Pitch–intensity coordination in wild chimpanzee (Pan troglodytes) vocalizations: a robust within-call pattern, not an ontogenetic one

Pitch–intensity covariation, or the tendency for fundamental frequency (F0) and vocal intensity to move in the same direction within a vocalization, has been proposed as a marker of maturing vocal-motor control, as the two parameters are driven by separate physiological systems that presumably must be actively coordinated rather than simply co-occurring. I tested this idea in wild chimpanzees using archival Gombe field recordings spanning infancy to adolescence (15 individuals). Voiced segments (n = 8,874) were classified with a ToBI-inspired pitch-accent × boundary-tone scheme, and two coordination metrics were computed: directional covariation across a segment's thirds, and the timing offset between peak F0 and peak intensity. Every subject in this sample showed higher covariation in the later portion of a call than the earlier portion (Wilcoxon W = 0, p < 0.001), and this held with no difference in magnitude across age classes (p = 0.73), which I take to reflect a developmentally invariant, likely physiologically grounded pattern rather than anything specific to maturation. Age-class differences in overall covariation did not reach significance under a conservative permutation test, though the adolescent-versus-infant contrast reflected a large effect size (d = 1.76) that this study was simply underpowered (66%) to confirm, which is a different conclusion than the infant-versus-juvenile contrast (d = 0.02), which looks like a true null. The peak-timing offset metric complicated this picture further by identifying a different age class as its outlier, which I take as a caution against overinterpreting either result on its own. Taken together, chimpanzee vocalizations appear to show a robust, age-invariant within-call coordination pattern, while the evidence for an ontogenetic trend remains inconclusive and would benefit from larger samples going forward.