Directionality of the sound beam in Leptonychotes weddelli (Mammalia: Pinnipedia)

The blind river dolphin (Platanista gangetica), first written about by Pliny the Elder in A.D. 72, was found (10 November 1968) to be the first known side-swimming cetacean. The rudimentary eye lacks the lens, but anatomical evidence suggests that the eye may serve as a light sensor. The underwater sound emissions of this species, although similar to those of the Amazon River dolphin (Inia geoffrensis), appear to be produced constantly.