gather. This helps hungry bats using a form of sonar to zero in on the frogs, scientists report today in the journal Science.

The research, conducted in Panama, sheds new light on the evolutionary arms race that has unfolded for eons between frogs and bats - one of the most interesting fights in the animal kingdom.

"Imagine the frog that's in a pond," says behavioural biologist Dr Wouter Halfwerk from the Smithsonian Tropical Research Institute in Panama. "It's like it's being spied upon by some agent that is spying on your communications."

"You try and make your love song and all of a sudden, yeah, you're screwed because someone is listening in on your call by using a completely different communication channel," says Halfwerk.

The brownish frogs, measuring less than 2 centimetres long, begin their mating calls when the sun sets in the rainforest. As the males make their calls, their vocal sacs inflate and deflate like a pulsing balloon, creating ripples on the water's surface.

The researchers conducted experiments to show that the frog's natural predator, the fringe-lipped bat, was attracted by the mating calls but also apparently employed echolocation - the use of sound waves and echoes to determine something's location - on the ripples to find the unfortunate amphibian.

The propagating ripples served as a watery bull's-eye, says Halfwerk, who is affiliated with Leiden University in the Netherlands and the University of Texas, Austin.

"When a bat flies by, the frog's first line of defence is to stop calling," says Rachel Page, a scientist with the Smithsonian Tropical Research Institute. "But the water ripples continue for another few seconds, effectively leaving a detection footprint for the approaching bat."

With one quick swoop by the bat, the male frog ended up without a mate - and without his life.

Nearly all the bats in the study preferred to attack frogs on ponds where the ripples were produced, compared to ponds where scientists made sure there were no such ripples.

The bats seemed to lose this hunting advantage when the pond was cluttered with leaves, interfering with the ripples.

"I wouldn't like being in the same situation," says Halfwerk. "It frightens me a little bit."