Divergence of a stereotyped call in northern resident killer whales

Abstract

Northern resident killer whale pods (Orcinus orca) have distinctive stereotyped pulsed call repertoires that can be used to distinguish groups acoustically. Repertoires are generally stable, with the same call types comprising the repertoire of a given pod over a period of years to decades. Previous studies have shown that some discrete pulsed calls can be subdivided into variants or subtypes. This study suggests that new stereotyped calls may result from the gradual modification of existing call types through subtypes. Vocalizations of individuals and small groups of killer whales were collected using a bottom-mounted hydrophone array in Johnstone Strait, British Columbia in 2006 and 2007. Discriminant analysis of slope variations of a predominant call type, N4, revealed the presence of four distinct call subtypes. Similar to previous studies, there was a divergence of the N4 call between members of different matrilines of the same pod. However, this study reveals that individual killer whales produced multiple subtypes of the N4 call, indicating that divergence in the N4 call is not the result of individual differences, but rather may indicate the gradual evolution of a new stereotyped call.

Cultural evolution of killer whale calls: background, mechanisms and consequences

“…N Pacific vs N Atlantic) and finescale analysis of similarity of different components of repertoires are promising directions for future research. Thomsen et al, 2002Deecke et al, 2000Miller, Bain 2000Miller 2002, 2006Miller et al, 2004Foote et al, 2004Foote et al, 2006Nousek et al, 2006Riesch et al, 2006Miller et al2007Riesch et al, 2008Weiß et al, 2006Rehn et al, 2007Foote and Nystuen 2008Holt et al, 2008Wieland et al, 2010Yurk et al, 2010Grebner et al, 2011 The first syllable may vary in frequency from <2 kHz ("l", low) to 2-4 kHz ("m", medium) and >4 kHz ("h", high). "h" and "l" variants may occur simultaneously, yielding two-voiced sounds ("I43 hlb" on this figure) The second syllable appears in at least four forms ("a"-"c") that occur in various combinations with variations of the first syllable.…”

“…Different pods produce distinct repertoires comprising a mix of unique and shared stereotyped call types, often referred to as a dialect. Calves adopt the call repertoire of their matrilineal unit, though their fathers may belong to another matrilineal unit, often from a different pod (Barrett-Lennard, 2000;Ford et al, 2011), and therefore have a different call repertoire than their offspring. If dialects or call repertoires were transmitted genetically through genes from both parents, offspring repertoires should be somehow intermediate between the mother's and father's dialect.…”

“…Divergence of call types could also result from predisposition to diverge from calls of other matrilines (Filatova & Miller, 2015). Grebner et al (2011) supposed that the evolution of killer whale calls may occur through increasing random variation in calls and then crystallizing call subtypes from the graded continuum. This would imply a deliberate tendency to increase the usage of calls with specific structure and discard calls with parameters intermediate between subtypes.…”

Evidence for vocal learning in juvenile male killer whales,Orcinus orca, from an adventitious cross-socializing experiment

“…Call types are thought to change through cumulative drift (Miller et al, 2004), but the source and directionality of the innovation is debated (Filatova et al, 2013). In one case, there was evidence of divergence instead of convergence in acoustic features of matched calls (Grebner et al, 2011).…”

“…The prevailing model for dialect development and persistence may be described as follows: young killer whales develop their dialect through vertical production learning (Janik and Slater, 1997;Janik and Slater, 2000); and the dialect is maintained by strong motivation to match calls, i.e. to emit calls similar to those of other group members in social exchanges (Deecke et al, 2000;Miller and Bain, 2000;Grebner et al, 2011;Filatova et al, 2011). However, use of calls by individuals is still poorly understood.…”