Commerson’s dolphins (Cephalorhynchus commersonii) can relax acoustic crypsis

Toothed whales use powerful ultrasonic biosonar pulses (i.e. clicks) for echolocation. Underwater acoustic recordings have suggested that the majority of toothed whale species can be grouped acoustically as either producing broadband clicks or narrowband high-frequency (NBHF) clicks. Recently, it has been shown that Heaviside’s dolphins, Cephalorhynchus heavisidii, emit NBHF clicks for echolocation but also clicks of lower frequency and broader bandwidth for communication. Here, we use acoustic recorders and drone video footage to reinforce previous findings that Commerson’s dolphins (C. commersonii) produce signals similar to Heaviside’s dolphins. We reveal that they use clicks with a lower frequency and broader bandwidth in the form of click trains and burst-pulses. These sounds were not recorded in the presence of smaller groups of Commerson’s dolphins, indicating that they may fulfil a communication function in larger groups. Also, we utilised a novel combination of drone video footage paired with underwater acoustic recordings to estimate the source level of echolocation clicks produced by Commerson’s dolphins. In addition, we compare the acoustic signals produced by Commerson’s and Heaviside’s dolphins to identify interspecific similarities and differences. Spectral differences were found in NBHF click trains, buzzes and burst-pulses between species; however, bandwidth and duration parameters were not significantly different for broadband click trains. Our findings make it likely that all four species of the Cephalorhynchus genus have the ability to generate both signal types, and further challenges the evolutionary concept of NBHF signal production. This study confirms the presence of a duel echolocation click (i.e. biosonar) strategy in Commerson’s dolphins, making them the second species of their genus known to produce two types of biosonar. We provide an in-depth quantitative analysis of Commerson’s dolphin acoustic signal types, and include a comparison of signal types between Commerson’s dolphins and the other species known to produce two types of biosonar, the Heaviside’s dolphin. In addition, this is the first study to combine drone footage with underwater acoustic recordings to measure the source level of toothed whale echolocation signals. We use this novel technique to provide source levels measured from Commerson’s dolphin echolocation clicks which are comparable to published values for this species calculated using an expensive and complicated array of hydrophones. Thus, we provide a simpler and more cost effective way to study sounds produced by marine mammals.