What do geladas and humans have in common? Apparently, vocal patterns.

T. gelada. Photo: Ron Waddington

They live in very large groups called herds, they primarily graze on terrestrial vegetation, and they roam on large open plains. In more than a few ways, geladas (Theropithecus gelada) are similar to cows. Geladas are more vocal than cows though. They have a diverse range of vocalizations for various behaviors and situations, such as contact, showing submission, aggression, and others (Kawai, 1979; Aich et al., 1990).  The diversity and complex communication exhibited by geladas is one of the reasons this is a very interesting primate species to study.

A new study on their vocal sequences shows that this species of Old World Monkey follows patterns that are similar to humans. As far back as the late eighties, Richman (1987) suggested the geladas use melody and rhythm in ways similar to humans. Now, Gustison and colleagues have shown that the longer the sequence of sounds is that a gelada makes, the shorter the sounds within that long sequence. This is the same pattern observed in humans and is called Menzerath's law. The longer our sentences, the shorter the words tend to be in those sentences.

The authors studied fifty-seven male geladas in the wild and recorded and analyzed over 1000 vocal sequences. In addition to discovering that longer sequences are made up of shorter calls, Gustison and colleagues report that long sequences start with short calls and short sequences start with long calls. Thus, the start of the sequence is an indicator of its overall length. Longer sequences also have faster tempos. Gustison and colleagues state this might be to reduce the possibility of one gelada being "talked over" by another. You can understand how this might be a problem in a chatty species that lives on open plains. If you have a lot to say, you better say it quickly.

Geladas grazing. Photo Alastair Rae

Interestingly, there was no negative relationship between the position of a call and its duration (calls later in the sequence were not shorter). Yet, the proportion of grunts where the gelada was exhaling decreased in longer sequences and the proportion of grunts where the individual inhaled increased. The authors believe this pattern is due to the fact that individuals make grunts with both inhalations and exhalations on the same breath as the sequence lengthens. Once geladas have more than 15 calls per sequence, the majority of their calls have both inhale and exhale grunts and the proportion of inhale and exhale grunts hardly varies.  Thus, respiration and energy demands may constrain gelada vocalizations, and be partially the reason for conforming with Menzerath's law.

This is the first time Menzerath's law has been studied in non-humans. Thus, other animals with expansive vocal repertories, such as songbirds, may also exhibit this law. It is hard to draw conclusions about the evolution of communication in non-human primates and our human ancestors without knowing if Menzerath's law holds true for species that are further separated from humans. For now, we know humans are not unique in adhering to Menzerath's law.  It is possible that this pattern existed before meaningful combinations of vocalizations had evolved. Tests in other species will only serve to improve our understanding of sequences, vocal patterns, and the evolution of language and communication in general.



Links of potential interest:
Menzerath's Law
IUCN page on geladas
YouTube video on gelada chatter

Works cited:

Aich, H., Moos-Heilen, R., & Zimmermann, E. (1990). Vocalizations of adult gelada baboons (Theropithecus gelada): acoustic structure and behavioural context. Folia primatologica, 55(3-4), 109-132.
Kawai, M. (1979). Auditory communication and social relations. Ecological and Sociological Studies of Gelada Baboons, 219-241.
Gustison, M., Semple, S., Ferrer-I-Rancho, R, Bergmann, T. (in press). Gelada vocal sequences follow Menzerath’s linguistic law. PNAS. www.pnas.org/cgi/doi/10.1073/pnas.1522072113 
Richman, B. (1987). Rhythm and melody in gelada vocal exchanges. Primates, 28(2), 199-223.