Sound variation and function in captive Commerson's dolphins (Cephalorhynchus commersonii)

Abstract

Commerson's dolphin (Cephalorhynchus commersonii), one of the smallest dolphin species, has been reported to produce only narrow-band high-frequency (NBHF) clicks and no whistles. To clarify their sound repertoire and examine the function of each type, we analysed the sounds and behaviour of captive Commerson's dolphins in Toba Aquarium, Japan. All recorded sounds were NBHF clicks with peak frequency >110kHz. The recorded click-trains were categorised into four types based on the changing pattern of their Inter-click intervals (ICI): Decreasing type, with continuously decreasing ICI during the last part of the train; Increasing type, with continuously increasing ICI during the last part; Fluctuating type, with fluctuating ICI; and Burst-pulse type, with very short and constant ICI. The frequency of the Decreasing type increased when approaching an object newly introduced to the tank, suggesting that the sound is used for echolocation on approach. The Burst-pulse type suddenly increased in front of the object and was often oriented towards it, suggesting that it was used for echolocation in close proximity to the object. In contrast, the Increasing type was rarely recorded during approach, but increased when a dolphin approached another dolphin. The Increasing and Burst-pulse types also increased when dolphins began social behaviours. These results suggest that some NBHF clicks have functions other than echolocation, such as communication.

Communication sounds of Commerson's dolphins (Cephalorhynchus commersonii) and contextual use of vocalizations

“…, Yoshida et al . , Reyes Reyes et al . ), with the exception of three studies that described low‐frequency pulsed sounds (Watkins and Schevill , Schochi et al .…”

“…The use of NBHF clicks for communication have been described for captive Commerson's dolphins (Yoshida et al . ) as well as Hector's dolphins ( Cephalorhynchus hectori) and harbor porpoises (Dawson , Clausen et al . ).…”

“…), little information exists about their acoustic communication signals (Yoshida et al . ). Echolocation signals of this species consist of NBHF clicks with peak frequencies between 129 kHz and 173 kHz, 3 dB bandwidth between 5 kHz and 21 kHz and duration between 56 μs and 80 μs (Kyhn et al .…”

“…The NBHF group includes at least four species of porpoises in the family Phocoenidae (Silber 1991, Li et al 2007, Villadsgaard et al 2007, Basset et al 2009), the four species of Cephalorhynchus dolphins (Kyhn et al 2009(Kyhn et al , 2010G€ otz et al 2010;Morisaka et al 2011;Reyes Reyes et al 2015), two species of Lagenorhynchus dolphins (Kyhn et al 2009(Kyhn et al , 2010, the Franciscana dolphin (Pontoporia blainvillei) (Melc on et al 2012), and the pygmy sperm whale (Kogia breviceps) (Madsen et al 2005). Some of these species have been shown to vary the emission rate of the NBHF clicks during the display of different behaviors, thereby managing to communicate with conspecifics by acquiring information from the repetition rate pattern of their clicks (Dawson 1991, Clausen et al 2010, Yoshida et al 2014. Available literature (Andersen and Amundin 1976, Madsen et al 2005, Morisaka and Connor 2007, Khyn et al 2013 supports the "acoustic crypsis" hypothesis, which assumes that killer whale predation risk was the primary selective factor favoring an echolocation and communication system in NBHF species restricted to sounds that killer whales cannot hear, above 100 kHz (Szymanski et al 1999).…”

Sensory Perception in Cetaceans: Part I—Current Knowledge about Dolphin Senses As a Representative Species

“…Most delphinids use whistles for communication, but some species use pulsed sounds (e.g., Commerson's dolphin, Cephalorhynchus commersonii; Yoshida et al, 2014). Why some delphinid and other odontocete species [e.g., the family of Phocoenidae (porpoises), the pygmy sperm whale, Kogia breviceps, and the genus of Pontoporia] do not produce whistles, but only pulse sounds was connected to the orca predation risk.…”