Free-ranging finless porpoises acoustically inspect their frontal area in advance

Abstract

Echolocation events, interpulse intervals, and swimming speeds of nine free-ranging finless porpoises in an oxbow of the Yangtze River, China were recorded by datalogger systems attached on the animals. Over 120 h of successful recording indicated that the finless porpoises acoustically inspected their frontal area in advance before swimming silently. The acoustical sensing distance estimated by the interpulse interval was significantly larger than the swimming distance without echolocation beforehand. Terminal phase which was already known in the echolocation behavior of bats could be found in free-ranging finless porpoises. The terminal phase is the decreasing interpulse intervals in an echolocation pulse train that are observed just before the prey capture. During the terminal phase of finless porpoises, linearly decreased interpulse intervals were recognized. In the mean time, the swimming distance and the change of the sensing distance were closely correlated with each other. This suggests that the finless porpoise knew precisely the distance to the approaching target in the time scale of subsecond order. Acoustical sensing effort was considered to be controlled appropriately by free-ranging finless porpoises to obtain underwater information they need. [Research supported by Promotion of Basic Research Activities for Innovative Biosciences, Bio-oriented Technology Research Advancement Institution, Japan.]