Chimpanzee (Pan Troglodytes) Caller Goals In The Issa Valley

Introduction

Vocalizations play a fundamental role in the social dynamics in a wide range of species by influencing the behavior of nearby listeners. Chimpanzees are our closing living relatives and yet the function and diversity of their vocal repertoire remains poorly understood after over 45 years of research. There are a number of reasons why it is so difficult to study the chimpanzee vocal repertoire. This includes the dense forest habitat that many chimpanzees occupy, making if difficult to identify the caller( producer of the call) and the receiver (call recipient). Furthermore, there is cultural variation in chimpanzee communities subject to long term-study, which cause vocal repertoires to vary between communities. In addition to chimpanzee communities expressing cultural variation, different researchers apply different methods to identifying vocalizations; this can result in varying numbers of total identified vocalizations in a community.

An adult male chimpanzee with a piloerection(raising hair) produces a pant hoot with other group members. Photo Credit GMERC.

Koret Slam Table 1

Research Question & Hypotheses

The chimpanzee vocal repertoire has five main call types: grunts, hoos, screams, barks, and other (Table 1.). These call types can be observed in a variety of contexts such as feeding, resting, traveling, alert, etc. The Caller-Goal Framework states that call types can be expressed in a variety of scenarios but that the call goal remains the same. Using this framework this study seeks to identify caller goals of chimpanzees in the Issa Valley and identify demographic trends. I seek to address the following hypotheses and respective predictions:

Hypothesis 1) Support solicitation vocalizations are expressed in equal frequencies by all group members.
Hypothesis 2) Appeasement vocalizations are most frequently expressed by females and juvenile/adolescent males.
Hypothesis 3) Social cohesion vocalizations are expressed by all demographic groups.

Materials

The camera trap video data analyzed for this study was recorded in the Issa Valley of Western Tanzania, East Africa. The study group consists of the group members in the semi-habituated Issa Chimpanzee Chimpanzee Community. This community was first researched from 2001-2003 and a long research presentence was established by Greater Mahale Ecosystem and Research(GMERC) in 2008. All data was collected by GMERC staff and mailed to the Primate Ecology and Molecular Anthropology Lab (PEMA) at the University of California Santa Cruz. The PEMA Lab delegates the camera trap footage to the Chimpanzee Video Coding team to process data by specific species, behaviors, and if applicable age and sex.

A Bushnell camera trap installed on a tree. The motion sensitive camera traps in this study would record one minute videos with ten second intervals.

Figure 1. Map of Western Tanzania that includes the Issa Valley and surrounding National Parks. The Gombe and Mahale Mountains host chimpanzee habitat (Piel, 2017)

Methods

This study worked with the compiled dataset of all chimpanzee videos (n=2913) from the year 2018 and was furthered refined to only contain videos with chimpanzee vocal events.(n=202).The videos that contained vocal events were screened to determine whether or not the caller and the receiver were visible. After the initial screening I categorized the vocal events by caller goal following the categories defined by Schamberg, Wittig, and Crockford 2018 as either appeasement, social cohesion, solicit support, or other. In each caller event the caller goal, the demographics of caller, and name ID were identified. After the callers were identified the associated receivers, participants, and instigators of the call were identified by their demographics and name ID. After the data was collected it was compiled for ANOVA statistical analysis to determine if there was any significant variation in the means of the results.

Solicit Support

Solicit support caller goals were distinguished by their high frequency aaah sound and the presence of a fear grimace. Chimpanzees with a fear grimace would lift up their lower and upper lips and display all of their teeth. Photo credit GMERC.

Appeasement

Appeasement grunts usually occurred in the presence of a fusion with an adult male. They could be distinguished by the expression of an open mouth without displaying teeth, their low frequency uh sound, or by their rapid shoulder movement. Photo credit GMERC.

Social Cohesion

Social cohesion calls were distinguished by their hoo sound. When they would produce this call their mouth would open in a round/circular shape. Photo credit GMERC

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