Science News: How an indri shapes a song to carry

High in Madagascar’s forest canopy, an indri opens its mouth as its song rises. A study published on 21 September turns that visible movement into a measurable question: does jaw position change with pitch in a way that could help the call carry?

Measured

Carugati and colleagues analysed 83 synchronised video recordings of 19 wild indris in PLOS Computational Biology. A markerless pose-estimation system tracked points on the upper and lower lips frame by frame. The researchers paired the distance between those points with the fundamental frequency—the acoustic rate perceived broadly as pitch—of the corresponding sound.

Wider vertical mouth openings were associated with higher fundamental frequencies. The team also constructed a computational vocal-tract model from skull measurements. Its estimated resonances, known as formants, shifted as the simulated mouth opened.

Inferred

The results support the hypothesis that indris coordinate jaw position with pitch and may tune the vocal tract so that its resonances align with the note or its harmonics. Such alignment can increase sound intensity. For an animal communicating across dense forest, that could improve long-distance transmission.

Not demonstrated

• The study did not show that the adjustment is deliberate.
• It did not directly measure how far each song travelled or how another indri responded.
• Two-dimensional video cannot fully recover three-dimensional jaw movement; head angle and vegetation introduced measurement problems.
• Other changes—such as lip or throat configuration—could contribute to the acoustic pattern.

The authors manually checked and excluded obstructed or badly angled frames. That improves the retained measurements but biases the sample towards near-frontal views. Their conclusion is therefore appropriately phrased as evidence consistent with tuning, not proof of a conscious technique.

In ordinary AE terms, a signal’s effectiveness depends on a relation among body, behaviour, forest and listener. The paper measures one part of that relation unusually well. It does not establish the whole communicative system, still less validate AE. A competing explanation—automatic biomechanics linking jaw opening and pitch—would remain plausible until experiments or richer three-dimensional observations distinguish active adjustment from physical coupling.

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