Oceanographic drivers of population differentiation in Indo-Pacific bottlenose (Tursiops aduncus) and humpback (Sousa spp.) dolphins of the northern Bay of Bengal

The Bay of Bengal is one of the most productive ecosystems in the northern Indian Ocean and it harbours a rich community of cetaceans, including Indo-Pacific bottlenose (Tursiops aduncus) and humpback (Sousa spp.) dolphins. The taxonomy of these genera has been controversial, but within the Indian Ocean both seem to be divided into phylogenetically discrete units that range from the east to the west. Within the Sousa genus, S. plumbea is distributed in the western Indian Ocean while S. chinensis is distributed in the eastern Indian and western Pacific Ocean. T. aduncus has a discontinuous distribution throughout the Indo-Pacific Ocean and two different phylogenetic units are known to exist, one along the eastern African coast and another one in the eastern Indian and west Pacific Ocean. In this study we investigate the phylogeography of Indo-Pacific humpback and bottlenose dolphins in the northern Bay of Bengal. We sequenced the mitochondrial DNA control region for 17 bottlenose and 15 humpback dolphins and compared the results with previously published sequences within each genus. In both cases, we found that Bangladesh dolphins are genetically different from neighbouring populations. While the Bangladesh T. aduncus seem to be more closely related to the African T. aduncus form than the Pacific form, Sousa spp. seem to be more closely related to individuals from Australia. The genetic uniqueness of these populations has important evolutionary implications, due to their isolation, coastal distribution in a geographic cul-de-sac characterized by an extreme infusion, redistribution and recycling of biological productivity, and conservation implications since their survival is threatened in particular by fatal interactions with fisheries. We suggest that the particular and extreme oceanographic conditions found in the Bay of Bengal may be driving speciation in these dolphins and other marine megafauna.info:eu-repo/semantics/publishedVersio

Clicking in Shallow Rivers:Short-Range Echolocation of Irrawaddy and Ganges River Dolphins in a Shallow, Acoustically Complex Habitat

Toothed whales (Cetacea, odontoceti) use biosonar to navigate their environment and to find and catch prey. All studied toothed whale species have evolved highly directional, high-amplitude ultrasonic clicks suited for long-range echolocation of prey in open water. Little is known about the biosonar signals of toothed whale species inhabiting freshwater habitats such as endangered river dolphins. To address the evolutionary pressures shaping the echolocation signal parameters of non-marine toothed whales, we investigated the biosonar source parameters of Ganges river dolphins (Platanista gangetica gangetica) and Irrawaddy dolphins (Orcaella brevirostris) within the river systems of the Sundarban mangrove forest. Both Ganges and Irrawaddy dolphins produced echolocation clicks with a high repetition rate and low source level compared to marine species. Irrawaddy dolphins, inhabiting coastal and riverine habitats, produced a mean source level of 195 dB (max 203 dB) re 1 µPapp whereas Ganges river dolphins, living exclusively upriver, produced a mean source level of 184 dB (max 191) re 1 µPapp. These source levels are 1–2 orders of magnitude lower than those of similar sized marine delphinids and may reflect an adaptation to a shallow, acoustically complex freshwater habitat with high reverberation and acoustic clutter. The centroid frequency of Ganges river dolphin clicks are an octave lower than predicted from scaling, but with an estimated beamwidth comparable to that of porpoises. The unique bony maxillary crests found in the Platanista forehead may help achieve a higher directionality than expected using clicks nearly an octave lower than similar sized odontocetes

Biosonar parameters of Irrawaddy dolphins (<i>Orcaella brevirostris</i>) and Ganges river dolphins (<i>Platanista gangetica gangetica</i>).

