Group-Specific Variation in Heterodyne Frequencies of Biphonic Calls in Resident (R-Type) Killer Whales Orcinus orca ater

Abstract

Biphonation—the simultaneous production of two independent fundamental frequencies within a single vocalization—is a prominent feature of killer whale calls, yet the factors influencing the presence and intensity of heterodyne frequencies, which arise from interactions between these fundamentals, remain poorly understood. We analyzed long-term acoustic recordings of killer whales from Avacha Gulf, Kamchatka, focusing on two widespread biphonic call types across eight families. For each call, we quantified the prominence of heterodyne frequencies and measured relative intensity parameters between the fundamentals (lower F0 and upper G0) and heterodynes. Our results reveal pronounced family-level differences in heterodyne expression. Six families exhibited consistently strong heterodyne frequencies, whereas two families showed absent or weakly expressed heterodynes. Relative intensity of F0 vs. G0 and heterodynes vs. G0 differed significantly between call types and families, and were positively correlated. Calls with poorly expressed heterodyne frequencies usually had lower relative intensity of F0, indicating a link between fundamental amplitudes and heterodyne strength. These findings suggest that killer whales can modulate heterodyne expression, potentially reflecting differences in sound production or propagation through the tissues. Moreover, the family-specific patterns of heterodyne expression indicate that heterodyne frequencies may serve as culturally transmitted acoustic markers, contributing to dialect identity and facilitating recognition among matrilineal social units.