Hyraxes – furry, small desert mammals whose Hebrew name means “rock rabbits) that are native to Israel and the rest of the Middle East as well as Africa – look cute and friendly, but they can cause harm. Blood-sucking sandflies that carry a parasite called Leishmania sting hyraxes and then bite humans, causing leishmaniasis – a disease that can cause mutilating skin ulcers and even death. 

Last year, an outbreak of leishmaniasis in the north of Israel inducted officials to consider suspending the animals’ protected status, allowing hunters to kill them. 

But researchers at Bar-Ilan University in Ramat Gan (near Tel Aviv) who have studied hyraxes’ vocal communications have found them to be helpful in understanding sex differences and the evolution of human language. 

In nature, social living is strongly connected to the ability to communicate with others. Maintaining social ties and coordinating with group mates require frequent communication. Thus, complex social systems are usually associated with well-developed communication abilities. Human language is, of course, the height of complex communication, but intensive and information-rich communication comes at a cost in terms of time spent transmitting information and the muscular effort invested in articulating signals to others. 

Almost nine decades ago, American linguist George Kingsley Zipf popularized this concept by articulating the Law of Brevity, a linguistic rule stating that word length is negatively correlated with its frequency of use in language. This principle was verified in almost 1,000 different languages and is regularly observed in the process of language evolution in which frequently used long words are often shortened, such as “television” to “TV.” So, while signalling systems have improved, informational content has been preserved.  

Is the Law of Brevity in human language derived from the evolutionary origins of animal communication? The relationship between the duration of calls and the frequency of usage has been tested in several animals, but results differed between 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 communicate mostly within short range (less than 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 contradicting pressure against the shortening of vocal signals. Additionally, long-range calls need to be louder, which probably adds to their production effort.  

With this in mind, the Israeli researchers set out 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 – that frequent calls should require the least effort to produce – they suggested that softer calls would be more frequent than louder ones.  

The researchers tested this in rock hyraxes (known to scientists by their Latin name Procavia capensis), which live in groups of up to 30 comprised of multiple females and their offspring and usually with just one adult resident male. Within the group, hyraxes frequently communicate using an extensive repertoire of calls.  But the bachelor adult males, who lead mostly solitary lives, interact with females only briefly during the short mating season and with other males mainly when fighting. Males frequently sing complex and loud self-promoting songs, transmitting their individual quality to both females and neighboring males. 

The hyraxes studied by the Bar-Ilan researchers live in the Ein Gedi Nature Reserve near the Dead Sea. Since 1999, this wild population has been monitored continuously as part of a long-term study of hyrax behavior and communication led by Prof. Eli Geffen, of Tel Aviv University, and recently also by Dr. Amiyaal Ilany and Prof. Lee Koren, of the Faculty of Life Sciences. As part of this study, 19 male and female hyraxes were fitted with individual, miniature audio recorders and all of their calls were logged for approximately one week. 

By listening and labeling all recorded calls, the researchers created a full vocal repertoire of the animals. Using this extensive data-set, they calculated usage frequency of all call types and measured the average duration and amplitude for each one. This allowed them 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. 

In their study, just published in the journal Evolution Letters, the scientists show how changing necessities can affect the development of different voices for various purposes and provide clues as to how the complexity of human language began to developed. 

They compared male and female repertoires and found that females produce more call types in general and more affiliative call types, such as a coo, in particular. According to one of the study’s lead authors, Dr. Amiyaal Ilany, this was not surprising, as hyrax females maintain stable social relationships within a group, while bachelor males have only limited opportunities for communication. 

The research team, which included Dr. Vlad Demartsev and Naomi Gordon, also discovered sexual differences in relation to the Law of Brevity. In females, longer calls are actually the more frequent ones, in contradiction to the Law of Brevity’s prediction. By comparison, 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 characterized by a short duration, as well as a minimum of amplitude. Male vocalizations are heavily influenced by the unique requirements of their self-advertisement songs, which must be loud in order to reach remote listeners.

“This raises the question of why human language isn’t optimized by amplitude,” noted Ilany. “Could it be because the development of artificial signaling 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,” he concluded.



Source: Israel in the News