The image of a gorilla beating its chest is one of the most iconic images related to the behaviour of these primates. However, despite being one of the most emblematic sounds in the animal kingdom, the well-known chest beats have received little attention from the scientific community.
A research team in which Jordi Galbany, expert from the Faculty of Psychology of the UB, takes part, has found a correlation between the body size of gorillas and the sound frequency of the typical chest beats. The biggest and most competitive male gorillas make lower frequency beats (deeper sounds). According to the experts, this visual and acoustic signal indicates the body size of gorillas to their social group –males and females– as well as to other neighbouring gorilla groups.
This international study has been published in the journal Scientific Reports and is led by Edward Wright and Martha Robbins, from the Max Plank Institute for Evolutionary Anthropology (Germany). Among the participants in the study are the researchers from the Dian Fossey Gorilla Fund (United States), the George Washington University (United States) and the Goethe University Frankfurt (Germany).
Gorillas in the Rwanda mountain mist
The mountain gorilla (Gorilla beringei beringei ) is one of the great African apes that lives in the volcanic hillsides in the Albertine Rift, in eastern Africa. This subspecies is listed as endangered by the International Union for Conservation of Nature (IUCN), and it is estimated that only about a thousand of them are in the Bwindi Impenetrable Forest National Park (Uganda) and the Virunga Mountains, a chain of mountains between the Democratic Republic of the Congo, Uganda and Rwanda.
The study focused on the analysis of ten social groups of gorilla mountains in the Volcanoes National Park of Rwanda. This habitat, which is quite reduced, is known thanks to the research and conservationist work the prestigious primatologist Dian Fossey started, and which is still ongoing by the homonymous Foundation.
Mountain gorillas live in social groups with one or more males that show a high competition towards the other males in the group and neighbouring groups. These present a sexual dimorphism –males weight twice the weight of females– and in general, females move across groups to choose the one they prefer depending on the males’ features.
The body size of gorillas is conveyed in the sounds
Despite the symbolic value of the gorillas’ chest beats, communication studies focused on the vocalizations of these apes. To date, researchers thought these beats were related to competition among males and the selection process of females in the group, but the type of conveyed information was unknown.
The study states that the strategy of chest beats is a reliable indicator of the body size of gorillas and it reveals their competitive ability towards the members of the same social group and others who are nearby. According to the experts, anatomic structures near the larynx in bigger males could reduce the frequency of the sound of these apes while chest beating. Therefore, rivals could be intimidated by the sound of these beats –which can be heard from a kilometer away– and would opt for avoiding aggressive encounters with the male who makes the sound, while females could use this information to choose their partners.
“The chest beating behaviour is typical of gorillas –especially male adults– and had always been described as a display of strength related to social status and threatening behaviours”, notes Jordi Galbany, member of the Department of Clinical Psychology and Psychobiology of the UB.
“This kind of behaviour –he continues– is the climax of a demonstration: the gorilla starts making vocalizations similar to short howls, it gets on its foot and runs while beating its chest with both hands, making an impressive drummer-like sound”.
“Male gorillas beat their chests once every twenty hours under observation, but they can do so every few minutes when there is interaction between two groups of gorillas”, notes Galbany. This behaviour is more common during the days when a female gorilla is in heat.
“The gorilla chest beat is one of those iconic sounds from the animal kingdom, so it is great that we have been able to show that body size is encoded in these spectacular displays”, says Edward Wright, the first author of the study.
Mountain gorillas: morphology, behaviour and sound recordings
As part of the study, the team recorded the chest beats of gorillas to analyse more sound parameters (length, number of beats, frequency, etc.). They also studied the body size –in particular, the width of the back– with non-invasive photogrammetric and techniques, a work in which the expert Jordi Galbany took part for four years in the Volcanoes National Park. The combination of morphological and behavioural data with these recordings enabled researchers to find the meaning of a movement that became an authentic identity sign of these primates.
“Conducting this study was challenging because the chest beats are relatively short in duration and the we needed to be in the right place at the right time to obtain the sound recordings, as well as staying clear from these large powerful animals”, says Eric Ndayishimiye, research assistant at the Dian Fossey Gorilla Fund.
Interestingly, the team also identified a great variation in the features of chest beats between different males (number of beats, length, etc.), which would open the door to explore the potential existence of individual signatures, belonging to each specimen.
The power of non-vocal acoustic signals in great apes
The results of the study stress the potential of non-vocal signals as a vehicle to transmit relevant information for great apes. “Other species of apes can communicate from distance through non-vocal signals, such as chimpanzees, that use buttress as drums, and make a great range of communicative signals with different lengths and features”, notes Jordi Galbany.
The relation between body size and acoustic features of signals has a special interest when it comes to species in which such size determines the ability to fight and successful reproduction. In this context, the team stated in previous studies that bigger male gorillas were more dominant socially and more successful in reproduction. In other primate groups, there is a relation between competition and vocalization features (for instance, in macaques, the baboon, the gelada and colobus monkeys). This relation has been observed in other vertebrate groups (red deer, fallow deer, American buffalo, panda bear, elephant seal and American caiman). However, there are not many examples of studies showing the relation between non-vocal acoustic signals and morphological features. “We would need to launch future research studies on gorillas to know whether the chest beating.
behaviour, despite providing reliable information on the body size, is properly decoded by the receptors, both males and females, and whether it has an impact on their behavioural response and decision-taking. Also, we should see the populational variants of this type of communication and therefore, expand the research study to other gorilla populations and subspecies”, concludes Jordi Galbany.
Video credit: Dian Fossey Gorilla Fund