*<p>Click parameter abbreviations: SL_pp : peak-to-peak source level; SL_RMS : RMS source level within a −10 dB energy window; SL_EFD: Energy flux density source level within a −10 dB energy window; D_-10dB: Click duration (−10 dB energy window); Fc: centroid frequency; Fp: peak frequency; BW: Bandwidth (−3 dB, −10 dB or root-mean-square); Q_RMS: Ratio of centroid frequency to RMS bandwidth; ICI: Inter-click interval.</p

Directionality of Ganges river dolphin biosonar.

<p>Composite horizontal directionality plot of biosonar signals from Ganges river dolphins with original data (black squares) redigitized from Bahl et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059284#pone.0059284-Bahl1" target="_blank">[65]</a>. Gray line is a best fitting piston model of an on-axis click transmitted through a circular piston with a radius of 9.7 cm. The symmetrical -3 dB beamwidth of the fitted piston model is 14.5 degrees.</p

Interclick intervals of Irrawaddy dolphins and Ganges river dolphin echolocation signals.

<p>Histograms show the distribution of interclick intervals for clean series of off-axis clicks from Irrawaddy dolphins (A) and Ganges river dolphins (B). Black interrupted lines show log-normal probability density functions fitted to the data. For Irrawaddy dolphins, median ICI was 30.1 ms (N = 923) while for Ganges river dolphins, median ICI was 27.8 ms (N = 614).</p

Field site and distribution of Irrawaddy dolphins and Ganges river dolphins.

<p>A) Map of the Sundarbans mangrove forest, Bangladesh, including sighting data of Ganges river dolphins (triangles) and Irrawaddy dolphins (circles). Adapted with permission from Smith et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059284#pone.0059284-Smith5" target="_blank">[73]</a>. Inserts show pictures of B) Irrawaddy dolphin, and C) Ganges river dolphin, taken by E. & R. Mansur, WCS.</p

Calibration of the acoustic localization procedure with a vertical 4-hydrophone array.

<p>Top: Localization range (mean ± SD) given by the acoustic localization procedure, as a function of the calibration distance. Precise localization indicated by the dotted line. Bottom: RMS error in the estimated transmission loss as a function of range from the array.</p

Species discrimination based on centroid frequency relevant for passive acoustic monitoring in the Sundarbans.

<p>A: Theoretical normalized probability density functions based on centroid frequency estimates (mean ± SD from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059284#pone-0059284-t001" target="_blank">Table 1</a>) from Ganges river dolphins (grey: PGG), and Irrawaddy dolphins (black: OB) and based on peak frequency estimates from Yangtze finless porpoise (NPA) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059284#pone.0059284-Li1" target="_blank">[42]</a> assuming normally distributed estimates. Abbreviations are for latin species names. Stacked bar plot indicates probability density of centroid frequency estimates for this study. Best separation criterion (stipled lines) provides a theoretical 98.7% correct classification of Ganges river dolphin clicks and 98.9% correct classification of Irrawaddy dolphin clicks. B+C: For off-axis clicks, spectral distortion increases low-frequency energy so centroid frequency estimates decrease (B: Irrawaddy dolphins, and C: Ganges river dolphins), reducing success rate of Irrawaddy classifications to 72.7% (N = 971) with the remainder being misclassified as Ganges river dolphins, and with Ganges river dolphins being classified successfully 99.2% of the time (N = 641).</p

Source levels of Irrawaddy dolphins and Ganges river dolphins.

<p>A) Estimated RMS source levels (SL) as a function of range between hydrophone array and estimated source position for both Irrawaddy dolphins (black) and Ganges river dolphins (grey). B) Normalized density estimates of the SL from both species, estimated using normal kernels with a 3 dB kernel width.</p

The unique cranial morphology of Ganges river dolphins.

<p>Cranial morphology of a Ganges river dolphin as seen from A) a left lateral and slightly anterior viewpoint, and B) an anterior viewpoint looking back along the anterior-posterior axis. Notice the unusual, highly porous bony maxillary crests that project anteriorly over the rostrum and nearly encircle the melon. Photos by A. Galatius.</p