Ripples in the water make croaking frogs vulnerable
The more a Tungara frog croaks, the greater his chance of finding a partner. But the vibrations produced by the noise on the water’s surface also attract predators. This is what research conducted by Leiden biologist Wouter Halfwerk and others has shown. An article about this finding appeared on January 24 in Science.
The Tungara frog (Physalaemus pustulosus) is found in Central America. During the mating season, the male of this species spends the night in a shallow pond or puddle and tries to attract females with his croaking. This reproductive behaviour is not only audible; the croaking also unintentionally produces ripples on the water’s surface. In doing so, the amorous frog gives off signals that both friends and foes can pick up with various senses. The females receive the signals, but so do male rivals, predators, and parasites.
The ripples on the water make the rival frogs croak faster, stop croaking, or attack. And that’s not the worst of it; there are also frog-eating bats to worry about. These animals pick up the sound made by the male frog and can use it to locate and catch their prey.
‘The Tungara frog has developed a defence strategy to escape from these predators,’ says Leiden biologist Wouter Halfwerk, who is the lead author on the article in Science. ‘The frog immediately stops croaking as soon as he detects a bat and, if he’s not too late, dives to the bottom of the pool. However, the rippling of the water continues for a few more seconds. Bats use that to track down their prey and outsmart the frog’s strategy. The research shows how complex interactions between animals can be. Frogs and bats perceive the ripples in ways that are fundamentally different, and that have major consequences for the evolutionary course of communication.’
Halfwerk suggests that bats use echoes to locate their prey. While they are not blind, they use their ears to ‘see’ more often than their eyes. They make noises and pick up the echoes that bounce off objects in their surroundings. In this way a bat can determine the location and shape of those objects. The researchers discovered that bats would attack the model of a frog more often if they placed it by a pool in which they made ripples. If the frog was surrounded by leaves, that masked the rippling and the bat’s preference disappeared. ‘This indicates that the environment plays a major role,’ Halfwerk claims. ‘Considering that frogs don’t use echoes to locate objects, the surroundings have a completely different influence on how they perceive the ripples. Vegetation is probably not very important, but other things that create vibrations in the water, like wind, rain or other animals are all the more important.’
Halfwerk carried out this research with a Rubicon grant from the NWO, in collaboration with the University of Texas at Austin, the Smithsonian Tropical Research Institute (STRI) and Salisbury University.
(29 January 2014)
Biology (7 specialisations)