While male Campbell’s monkeys may not have a large enough vocabulary to come up with one of those word-a-day calendars, new research published in the latest edition of the Proceedings of the Royal Society B reveals that the primates are able to communicate using six main sounds.
The key to those sounds, Camille Coye of the University of St. Andrews and an international team of colleagues explained, are the use of the suffix “-oo,” which partially offsets the lack of control that they have over their acoustic flexibility. The same researchers previously reported that male Campbell’s monkeys use at least six primary calls: boom-boom, krak, krak-oo, hok, hok-oo and wak-oo.
Each of those calls have a unique meaning, according to Discovery News, and can be understood by both male and female members of their own species, as well as by members of a related group of primates known as Diana monkeys. Furthermore, their research confirms previously suspected translations of what each of those calls mean.
For instance, when a male Campbell’s monkey makes the “krak” sound, it means that there is a leopard nearby, but by adding the suffix “-oo” to make “krak-oo,” the meaning changes to indicate the presence of a non-leopard disturbance such as another animal passing by. They can combine the sounds, so that the noises “boom-boom-krak-oo” form a sentence meaning “Look out for that falling tree branch!”
Experimentally testing non-humans use of suffixation
“This study is the first to experimentally test the existence and saliency to receivers of ‘suffixation’ – a mechanism analogous to the one observed in humans – in the natural communication of wild animals,” Coye told redOrbit via email. “There are only a few studies describing suffixation-like systems in the vocal repertoire of their species, and ours is the only one to demonstrate experimentally the cognitive relevance of this combinatorial system in wild animals.”
However, she added that her team believes this phenomenon may be “widespread” and that experts only lack “comparative investigations at the moment. In line with this idea, there are more and more studies finding call combinations at the vocal sequence level (e.g. in hyraxes and bonobos). We hope that our study will inspire other researches of this kind.”
Coye and her colleagues used a playback experiment, broadcasting both actual and artificially modified male Campbell’s monkey calls to see how recipients would react to those suffixation patterns. The calls were played to a group of 42 male and female Diana monkeys, and the way that the latter group responded indicated that they understood the meaning of the calls for both natural and artificially recombined calls, hence confirming the combinatorial hypothesis.
Other root calls used by the primates include “hok” for crowned eagle and “wak,” which has a less clear-cut meaning. They know the context in which the monkeys use these calls, but they are always used in complex sequences with other calls, Coye explained. Like “krak,” each of these terms can be altered by adding “-oo,” changing “hok” from an alert about birds of prey to “boom boom hok-oo hok-oo,” which Discovery News said appears to involve claiming territory.
Similar to, but not quite, a human-like language
The researchers emphasize that the use of the term “language” should be reserved for people, though several similarities between humanity’s method of communicating and the calls of the Campbell’s monkeys. In addition the use of suffixes to change call meanings, they have been shown to engage in conversations and possess “vocal convergence” similar to accents.
“Language is a uniquely human capacity, involving complex cognitive functions, memory and developed vocal abilities, and we still do not know how it appeared in our species,” Coye told redOrbit. “Its outstanding complexity led the specialists to the conclusion that it could not have evolved from scratch and that some precursors of the cognitive abilities and communicative mechanisms observed in humans may have evolved earlier in the primate lineage.”
As such, she added, these abilities and mechanisms “may be found in other primate species. Hence, our goal is to understand which abilities existed before language, abilities which may have been the ground of language evolution. To this end, we try to describe the parallels and differences observed between human language and non-human primates’ communication.”