Individual identity encoding and environmental constraints in vocal recognition of pups by Australian sea lion mothers

Colonial living imposes strong selection pressures on the communication systems of species with many animals communicating on the same sensory channels simultaneously. Colonial species often exhibit complex individual vocal signatures that encode a caller's identity in their vocalizations. During lactation, Australian sea lion, Neophoca cinerea, mothers and pups are repeatedly separated when mothers leave the colony to forage. After maternal foraging trips, mothers and pups must reunite in the colony. Both mothers and pups produce individually distinctive vocalizations during reunions and can recognize their counterpart's calls. Using playback experiments with modified calls, we investigated the acoustic parameters involved in the recognition of pup vocalizations by mothers. We also examined the efficiency of the vocal signature in three habitat types within the colony using propagation tests. We found that Australian sea lion females used a combination of temporal, amplitude and frequency parameters to recognize their pup's vocalizations. Pup vocalizations were severely degraded in the herbaceous dune habitat type with no vocal signature components reliably propagating to any measured distance. By contrast, calls propagated comparatively well in the shrubby dune habitat of the colony, as the vegetation appeared to act as a windshield. This study shows that the vocal signatures of Australian sea lion pups are both shaped and constrained by their environment and ecology, and that these signatures are of a moderate level of complexity when compared to those of other colonial vertebrates.