Menacing mating game: Frogs fear bats!

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Menacing mating game: Frogs fear bats!
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bat hangs upside down with whole frog in it's mouth
A frog-eating bat with its prey. These bats detect frogs on two different channels, according to a new report.
Photo courtesy Christian Ziegler

Like any foolish fellow, the túngara frog lives and loves dangerously. To those in the tropical-bio-biz, it’s old news that this resident of shallow ponds ranging from Venezuela to Central Mexico is prey to frog-eating bats.

That croaking mating call makes a great target for the flying mammals with an appetite for frogs’ legs. But now we hear another reason why life is hard for the feckless frog.

In a study released today, scientists revealed that the croaking frog sends two separate signals to the bats: First, the mating call, which deters competing males while attracting females — and those hungry bats.

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frog in shallow water, surrounded by concentric ripples
Calling male túngara frog making ripples.
Photo courtesy Ryan Taylor

But the frogs power their croak by inflating and deflating an enormous air sac, which forms ripples on the pond that survive even after the frog chokes off its croaks after seeing a bat against the night sky

To test the interaction between the bats and frogs, first author Wouter Halfwerk of Leiden University in The Netherlands set up an experiment in Panama, using artificial frogs to simulate the appropriate sound, with or without ripples

Halfwerk and co-author Michael Ryan, professor of zoology at the University of Texas, found that bats would attack in response to the mating call alone, but the attacks increased 36 percent when ripples accompanied the soundtrack.

Frogs croak, and then croak!

This makes life difficult for the frogs, Ryan notes. To reproduce, they must call. “The males can stop calling, but they can’t take these ripples back, so the danger of calling extends for a few seconds. It’s amazing that bats are able to figure this out.”

Bats (starting from a perch outside the video frame) prefer to attack frog models if pools are rippled. Video was shot in darkness, with an infrared camera.
Video courtesy Wouter Halfwerk

The tests were held in darkness, so the bats must have been using their sonar — echolocation — to find the pattern of ripples.

The mating call is primarily to attract females, but it also shouts “Keep away!” to other males, and the competition doubled their “I’m here too!” responses when ripples followed the croaks. “If you look when they are calling, the frogs are spaced out,” Ryan says. “If another male is too close, they start to make aggressive calls, and sometimes they fight; I have seen them kill each other.”

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illustration showing a male Goldie's bird of paradise displays to a female. The male has a yellow and dark green plumage, a lavender grey breast and two long crimson tails. The female has an olive-brown plumage with cinnamon-brown below.
Top: This showy Paradesia decora male may attract the dull-colored female at bottom, but that dramatic costume makes him rather prominent to predators.
John Gerrard Keulemans, Wikimedia

Ryan, who first noticed the frog-bat system as a graduate student in the 1980s studying with bat biologist Merlin Tuttle, notes, “We have known for a long time that the bats can find the frogs.” The new study, however, is the first to show how waves created when the frog sounds off affects bats — and other frogs.

Now that they know that competing males are more responsive when ripples are present, the researchers plan to see how females act with and without ripples.

Live to love, but love to live!

If the bats have “cracked the code” of the frogs behavior, turning a mating move into a death dance, that could be shaping frog behavior. “We know frogs prefer calling under cover, compared to out in the open,” says Ryan. Not only do bats have trouble flying to the sheltered frogs, but they may also have difficulty detecting ripples with echolocation in the brush.

We mused: the frogs, like certain guys we could name, allow mating to trump everything, even mortal danger. “Through the entire animal kingdom — there are exceptions — but most of the attributes that make an animal sexy or beautiful can be very costly. Men die before women in part because testosterone has a negative effect on the immune system.”

Every time an animal communicates, it creates a disturbance in the environment, and that’s especially true for the “look-at-me” mating messages. “The question I have,” Ryan adds, “is how many of these incidental things that we animals do become fodder for another animal that is looking to parasitize, to lay eggs” or grab dinner?

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a frog lying in a shallow pool, with its vocal sac fully inflated
Photo courtesy Ryan Taylor
Calling male túngara frog with fully inflated vocal sac.

Do some showboating, and a biological big brother may be bugging your channel, Ryan says. “Males have to make themselves more conspicuous to females; to call louder, to wear brighter colors, do fancy dances. But all of this also makes them more conspicuous to predators.”

– David J. Tenenbaum

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Terry Devitt, editor; S.V. Medaris, designer/illustrator; Yilang Peng, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive

Bibliography

  1. Risky Ripples Allow Bats and Frogs to Eavesdrop on a Multisensory Sexual Display, by Wouter Halfwerk et al, Science, 24 January 2014.
  2. What is sexual selection?
  3. Sexual selection in the fossil record
  4. Frogs’ Songs Attract Females, and Predators
  5. For The Tungara Frog, Two Cues Are Better Than One