Individuality in Roars of Black-and-Gold Howler Monkeys (Alouatta caraya)

Information contained in vocal signals can include a caller’s individual identity, which can be potentially used for individual recognition. Individual recognition at a distance allows individuals to reduce the costs of aggressive interactions with neighbors by adjusting responses based on familiarity and/or past experiences. Howler monkey roars are long-distance vocal signals given mostly by males that function in group spacing. Although multiple aspects of howler monkey roaring behavior have been studied, detailed examinations of roar structure are scarce. With the goal to evaluate individuality in roars, we recorded eight, adult, male, black-and-gold howler monkeys (Alouatta caraya) in northeastern Argentina. Howler monkey roars are composed of respiratory cycles, in which an exhalatory and an inhalatory section can be distinguished. We selected 14–15 respiratory cycles per male, measuring one temporal and six spectrum-based parameters, and frequency from the first to the sixth formant. To assess individual distinctiveness in roars, we used stepwise Discriminant Function Analysis followed by leave-one out-cross validations for the complete respiratory cycle, for the exhalatory section and for the inhalatory section. We measured 119 respiratory cycles and the DFA correctly classified 59% of complete respiratory cycles, 56% of exhalatory sections, and 63% of inhalatory sections. These classifications were significantly higher than expected by chance. Our results show that black-and-gold howler monkey roars are individually distinctive, particularly the inhalatory section, and that formant frequencies and duration are key acoustic parameters to individualize roars.