Comparing recordings from four chimp populations, both wild and captive, researchers have now found chimpanzees also produce lip-smacks at an average speech-like rhythm of 4 hertz.
In great apes, however, the fastest mouth rhythms tend to keep a steady rhythm around a single hertz, so the authors think the variability of lip-smacking frequencies in chimp populations may imply social factors instead of hard-wired signals.
Still, between captive and wild populations, the authors found no systematic difference in mouth signals, probably because of a "substantial overlap in the range of rhythms present" among individuals in different groups.