Maintaining contact : design and use of acoustic signals in killer whales, Orcinus orca

Abstract

This thesis presents data on the structure and use of acoustic signals produced by free-ranging resident killer whales. The analysis focuses on signal features that might be useful for animals to maintain contact and coordinate activities with preferred associates, including: distinctiveness by group or individual, call amplitude, and directionality cues that might cue the direction-of-movement of the signaler. Research was conducted in Haro and Johnstone Straits off Vancouver Island, British Columbia, where killer whales have been the focus of a long-term photo-identification effort. Extensive previous research on this population has demonstrated stable kin-based matrilineal social groups and pod-specific calling behavior. Individually-distinctive markings and pigmentation patterns were used to identify groups or individuals from which sounds were recorded. Recordings from each of the three matrilineal groups composing pod Al were made when each subgroup was isolated from the two other subgroups. Analysis of call use and structure revealed subgroup-specificity that was qualitatively similar to previously observed differences between pods, although more subtle. This finding suggests that pod-specific calling arises primarily as a consequence of accumulated drift or divergence of calls between highly cohesive matrilineal subgroups as they gradually separate into different pods. A new towed array beamforming system was developed to identify vocalizing killer whales concurrent with focal behavioral observations. Carefully positioning the array relative to the animals and linking visual observations of whale position with the angle-of-arrival of sounds on the towed array allows reliable identification of signalers in many circumstances. Using this new system, a sample of 140 calls was recorded from identified individuals within W-pod to compare the call-type repertoires of individuals within a matrilineal subgroup. The three individuals composing W-pod shared at least four different call types and call-type frequency did not differ by individual, suggesting each matrilineal group member uses the same call types in a similar fashion. To measure signal source levels, the range from the array to a signaler was calculated by triangulating the angles-of-arrival of the sound on two beamforming arrays towed in series. Source levels of 819 calls and 24 whistles were combined with a model of sound propagation and perception to estimate the maximum range at which another killer whale could detect each sound in quiet conditions. The estimated maximum range of detectability of all sounds ranged from 4.5 to 26.2 km, suggesting killer whales can maintain acoustic contact with each other over long ranges. Whistles and variable calls have a smaller active space than stereotyped calls which appear to consist of two groups: long- and short-range call types with a mean estimated active space of 14.5 and 8.8 km, respectively. Directionality features of calls were described by recording sounds in front of, and behind, groups of animals as they passed the towed-array system. The frequency structure of the sample of 263 calls recorded in these conditions was clearly dependent on the orientation of the signaler to the receiver, with high-frequency components strongly attenuated when the whales were oriented away from the array. This directionality pattern appears to provide a simple and reliable cue of the direction-of-movement of signalers, and may be an important structural feature of calls helping killer whales regulate their spacing relative to each other.