When Ross et al. started out to reconstruct the evolution of human laughter, apes were just tickled to help. Really. Expressions of human emotion, like laughing, are argued to have evolved from ancestral nonhuman primate displays. Ross et al. tested this argument by tickling 21 infant and juvenile orangutans, gorillas, chimpanzees, and bonobos, as well as three human infants, and examining the vocalizations produced by the tickling. The acoustics of these vocalizations were quantified with 11 measurements and compared between humans and apes. These eleven measurements were based on three variables, spectral variables: voiced segments, cell peak frequency, spectral slope, first spectral moment, second spectral moment, vibration regimes per call, temporal variables: call duration, intercall interval, calls per series, bouts per series, and airflow variables including alternating egressive-ingressive bouts.
Some of the results that were found included humans producing more voiced sounds with regular vocal-fold vibration compared to any other species. Only one ape, which happened to be a bonobo, made voiced sounds when tickled. The remaining apes had irregular and noisy vocal-fold vibration patterns. However, when the spectral-temporal patterns were examined they revealed that all species, including humans, could emit calls with two or more vibration regimes (patterns of energy distribution over time). Airflow results showed that humans emit egressive laughter. All great apes showed at least one instance of egressive tickle-induced calls . The longest egressive calls came from orangutans and then chimpanzees. After measuring all 11 acoustic traits in the sound produced from each species, the researchers mapped out how these sounds appeared to be related to each other, resulting in a family tree. The acoustic family tree actually matched the way the species themselves are related.
The results of the experiment suggest that the origins of human laughter can be traced back to 10-16 million years ago to the last common ancestor of human and modern great apes. The sounds of the common ancestor were likely to be longer and slower than those of humans and include a smaller number of more uniformly noisy calls that have fewer changes in vibration regimes. It was concluded that there is a large acoustic difference between the tickle induced laughter of great apes and the features of human laughter. However, these differences in laughter could have been shaped through selection and exaggeration of preexisting traits.
Why were the current acoustic properties of the human laugh selected for throughout human evolution? Also what functions may the acoustic components of the laugh have served as laughter became a large component of human social communication?
Posted by Tiffany Mallet (2)
Really interesting find! Would the differences in anatomy (if there are any large differences at all) account for the differences in the sound of a laugh between humans through bonobos? And why would this feature even be present in our genealogy? Are their any benefits that the sender gets by emitting this sound? It would be an interesting study to find out why this trait has even evolved since it only serves the purpose of showing happiness. Or is it more like an innate knee-jerk response rather than a chosen behavior when being tickled?
ReplyDeleteGreat article!!
Posted by Christine Rega
Thank you! From what I understand it is considered an innate knee-jerk response rather than a chosen behavior since it is a reaction that clearly sometimes the monkeys cannot control. Tickling is considered to be an involuntary twitch. The sender actually does benefit from this sound because it tells the"tickler", receiver to continue tickling. Tickling releases endorphins that simulate good moods which would be another benefit to the sender. Upon further research there has been no research on whether difference in anatomy would account for the difference in sound. Good questions!
ReplyDelete-Tiffany Mallet