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Evidence of Variable Agonistic Behavior in Killer Whales (Orcinus orca) Based on Age, Sex, and Ecotype
Abstract: We used photo-identification catalogs to assess the occurrence of rake marks in northern resident (NRKW) and transient (TKW) killer whale ecotypes from the northeastern Pacific. Rake-mark coverage on visible surface areas, excluding dorsal fins, was grouped into four categories: (1) None (0%), (2) Mild (1 rake mark to < 25%), (3) Moderate (25 to 50%), and (4) Severe (> 50%). We conducted pairwise comparisons on density of rake marks with sex, age, and ecotype using mixed model, ordinal logistic regression. In NRKWs, rake marks increase with age in males (p = 0.0007), and males had a higher frequency and density than females (p < 0.0001) and juveniles (p = 0.02), with no difference between females and juveniles. Male residents > 30 years had an 8-fold higher frequency of severe rake marks than any younger 5-year age group (p < 0.0001), while no such relationship was seen in females (p = 0.30). In TKWs, rake marks also increase with age in males (p < 0.0001) and females (p < 0.0001), though no difference was found (p = 0.1) between males and females. NRKWs and TKWs had differences in rake-mark frequency and distribution (p = 0.004) due mostly to an increase in density (p = 0.003) in female transient vs female resident killer whales. While final confirmation will require behavioral observations, and based on our indirect results, we hypothesize that the increased aggression in older males from both ecotypes is due to sexual competition; and in transients, it is potentially due to prey competition. The increased aggression observed in transient adult females compared to resident females may be due to sexual coercion and/or an increased need to reestablish dominance in the more fluid social hierarchy, while increases observed for transient senescent females may be due to an increased need for calf protection.
Key Words: aggression, agonistic behavior, killer whale, orca, Orcinus orca, rake marks, dominance hierarchy
Page Numbers: 430-446