Evolutionary psychologist Adriano Lameira conducted a study to test two hypotheses regarding the volume of the vocal repertoire in primates. The analysis revealed that the number of animals in a group does not influence the richness of their vocal repertoire.
The analysis covered the vocal repertoires of 77 primate species and showed that their vocal stock depends on several factors: where the species lives predominantly (on land or in trees), its body mass, and the interaction of these two characteristics.
It was established that in arboreal primates, the volume of the vocal repertoire increased proportionally with an increase in their body size. For other species, there was no dependence on body size, but the habitat became the determining factor: a terrestrial lifestyle was associated with a smaller repertoire than expected, while a mixed lifestyle was associated with a larger one.
Previously, according to the social brain hypothesis, it was believed that a species' brain size correlates with its social complexity, i.e., group size. This link has been observed in primates, some cetaceans, birds, and carnivorous mammals. In 2005, scientists demonstrated that in 42 primate species, group size was related to vocal repertoire: the larger the group, the more diverse the sounds the species made. This was explained by the need for more precise communication to maintain complex social bonds, which in turn resembled the process of human speech emergence.
However, modern revisions of many primate species' dictionaries show that the direct link with group size is not always obvious. Furthermore, there are other potential factors influencing the size of the vocal repertoire, such as the animal's physique and its habitat.
Adriano Lameira from Warwick University analyzed the vocal repertoires of 77 primate species, including humans, studying their relationship with group size, brain size, body mass, and type of lifestyle (arboreal, terrestrial, or mixed), taking into account phylogenetic relationships between species.
The study found that neither group size nor brain size was a predictor of vocal repertoire size (p > 0.05). Instead, the habitat played a decisive role. In arboreal species, the habitat interacted with body size: every doubling of body mass increased the repertoire by approximately 28 percent (p = 0.027), whereas this dependence was not observed in terrestrial and mixed species.
Habitat also had an independent effect: the repertoires of species with a mixed lifestyle were 17 percent higher than predicted by a model based only on group size, brain size, and body size. Arboreal species' repertoires exceeded the prediction by 8 percent, while terrestrial ones lagged by 6 percent.
According to Lameira, the data refute the idea that primate vocal diversity is due to social complexity. The main factors appear to be physical parameters—body size and environment. The researcher hypothesized that the emergence of language in humans might have been a consequence of our ancestors' arboreal lifestyle, rather than a result of social structure.
Anthropologists previously noted that chimpanzees in the Issa Valley, living in a mosaic environment where dense forests neighbor open areas, more often move on two limbs, but do so along branches rather than on the ground. This observation led scientists to suggest that the development of bipedalism in the ancestors of modern humans was not directly linked to the transition to a terrestrial lifestyle.