Honestly exaggerated: howler monkey roars are reliable signals of body size and behaviourally relevant to listeners

Acoustic signals are key components of animal social behaviour, potentially conveying fitness-relevant information about signallers. Howler monkeys produce extremely loud, low-frequency roars, which exaggerate the acoustic impression of body size relative to other species. However, whether howler monkey roars contain reliable information about body size within species, and whether conspecific listeners use this information and adjust their responses accordingly, remain unclear. In this study, whether the roars of black-and-gold howler monkeys, Alouatta caraya , serve as honest signals of body size was first examined by analysing the relationship between formant spacing and body mass in 11 adult males. The results revealed a strong negative correlation: larger males produced roars with lower formant spacing. Subsequently, the behavioural relevance of the variation in formant spacing was tested using playback experiments. These were conducted with 23 conspecific listeners, simulating the roars of unknown males with small, average and large body sizes. Listeners showed significantly different responses to calls of different body sizes. When presented with calls simulating larger males, they were more likely to approach and spent longer orientated towards the playback speaker. The effect of simulated body size on the likelihood of listeners vocalizing in response was not significant, although males spent more time vocalizing in response to playbacks than females. These findings indicate that formant spacing in howler monkey roars serves as an honest indicator of body size and plays a critical role in mediating social interactions. Our study highlights the adaptive importance of acoustic cues to body size, which can provide receivers with accurate information that can be used to assess rivals or choose mates. • Body mass is negatively correlated with formant spacing in male howler monkeys. • Listeners spent longer oriented towards playback simulating larger males. • Listeners were also more likely to approach simulations of larger males. • Males were more likely to vocalize in response to playback than females. • Formants advertise body size and mediate social interactions in howler monkeys.