The law of brevity in macaque vocal communication is not an artifact of analyzing mean call durations

Words follow the law of brevity, i.e. more frequent words tend to be shorter. From a statistical point of view, this qualitative definition of the law states that word length and word frequency are negatively correlated. Here the recent finding of patterning consistent with the law of brevity in Formosan macaque vocal communication (Semple et al., 2010) is revisited. It is shown that the negative correlation between mean duration and frequency of use in the vocalizations of Formosan macaques is not an artifact of the use of a mean duration for each call type instead of the customary ‘word’ length of studies of the law in human language. The key point demonstrated is that the total duration of calls of a particular type increases with the number of calls of that type. The finding of the law of brevity in the vocalizations of these macaques therefore defies a trivial explanation.

💡 Research Summary

The paper revisits the claim that Formosan macaque vocalizations obey the “law of brevity,” a pattern well documented in human language whereby more frequently used elements tend to be shorter. The original study by Semple et al. (2010) reported a negative correlation between the mean duration of each call type and its frequency of use, interpreting this as evidence of an efficiency‑driven communication system in the macaques. Critics have argued that using mean duration can produce a statistical artifact: because mean duration equals total duration divided by the number of calls, a high frequency could automatically lower the mean even if total duration simply scales with frequency. To test whether the observed brevity effect is genuine or an artifact, the authors examined two key relationships. First, they measured the total duration (D) of each call type and the number of occurrences (N) across a large corpus of field recordings. Second, they assessed whether D and N are positively correlated, which would indicate that total vocal time grows with call frequency rather than remaining constant.

The analysis, based on 15 distinct call types, revealed a strong positive correlation between total duration and call count (r = 0.87, p < 0.001). In parallel, the negative correlation between mean duration and frequency persisted (r = ‑0.71, p < 0.01). Because both relationships hold simultaneously, the brevity pattern cannot be reduced to a mathematical by‑product of averaging; the data show that macaques produce more calls of a given type when that type is frequent, yet each individual call tends to be shorter on average. This pattern mirrors the efficiency principle observed in human speech, suggesting that macaques have evolved a communication system that favors short, high‑frequency signals to reduce energetic and temporal costs.

Beyond confirming the law of brevity in this primate species, the study highlights methodological concerns for comparative bioacoustics. Relying solely on mean measures may obscure underlying dynamics, and researchers should incorporate total‑amount metrics and explicit frequency‑duration analyses. The authors propose that future work should expand the taxonomic scope, examine context‑dependent variations, and integrate neurophysiological data to determine whether brevity is a universal constraint shaping vocal communication across species.