Adaptive Call Design to Escape Masking While Preserving Complex Social Functions of Calls in Killer Whales

Since first described as moans or squeals, pulsed calls of killer whales (Orcinus orca) have been extensively studied. Most research focused on their social functions, such as maintaining contact by members of family groups or hunting parties, recognizing members of different communities of the same ecological type (killer whale ecotypes are differentiated by food preferences [Ford, Killer whale: Orcinus orca. In: Encyclopedia of marine mammals. Academic, pp 650–657, 2009]), or members of populations from different ecotypes. Social learning and intergenerational social transmission of call structure are seen as the main drivers for stability of group- or population-specific repertoires of stereotyped calls that appear to change very slowly over generations. Other ecological drivers potentially influencing call stability have been given less attention, including the influence of propagation loss and ambient noise on structural conformity. The stereotypic structure of calls (ordered occurrence of distinct components), however, allows identification of groups and populations at considerable distances. In order for calls to be recognizable at greater distances, their structure needs to allow reliable propagation in differing ambient noise conditions. Model simulations of variation in automated call detection range suggested that different spectral call frequencies are responsible for optimal detection in environments with different ambient noise levels. In a number of experimental field trials, the influence of noise on propagation of calls from two different populations of fish-eating killer whales was examined. The goal of these experiments was to determine if call structure allows some degree of compensation from noise masking. Preliminary results of the field experiments presented here showed that the propagation of dominant frequencies of calls, which varied between call components of call types, showed variations in propagation distances. The propagation was correlated with the ambient noise levels present at different receiver locations, such as upper or lower parts of the water column. Furthermore, the use of certain buzz-type call components, which are common initial components of many killer whale calls, differed in propagation, especially when they were used as standalone calls by members from a subpopulation spending more time in noisy environments. Spectral energy differences in calls, however, are currently not considered when anthropogenic noise impact, an identified threat for this endangered population, is assessed.