Wild western lowland gorillas signal selectively using odor

Abstract

Mammals communicate socially through visual, auditory and chemical signals. The chemical sense is the oldest sense and is shared by all organisms including bacteria. Despite mounting evidence for social chemo-signaling in humans, the extent to which it modulates behavior is debated and can benefit from comparative models of closely related hominoids. The use of odor cues in wild ape social communication has been only rarely explored. Apart from one study on wild chimpanzee sniffing, our understanding is limited to anecdotes. We present the first study of wild gorilla chemo-communication and the first analysis of olfactory signaling in relation to arousal levels and odor strength in wild apes. If gorilla scent is used as a signaling mechanism instead of only a sign of arousal or stress, odor emission should be context specific and capable of variation as a function of the relationships between the emitter and perceiver(s). Measured through a human pungency scale, we determined the factors that predicted extreme levels of silverback odor for one wild western lowland gorilla (Gorilla gorilla gorilla) group silverback. Extreme silverback odor was predicted by the presence and intensity of inter-unit interactions, silverback anger, distress and long-calling auditory rates, and the absence of close proximity between the silverback and mother of the youngest infant. Odor strength also varied according to the focal silverback's strategic responses during high intensity inter-unit interactions. Silverbacks appear to use odor as a modifiable form of communication; where odor acts as a highly flexible, context dependent signaling mechanism to group members and extra-group units. The importance of olfaction to ape social communication may be especially pertinent in Central African forests where limited visibility may necessitate increased reliance on other senses.

Figure 1. The predictive effect of inter-unit interaction intensity on silverback extreme smell.

For no interactions n = 212. For low intensity interactions n = 21 (one interaction omitted due to missing data). For medium intensity interactions n = 11. For high intensity interactions n = 10 (one interaction omitted due to missing data). Error bars: ± 1s.e.

Figure 2. Total monthly interaction rates in relation to silverback extreme smell.

A total of n = 79 interactions occurred in 2007.

Figure 3. Silverback auditory responses during high intensity inter-unit interactions in relation to his extreme and low smells.

For quiet response n = 4. For loud response n = 6; one loud interaction was omitted due to missing data. Error bars: ± 1s.e. Figure 3A relates to silverback extreme smell. Figure 3B relates to silverback low smell.