A study published in a new issue of Scientific Reports suggests that the evolutionary pressures of the Miocene hominid from dense forests to open plain life may have influenced them to switch from vowel-based calls to consonant-based calls. These findings shed new light on the evolutionary development of early humanoid communication.
Climate change in the middle and late Miocene (5.3 million to 16 million years ago) replaced Africa's forests with vast open grasslands, shifting hominids from trees to surface life. However, it is unclear how this geomorphological shift affected their vocalization, as the fossil record does not contain the soft tissue needed to preserve the research. Orangutans produce non-pronounced consonant-like sounds (air produced through the mouth) and vowel-like vocalizations (produced by vocal cord vibrations) vocalizations in a complex sequence. As the last remaining arboreal apes, they are ideal for studying tree-to-earth transitions.
A team of researchers from the University of Warwick in the United Kingdom investigated recordings of the calls of orangutans in the savannah habitat of South Africa. Their calls are particularly complex, similar to syllables. The team recorded 487 calls (including a kiss-like squeak and a vowel-like muffled grunt) from 20 Sumatran and Bornean orangutan populations. These calls were re-recorded every 25 meters for a total distance of 400 meters to test how well they sounded at different distances.
The team reported that vowel-based calls were significantly harder to hear than consonant-based calls at a distance of 125 meters, while consonant-based calls showed a moderate decrease in audibility at 250 meters. In addition, less than 20% of vowel-based calls can be heard at a distance of 400 meters, compared to about 80% of consonant-based calls at this distance.
Overall, this suggests that consonant-based calls are significantly more effective in open venues. Given the prominence of consonants in modern human language, the team believes that migration to the open plain may have played an important role in the development of humanid vocal communication.