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Behavior

Facing the music: calls of one of the worlds most endangered dolphins

The Study: Melo-Santos, G., Figueiredo Rodrigues, A.L., Tardin, R.H., de Sá Maciel, I., Marmontel, M., Da Silva, M.L., May-Collado, L.J. 2019. The newly described Araguaian river dolphins, Inia araguaiaensis (Cetartiodactyla, Iniidae), produce a diverse repertoire of acoustic signals. PeerJ: e6670. https://doi.org/10.7717/peerj.6670

River Dolphins

Figure 1. Images of Araguaian river dolphins depicting the two forms of behaviour examined in this study; socializing (A,B) and feeding (C,D) (Melo-Santos et al., 2019)

Inia araguaiaensis was first discovered in 2014 and is one of only five extant species of “true” river dolphins, four of which have “threatened” status (under IUCN). True river dolphins, also known as botos, are genetically distinct from other river-dwelling and marine dolphin species. Botos have flexible necks and backbones, large low dorsal fins, and a long slender rostrum (beak/snout). Exclusive to the Amazon, Orinoco, and Tocantins, these evolutionary relics are some of the rarest and most endangered dolphins in the world. Unfortunately, much less is known about botos than other dolphin species, partly due to the complexities associated with studying them. In a murky and complex habitat, these solitary cetaceans are difficult to detect. Our limited knowledge, coupled with their imminent extinction, makes boto research an urgent priority.

 

Figure 2. A spectrogram of dolphin vocalizations. Source: https://en.wikipedia.org/wiki/Spectrogram

Dolphin Vocalizations

Traditional vocalization studies, conducted on captive river dolphins, used spectrograms. Spectrograms are a visual representation of sound, graphed as frequency versus time (Fig. 2). These methods lack the ability to provide characteristic or quantitative information without further analytical tools. Previous research recognized few sound types in river dolphins, with many studies reporting only three sounds, and others even suggesting that Inia are silent. Later studies found up to six different types of sounds, but the number and type of sounds have caused contention in the scientific community.

 

Despite controversy over this topic, one thing is certain:

boto communication is vastly understudied.

The Study was cleverly designed to take advantage of a population of botos that regularly visit a fish market in the Tocantins River town Mocajuba, Northern Brazil. This sampling method reduced the difficulty associated with finding them while ensuring they remained free-rangeing. Throughout 2013-2016 boto acoustics and behaviors were monitored for three to fifteen days at a time. Sound recordings were taken using hydrophones (underwater microphones); video recordings were taken using GoPro’s (action cameras). Video recordings were used to acquire visual data including the number of individuals, age class, sex, individual identification (based on markings), and behavior. Two types of behavior were studied: socializing (physical contact or swimming alongside) and feeding (open-mouthed food soliciting, poking humans with snout) (Fig. 1).

To analyze the data the authors utilized the following acoustic programs and ecological principles:

  • ARTwarp program written in MATLAB
    • Categorize acoustic signals
  • Raven Pro software from Cornell Lab of Ornithology
    • Measures and analyzes sounds
  • Beluga program written in MATLAB
    • Further sound analysis
      • Call duration
      • Contours (pitch shifts)
      • subharmonics (frequency oscillation)
      • biphonation (two pitches at the same time).
    • Whittaker rank-abundance curve (Fig. 3)
      • Shows frequency of expression of each call
    • Rarefaction curve (Fig. 4)
      • Shows whether sampling efforts have saturated (i.e. further sampling would not be expected to find new features)

Figure 3. Whittaker rank-abundance plot of 237 unique sound-types ranked by abundance (Melo-Santos et al., 2019)

Figure 4. Rarefaction curve of all call types discovered over 15.75 total cumulative hours of acoustic recording (Melo-Santos et al., 2019)

 

 

 

 

 

 

 

 

 

 

 

What does it all mean?

Figure 5. Frequency contours showing 237 sound-type resulting from ARTwarp analysis of Araguaian botos from Mocajuba, Tocantins River.

From 727 sound clips, the ARTwarp analysis found 237 unique sound-types (calls), showing that Inia araguaiaensis have a significantly more diverse acoustic repertoire than ever before reported for the entire genus (Fig. 5). The Beluga and Raven programs showed that the majority of sounds were complex and non-linear, suggesting that calls contain information about individual identity, emotional state, and social dynamics (as previously shown in whales). Similar to Pontoporia (sister genus), call lengths were short and of intermediate bandwidth. Short, low-frequency calls have likely been selected for to avoid obstacles in a complex riverine habitat, while travelling father than calls of more gregarious cetaceans. The Whittaker plot showed that few calls were highly abundant (calf-mother interactions), while the majority were rare. This finding agrees with expectation since botos are typically solitary species with the exception of mother-calf interaction. In a complex, murky river mother-calf recognition may be facilitated by vocalizations. The rarefaction curve showed no asymptotic behavior, indicating that the 237 calls are likely not a full representation of the boto language.

 

Previous studies have shown that dolphins are highly intelligent and communicative animals, but little research has focused on out-groups, such as river dolphins. This work begins to untangle a possible link in the evolutionary development of cetacean communication. Gaining insight into both evolutionary linkages and animal behavior can be extremely beneficial in terms of conservation and management.

 

Video clip: https://www.eurekalert.org/pub_releases/2019-04/uov-mrd041619.php

Sound clips: https://peerj.com/articles/6670/#supplemental-information

 

 

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