Vocal individuality and species divergence in the contact calls of banded penguins

Abstract

Penguins produce contact calls to maintain social relationships and group cohesion. Such vocalisations have recently been demonstrated to encode individual identity information in the African penguin. Using a source-filter theory approach, we investigated whether acoustic cues of individuality can also be found in other Spheniscus penguins and the acoustic features of contact calls have diverged within this genus. We recorded vocalisations from two ex-situ colonies of Humboldt penguin and Magellanic penguin (sympatric and potentially interbreeding in the wild) and one ex-situ group of African penguins (allopatric although capable of interbreeding with the other two species in captivity). We measured 14 acoustic parameters from each vocalisation. These included temporal (duration), source-related (fundamental frequency, f0), and filter-related (formants) parameters. They were then used to carry out a series of stepwise discriminant function analyses (with cross-validation) and General Linear Model comparisons. We showed that contact calls allow individual discrimination in two additional species of the genus Spheniscus. We also found that calls can be classified according to species in a manner far greater than that attributable by chance, even though there is limited genetic distance among African, Humboldt, and Magellanic penguins. Our results provide further evidence that the source-filter theory is a valuable framework for investigating the biologically meaningful information contained in bird vocalisations. Our findings also provide novel insights into penguin vocal communication and suggest that contact calls of the penguin family are affected by selection for individuality.

Acoustic correlates of body size and individual identity in banded penguins

“…Moreover, recently, in the African penguin it has been demonstrated that both the source- and filter- related components can encode individual identity information [ 10 ]. Overall, our findings support the hypothesis that in nesting penguins, the acoustic cues to identity are the fundamental frequency and the energy distribution across the spectrum [ 33 , 55 ] and that studying the anatomical constraints that influence the vocal output with a source-filter theory approach can lead to a better understanding of the individual identity information encoded in their vocalisations [ 10 , 31 ].…”

“…These measurements included temporal measures, such as call duration which is related to lung capacity [ 45 ], source-related vocal features ( f 0 ) which are related to the vibrating mass in the syrinx [ 46 ], and filter- related features (formants), related to the supra-syringeal vocal tract [ 18 ]. Specifically, following the visual inspection of the spectrograms and previous acoustic response of vocal tract models for Spheniscus penguins [ 10 , 31 ], we extracted the contour of the first four formants ( F 1 – F 4 ) of each vocalisation. We used a Linear Predictive Coding and we set to 5 the maximum number of formants to be tracked by the Praat software below 4000Hz.…”

“…Namely, a contact call emitted by isolated birds, an agonistic call used in aggressive interactions, an ecstatic display song uttered by single birds during the breeding season, and a mutual display song vocalised by pairs at their nests [ 26 , 27 , 29 ]. The role of learning in penguin call development is thought to be absent, but the acoustic features of vocalisations are species-specific [ 30 , 31 ] and have evolved under different environmental constraints, ecological pressures, sexual and social sources of selection [ 26 , 32 ]. Moreover, the mechanisms used to encode individual identity information in vocalisations vary widely according to breeding ecology (i.e.…”

“…nesting vs non-nesting) of the different species [ 33 , 34 ]. Accordingly, it has been recently demonstrated that studying the anatomical constraints that influence nesting penguin vocalisations from a source- filter perspective, can lead to a much better understanding of the acoustic cues of individuality contained in their calls [ 10 , 31 ]. Finally, in a few penguin species, the display songs have been demonstrated to provide acoustic cues to the body size and condition of the emitter.…”

Variation in the ecstatic display call of the Gentoo Penguin (Pygoscelis papua) across regional geographic scales

“…Vocal characteristics in penguins are relatively understudied compared to other bird taxa, but studies have shown gradual interspecies differentiation over time (Thumser et al 1996, Favaro et al 2016 Seddon andvan Heezik 1993, Favaro et al 2016), Magellanic Penguins (S. magellanicus; Clark et al 2006), and Macaroni Penguins (E. chrysolophus; Searby et al 2004). These studies all indicate that while there may be differences in complexity between species, vocalizations play an important role in behavior across all penguins.…”