Good Vibrations Help a Frog Locate Tasty Prey

Living in southern Africa, the aquatic frog Xenopus catches insects by detecting critters' vibrations on the water surface. Not able to see well in a liquid environment, the frog gets a wealth of information from the water waves that insects produce as they slosh around. The waves tell Xenopus the direction in which the insect is located. They even give the frog a general idea of the type of insect that is making the waves. To detect the water waves on its skin, the frog has about 180 receptors known as "lateral-line" organs, which are found on the skin along both sides of the body, around the eyes, and also on the head and neck. Now, researchers in Germany (Leo van Hemmen, TU Munich, +49-89-289.12362) have developed a simple model that explains how the lateral-line organs enable Xenopus to locate and classify its prey. Strikingly, the model suggests that the frog can reconstruct the shape of the water wave (its "waveform") from limited information, namely the movement of water recorded by the 180 simple sensory organs. In the frog, water gets deflected by 4-8 flag-like structures (called "cupulae") in the lateral line organs. Each deflection stimulates nearby hair cells to generate electrical spikes that are synchronized in time with the deflection. The timed electrical spikes from the 180 sensory organs, the researchers show, contain enough information for the frog to "estimate" the shape of the water wave pretty accurately. This is true even if some of the lateral-line organs are not functioning properly. Furthermore, they show how the frog can localize and distinguish between two different water waves coming simultaneously from two insects in different directions. This model may also be applicable to the mechano-sensory systems of other animals, such as crocodiles (Soares, Nature, 16 May 2002), which have similar receptor organs (Franosch et al., Physical Review Letters, upcoming).