In general, water is an excellent medium or sound propagation. Loss due to transmission is far less than in air. The characteristics of the sound are, however, different because water is less compressible. At a given energy, sound pressure is 60 times, or by 36 dB, higher in water than in air, and correspondingly particle velocity 60 times smaller. Also sound propagation is about five times faster (1500 m/s) and, consequently, wave length and the range of the near field five times larger in water. Shallow water, however, is a very poor medium for sound propagation. Underwater sound is totally reflected at the water surface and also reflected on the bottom. Consequently, if the depth of a water body is less than approximately half the wave length of a particular frequency, that frequency cannot propagate far. Shallow water, thus, acts as a high-pass filter with the cut-off frequency being inversely related to water depth. In addition, the cutoff frequency depends on the hardness of the ground underneath, i.e., whether it is muddy or rocky. Xenopus communicates acoustically like other frogs and its set of calls corresponds to that found in other species. However, calling and acoustic communication take place on the bottom of water bodies. Since Xenopus is known to live in shallow ponds with depths below half a meter, the advertisement call cannot propagate far under such conditions and females cannot be attracted from a distance. Therefore, I have studied acoustic communication of Xenopus in a natural pond in South Africa. Calls are emitted from dusk to midnight. Some males were observed calling in water only 15 cm deep. While calling, males are not stationary, but rather swim around from one place to the other. By doing so, they encircle an area of 1-10 m2 that is defended as a territory by fighting against male intruders. This form of marking the territory is obviously enforced by the sound characteristics in the water. Acoustical spacing, the typical form of marking territories on land, would entail that individuals live the closer to each other the shallower the water. The borders between territories can be shifted by fighting between neighbours, but the location of the territory of a given individual remains constant throughout the season and even between seasons. Females are much less active. Some of them keep motionless for hours at the very shallow edges of the pond, often with their heads protruding slightly out of the water. Sound cannot enter into these shallow areas, and it might be that the females in those areas are unreceptive and escape from hearing the males' calls. Other females roam around very slowly, moving through less than 5-10 m altogether within a night. In three seasons, we never observed a phonotactic approach of a female towards a male. If a female happens to get into the territory of a male, the male approaches the female and begins an amplexus. The female replies by stretching and by emitting a release call. Mostly (i.e., in 90% of about 100 amplexus observed) the male finally gives up without success. If, however, the female accepts the male, they lie down on the bottom for about half an hour and then swim around all over the pond, depositing eggs in small portions at solid structures such as grass, stones or submerged twigs.
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