Exploring Baboon Vocalizations with Speech Science Techniques
5. Articulatory question: Is production similar?
The spectral and temporal similarities that we established in the lower range of the spectrum (below 1.5 kHz), did not take into account the P2 peak of {a} around 2.3 kHz (Table 1). To explain the correspondence of this peak with the second formant in [a] we have to invoke an acoustic production model (Boë et al., 2013). This model lets us determine the maximal acoustic space within which the first two formants of all vowels occur for a vocal tract of the given length (Boë et al., 2017). We have estimated 13.4 cm as the length of the vocal tract of a male baboon. Figure 3 shows (in grey) the maximum acoustic space (MAS), which is the area enclosing all paired (F1, F2) values that could theoretically be produced by such a vocal tract, assuming total control of its shape and configurations (see Boë et al., 2017, for discussion). The spectral peaks P1 and P2 of {w}, {a}, and {hoo}, pro-vided in Table 1 and interpreted as formants, are shown as they would be located inside this maximal acoustic space.
Figure 3: F1 and F2 for [w], [a], and [u], as shown in the maximum acoustic space of a 13.4 cm vocal tract, corresponding to that of an adult male baboon.
We observed that, in agreement with our production hypothesis, {a} uttered by baboons would correspond to an open front vowel. The {hoo} is located where we would expect the mid- high back rounded [o]. The fact that it does not seem to correspond really to the high back rounded [u] might be due to the fact that lips are not sufficiently closed and protruded. For {w}, the first peak, P1, is in continuity with P1 of {a} whereas the continuity between P2 of {w} and P2 of {a} is absent. Though this means we do not have an actual trajectory for [wa.u] in F1-F2 formant space, we have indicated an approximate trajectory schematically by arrows (Figure 3).
Figure 4: Distances a and b measure the displacement of the thoracic cage;
distance c measures lip opening; distance d is from the center of the ear canal to the distal extremity of the upper lip and quantifies lip protrusion (images 6 and 15 of the video).
Figure 5: Articulatory analysis of the sample of “wahoo” extracted from the video. Synchronization of sound and image with spectrum (Praat, 50 ms analysis window), lip opening, rib cage variations, and lip protrusion (fitted with a 3rd degree polynomial curve).
We also directly studied the wahoo production on the video clip. We manu-ally estimated and reported three types of measurements from the video, image by image: (a) lip opening, (b) movement of the rib cage, and (c) lip protrusion (Figure 4). These measurements are synchronized with the sound, so as to allow analysis of the production sequence. From anatomical data (Boë et al., 2017), knowing that distance between the center of the ear canal and upper lip in the rest position is about 17 cm, we estimated the conversion factor at 5 pixels/cm. The lip opening gesture takes about 180 ms (5.5 images) and includes {w} and the beginning of {a}, with lips plateauing until its end (image 12), when the lips close to the beginning of the {hoo} (image 15). Ingressive airflow is shown by an increase of thoracic volume synchronized with {wa} and a plateau that ends at the beginning of {hoo} (image 12) together with the closing of the lips. The lip protru-sion measurement is highly variable or noisy, with a trajectory fit (using a 3rd degree polynomial curve) which reaches its maximum when the lips close (image 15).
For the 3 types of measurement we can therefore estimate, relative to the rest position, a range of variation through 1 cm for lip protrusion, 8 cm for lip opening, and 1 cm for the rib cage movement (Figure 5). The {w}
lip opening gesture is accompanied by a rise in F0 as well as a rise in the first formant. This effect is well- known for the [w] (Potter et al., 1947) and for stop consonants in general (Wang and Fillmore, 1961), and reflects the strong coupling between source and vocal tract. The fundamental frequency is correlated with the airflow at the glottis and the transition of the first formant corresponds to the opening of the vocal tract. The measurement of the protrusion supports the hypothesis of lip protrusion for {wa}. This protrusion also seems to involve a forward projection of all the tissue of the muzzle.
Overall, except for the difficulty of showing a clear F2 transition for {w}, our hypothesis of similar production mechanisms for baboon vocalizations and human speech is supported by our observations, with the caveat of course that pulmonic ingression is only paralinguistic for human speech.
6. Conclusions
There is an increasing interest from various research communities in animal vocalizations that are used for communication purposes. This is particularly true for researchers in primatology, due partly to increased interest in lan-guage evolution. We have here compared an understudied baboon vocaliza-tion, the “wahoo”, and its onomatopoetic name from human speech. We have used standard methods, commonplace in speech research, to analyze and compare certain aspects of baboon productions to similar processes in human speech. In particular, we used acoustic analysis of F0 and of spectral characteristics of the baboon wahoo to understand how it is likely perceived by humans, and we showed that those acoustic traits indeed provide support to its onomatopoetic name. We also used a video of a baboon producing a wahoo to extract quantitative articulatory data allow-ing us to understand several interestallow-ing aspects of the baboon’s production mechanism, and we showed that many of them are quite similar to human speech production mechanisms. We also verified that ingressive vocaliza-tion, which is found paralinguistically but is unusual in human speech, is common in the baboon wahoo. We thus transcribe baboon wahoo as [wa↓.
u↑] with the ↓ down arrow indicating an ingressive initial syllable, and ↑ for the egressive second syllable.
We believe we have demonstrated that standard phonetic and acoustic methods developed for speech can be profitably used for the analysis of vocalizations in non- human animals, and we recommend further explora-tory efforts in the same vein.
Acknowledgements
The authors are grateful to: Tom Larimer for the YouTube baboon video;
Thierry Legou, Arnaud Rey, Caralyn Kemp, Yannick Becker for recording and labeling the vocalizations of Papio papio baboons; Guillaume Captier for the anatomical study; and Pierre Badin, Pascal Perrier, and Jean- Luc Schwartz for helpful discussions. This research was funded partly by ANR SkullSpeech grant ANR-13-TECS-0011 “e- SwallHome- Swallowing & Res-piration: Modelling & e- Health at Home” in the “Technologies pour la Santé” program.
Research supported by grants 16-CONV-0002 (ILCB), ANR-11-LABX-0036 (BLRI) and ANR-11-IDEX-0001–02 (A*MIDEX).
Technical support from the staff of the Rousset- sur- Arc primate center is acknowledged.
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Department of Psychology and Neuroscience, St. Andrews University, St. Andrews, Scotland, UK
2Department of Anthropology, Durham University, Durham, UK