Temporal patterns in Cape fur seal (Arctocephalus pusillus pusillus) mother and pup attraction calls

Abstract

Vocal recognition is widespread in the animal kingdom, and a necessary tool for offspring survival in some groups. Temporal patterns of animal vocalisations can facilitate communication and convey information such as identity, emotional state, or motivation of the caller. While pinniped (i.e., walrus, eared and true seals) vocalisations are generally well studied, and captive pinnipeds show strong timing abilities, little is known about the temporal structure of their calls in the wild. Here, we followed Cape fur seals (Arctocephalus pusillus pusillus) during spontaneous search efforts at the Cape Cross Seal Reserve in Namibia. To investigate the temporal and rhythmic patterning of Cape fur seal attraction calls, we analysed call bouts of 80 mothers and 148 pups. We assessed the relative vocal efforts undertaken by mothers and pups by calculating calling rates, inter-call intervals (periods of silence between vocalisations), and the total time spent calling per bout. To explore the rhythmic structure of the calls, we visualised their temporal patterns by plotting the calling events and frequency histograms of the inter-onset-intervals between each two consecutive vocalisations in a bout. A normalized Pairwise Variability Index was calculated for each individual to investigate underlying patterns and compared between mothers and pups. Pups produced more calls per search, vocalised at higher rates, and took shorter breaks between consecutive vocalisations than females. Even though female vocalisations were much longer, there was no significant difference in the total time females and pups spent vocalising per bout. All animals vocalised at seemingly random intervals, with no distinguishable rhythmic pattern, suggesting that these do not encode identity information during mother-pup reunions. However, numerical analysis showed a potential asynchronous patterning within the age classes, possibly used in anti-masking. Our results indicate that Cape fur seal females and pups invest their energy differently during a search, and while their total calling effort is comparable, reunions seem to be driven mostly by the young.