Underwater click train production by the hippopotamus (Hippopotamus amphibius) suggests an echo-ranging function

Abstract

Abstract Common hippos (Hippopotamus amphibius) live in murky waters and produce a variety of acoustic signals including underwater click trains considered to be social in function. We tested the hypothesis that click trains may function for underwater detection. We used observational and experimental methods involving 16 captive hippos to document the occurrence of click trains in different contexts and describe the acoustic parameters of the clicks. Male and female hippos produced click trains correlated with searching underwater for food items placed in their pools. Males produced click trains when alone supporting the hypothesis that these signals function for detection and are not only social in function. The frequency bandwidth of individual clicks varied and most were below 10 000 Hz. Click train production by hippos during underwater searches suggests a rudimentary form of echo-ranging that may function when other sensory systems are limited in their aquatic environment.