Feel the beat Cape fur seal males encode their arousal state in their bark rate

The Cape fur seal is one of the most colonial mammal species in the world. Breeding colonies are composed of harems held by mature males (older than 10 years) with up to 30 females and their pups, while subadult males (younger and socially immature) are kept away from bulls’ territories. As with other pinnipeds, Cape fur seals are highly vocal and use acoustic signals in all their social interactions. Previous research revealed that barks - short vocalizations always produced in sequences – which are produced by males for territorial defence, mating behaviours and agonistic interactions, convey information about the sex, age-class and individual identity. This study investigated whether motivational cues such as the arousal state can be encoded in territorial males’ barks by analysing their acoustic features in multiple contexts of production. The bark rate (number of calls per unit of time) and the fundamental frequency were found to significantly increase during high arousal state interactions (confrontation vs. standard activity). Secondly, playback experiments revealed that Cape fur seal subadult males responded with a higher level of vigilance when territorial males’ barks had a faster bark rate. Decoding the arousal state of bulls' from their barks is a mechanism that will likely constitute an advantage for both bulls and by the subadult males by avoiding or reducing physical conflicts, and thereby reducing energy expenditure and the risk of injury. This study is the first evidence of Cape fur seals’ ability to use rhythm in their vocalizations to modulate their social interactions