A new study published in Evolution Letters suggests vocal repertoire efficiency in rock hyraxes is driven by call amplitude rather than duration, challenging predictions of the Law of Brevity. Authors Dr Vlad Demartsev and Dr. Amyiaal Ilany tell us more.
Social living in nature is tightly bound with the ability to communicate with others. Maintaining social ties and coordinating with group mates require frequent communication; therefore, complex social systems are usually associated with well-developed communication abilities. The apex of communicational complexity is undoubtedly human language; characterised by a nearly unlimited capacity for information transmission, it is considered to be one of the main factors facilitating the formation of complex human society and culture. However, intensive and informationally rich communication comes at a cost in terms of time spent transmitting the information and of muscular effort, invested to articulate the signals. Thus, signalling systems are likely to become optimized by minimizing efforts for signalling unit production, while maintaining its informational content. American linguist George Kingsley Zipf has popularized this concept in the 1930s by articulating the Law of Brevity, stating that a word length is negatively correlated with its frequency of use in a language, i.e. the most frequently used words are shorter. This principle was verified in almost a thousand languages and it is regularly observed in the process of language evolution. Sparing their effort, speakers tend to shorten long words, when they become more frequently used; e.g. the word “television” has been shortened to TV.
The universal nature of the Law of Brevity raised the question of whether it is a general characteristic of any communicational system and whether its existence in human languages stems from the evolutionary origins of animal communication. The relationship between call duration and usage frequency was tested in several animals, but the results differed between the species. A proposed explanation for the lack of a clear fit of animal repertoires to the brevity principle is the abundance of long-range calls. Humans mostly communicate from a short-range (< 3.5 meters) while animals frequently need to transmit their signals much further. Longer calls are more efficient for long-range communication, as they are less likely to be masked by noise. This might have resulted in a contradicting pressure against shortening of vocal signals. Additionally, long-range calls need to be louder, which is likely adding to their production effort.
Following this direction, we wanted to examine whether call amplitude, rather than call duration, might be the main factor by which animal vocal repertoires are optimized. By adopting the “least-effort” logic, i.e. frequent calls should require least effort to be produced, we have hypothesized that softer calls would be more frequent than louder ones.
We tested this in rock hyraxes, a medium-sized mammal native to Africa and the Middle East. Rock hyraxes are social animals, living in groups of up to 20 individuals. The groups consist of multiple females and their offspring with usually one adult resident male. Within the group, hyraxes frequently communicate, using an extensive repertoire of calls. On the other hand, bachelor adult males (mostly leading solitary lives) interact with females only briefly during the short mating season and have mainly aggressive encounters with other males. Males frequently sing complex and loud advertisement songs, transmitting their individual quality to both females and neighbouring males.
Our hyrax study population is residing at the Ein-Gedi Natural Reserve near the Dead Sea in Israel. This wild population has been continuously monitored since 1999 as a part of a long term study of hyrax behaviour, communication and sociality, led by Prof. Eli Geffen (School of Zoology, Tel-Aviv University) and recently also by Dr. Amyiaal Ilany and Dr. Lee Koren (The Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University). As part of this study, we have fitted 19 hyraxes (males and females) with individual, miniature audio recorders and logged all their calls over approximately one week. By listening and labelling all recorded calls, we created full rock hyrax vocal repertoire. Using this extensive dataset, we have calculated the usage frequencies of all call types and have measured the average duration and amplitude for each one. This allowed us to examine if hyrax vocal repertoire corresponds with the classic Law of Brevity (call duration/usage) relationship, or, whether the optimization factor of the vocal performance is call amplitude.
When comparing male and female repertoires, we found that females produce more call types in general and more affiliative call types in particular. This was not surprising, as hyrax females maintain stable social relationships within a group, while bachelor males have only limited communication opportunities. There are sexual differences also in relation to the fit to Law of Brevity. In females, longer calls are actually the more frequent ones, in contradiction to the Law of Brevity’s prediction. In contrast, amplitude seems to follow the “least effort” paradigm, as soft calls (requiring less effort to produce) are more frequently used. The male repertoire, on the other hand, is optimised for minimizing duration, but not amplitude. Male vocalizations are heavily influenced by the unique requirements of their advertisement songs, which have to be loud in order to reach remote listeners. Therefore, optimization of male repertoire for amplitude would have likely impaired the songs function. When songs are excluded from the analysis, amplitude-based optimization is detected for the remaining male repertoire.
In hyraxes, the two sexes were under different selective pressures in terms of repertoire optimization. Female repertoire, tuned for frequent communication over a wide range of distances, is optimized by minimizing call amplitude. However, while amplitude is the main candidate for optimization, this can be constrained by contradicting selection pressures. Males are committed to their loud songs, so amplitude reduction is prevented by the needs for long-range transmission, resulting only in duration-based optimization of their repertoire, much like in human languages. It is thus an intriguing question; why aren’t human languages optimized by amplitude? Could it be because the development of artificial signalling means for long-range communication made high amplitude calls less needed? Perhaps the high pressure for increased informational content in the emerging human languages capped the amplitude of the vocal signals, as loud calls have less capacity for informational content. Both scenarios could lead to duration-based optimization that is now widespread.