Dugong (Dugong Dugon)

Dugongs (Dugong dugon) are members of the order Sirenia, which also includes manatees. However, they are quite different from manatees in a number of morphological, physiological, and behavioral respects. Dugongs are strictly marine and possess a dolphin-like fluke rather than paddle-like tails of manatees. Adult males have substantial tusks, which are not present in other sirenian species. Like other sirenians, dugongs have a robust fusiform body with no dorsal fin and possess paddle-like forelimbs.

Dugongs are found in the waters of approximately 40 countries along an estimated 140,000 km of coastline. The animals are found in shallow, tropical waters of the Pacific, with some of the largest populations living in northern Australia waters. The World Conservation Union (IUCN) Red List currently lists the dugong as Vulnerable (to extinction). Despite being legally protected in many countries, the main causes of dugong population decline are anthropogenic and include hunting, habitat degradation from pollution, and fishing-related fatalities. Researchers are now using passive acoustic monitoring to assess the presence and behavior of animals. Boating activity is high in the shallow waters where dugongs live; researchers are investigating potential effects of boat noise on dugong vocalizations.

Dugongs produce ‘bird-like’ vocalizations called ‘chirps’ and ‘trills,’ as well as ‘barks,’ ‘squeaks,’ ‘quacks,’ and ‘croaks.’ Chirps (or “short duration calls”) are frequency modulated, narrow band signals ranging 3–18 kHz that last less than 60 ms. These vocalizations are the most frequently detected type of dugong call. Trills are “long duration calls” that are also frequency modulated signals that consist of a rising cascade of notes in the 3–18 kHz range and last as long as 2.2 s. Barks are broadband signals of 500–2,200 Hz that last up to 120 ms. These are not observed as frequently as the other call types. The mean source level for all dugong calls has been estimated at 139 underwater dB. This relatively low source level, in the context of ambient noise levels in their habitats and transmission losses associated with the shallow waters in which they live, limits communication ranges of dugongs to short distances (likely less than tens to hundreds of meters).  

References:

• Anderson, Paul K. and Robert M.R. Barclay.  1995.  Acoustic signals of solitary dugongs: physical characteristics and behavioral correlates.  Journal of Mammalogy.  76(4): 1226-1237. https://doi.org/10.2307/1382616
• Ando-Mizobata, N., K. Ichikawa, N. Arai, and H. Kato.  2014.  Does boat noise affect dugong (Dugong dugon) vocalization?  Mammal Study.  36(2): 121-127. https://doi.org/10.3106/041.039.0208
• Ichikawa, Kotaro, Tomonari Akamatsu, Tomio Shinke, Kanjana Adulyanukosol, and Nobuaki Arai.  2011.  Callback response of dugongs to conspecific chirp playbacks.  Journal of the Acoustical Society of America.  129(6): 3623-3629. https://doi.org/10.1121/1.3586791
• Ichikawa, Kotaro, Chika Tsutsumi, Nobuaki Arai, Tomonari Akamatsu, Tomio Shinke, Takeshi Hara, and Kanjana Adulyanukosol. 2006.  Dugong (Dugong dugon) vocalization patterns recorded by automatic underwater sound monitoring systems.  Journal of the Acoustical Society of America.    119(6): 3726-3733. https://doi.org/10.1121/1.2201468
• Ichikawa, Kotaro, Tomonari Akamatsu, Tomio Shinke, Kotoe Sasamori, Yukio Miyauchi, Yuki Abe, Kanjana Adulyanukosol, and Nobuaki Arai. 2009.  Detection probability of vocalizing dugongs during playback of conspecific calls.  Journal of the Acoustical Society of America.   126(4): 1954-1959. https://doi.org/10.1121/1.3203805