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

Submitted in partial fulfillment of the requirements for the degree ofDoctor of Philosophy at the Massachusetts Institute of Technology and theWoods Hole Oceanographic Institution September 2000

This thesis presents data on the structure and use of acoustic signals produced byfree-ranging resident killer whales. The analysis focuses on signal features that might beuseful for animals to maintain contact and coordinate activities with preferred associates,including: distinctiveness by group or individual, call amplitude, and directionality cuesthat might cue the direction-of-movement of the signaler. Research was conducted inHaro and Johnstone Straits off Vancouver Island, British Columbia, where killer whaleshave been the focus of a long-term photo-identification effort. Extensive previousresearch on this population has demonstrated stable kin-based matrilineal social groupsand pod-specific calling behavior.Individually-distinctive markings and pigmentation patterns were used to identifygroups or individuals from which sounds were recorded. Recordings from each of thethree matrilineal groups composing pod Al were made when each subgroup was isolatedfrom the two other subgroups. Analysis of call use and structure revealed subgroup-specificitythat was qualitatively similar to previously observed differences between pods,although more subtle. This finding suggests that pod-specific calling arises primarily as aconsequence of accumulated drift or divergence of calls between highly cohesivematrilineal subgroups as they gradually separate into different pods. A new towed arraybeamforming system was developed to identify vocalizing killer whales concurrent withfocal behavioral observations. Carefully positioning the array relative to the animals andlinking visual observations of whale position with the angle-of-arrival of sounds on thetowed array allows reliable identification of signalers in many circumstances. Using thisnew system, a sample of 140 calls was recorded from identified individuals within W-podto compare the call-type repertoires of individuals within a matrilineal subgroup. Thethree 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 thesame call types in a similar fashion.To measure signal source levels, the range from the array to a signaler wascalculated by triangulating the angles-of-arrival of the sound on two beamforming arraystowed in series. Source levels of 819 calls and 24 whistles were combined with a modelof sound propagation and perception to estimate the maximum range at which anotherkiller whale could detect each sound in quiet conditions. The estimated maximum rangeof detectability of all sounds ranged from 4.5 to 26.2 km, suggesting killer whales canmaintain acoustic contact with each other over long ranges. Whistles and variable callshave 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 frontof, and behind, groups of animals as they passed the towed-array system. The frequencystructure of the sample of 263 calls recorded in these conditions was clearly dependent onthe orientation of the signaler to the receiver, with high-frequency components stronglyattenuated when the whales were oriented away from the array. This directionalitypattern appears to provide a simple and reliable cue of the direction-of-movement ofsignalers, and may be an important structural feature of calls helping killer whalesregulate their spacing relative to each other.

Funding for the research in this thesis was provided by grants from WHOI'sRinehart Coastal Research Center in 1995 and 1999, a grant from WHOI's OceanVentures Fund in 1998, an NSF graduate fellowship to the author, grant CC-S-6L 1002-001-C from the Office of Naval Research, and broad financial support from WHOI'seducation department.