Male baboon responses to experimental manipulations of loud “wahoo calls”: testing an honest signal of fighting ability
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Among male chacma baboons (Papio hamadryas ursinus), rank positions in the dominance hierarchy are fiercely contested. Physical fighting is costly but relatively rare in this species. Instead, disputes are frequently resolved using displays that include loud, repetitive “wahoo” (two-syllable bark) vocalizations. We previously found that males of all ranks adjust their contest behavior based on the relative fighting ability of opponents and that length of the second syllable (“hoo” duration), calling rate, and fundamental frequency reliably indicate fighting ability. To test whether males indeed attend to hoo duration when assessing opponents, we designed two sets of playback experiments in which call sequence pairs were identical except for this single modified feature. In experiment 1, we used calls recorded from high-ranking males unfamiliar to all subjects. In experiment 2, callers were familiar rivals that ranked one position below subjects in the dominance hierarchy. In paired analyses, subjects in both experiments responded more strongly to sequences with more intense signal features (most commonly associated with high-quality males) compared to sequences with relatively less exaggerated features (most often associated with low-quality males). Results suggest that males can use acoustic features to both indirectly evaluate strangers and to monitor the changing condition of those rivals that present the biggest intragroup threat to their position in the dominance hierarchy. Taken together with our previous research, baboons appear to follow a classic assessor strategy—signal features related to rank and condition are salient to males and directly affect their propensity to respond to rivals.
KeywordsAcoustic communication Assessor strategy Game theory Intrasexual competition Papio Playback experiments
We are grateful to the Office of the President of the Republic of Botswana and the Botswana Department of Wildlife and National Parks for permission to conduct this research. J. Nicholson provided invaluable logistical support, insights into methods, and assistance with data collection. A. Mokupi, R. Hoffmeier, M. Heesen, W. Smith, and C. Shaw also provided generous assistance in the field. We thank two anonymous reviewers for their careful feedback. We are grateful to I. Clark and the staff at Eagle Island Camp and Game Trackers for their friendship and support. Research was supported by the University of Pennsylvania, The Ohio State University, and a National Institutes of Health Grant (#MH62249).
This research adhered to the Animal Behavior Society's Guidelines for the Use of Animals in Research, the legal requirements of Republic of Botswana, and all institutional guidelines.
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