The influence of ecology on sociality in the killer whale (Orcinus orca)

Abstract

Figure 3. Cumulative number of bifurcations across AI levels for the data collected in nearshore Shetland and Eastern Icelandic waters. Significant knots that maximized Z statistics from comparisons of stepwise changes above and below the knot value and which signify social tier delineation were at an AI of 0.84 for Pacific and Scotland mammal-eating killer whales and, although non significant, at 0.86 for the Icelandic data. BECK Andy Foote/NAKID IMAGES RESULTS & DISCUSSION Both Atlantic killer whale groups had stable long-term primary social tiers (AI= 0.8). There were further bifurcations at a range of AIs in the Icelandic data, suggesting more frequent or more persistent associations at a secondary social tier. The mean and typical group size of Atlantic killer whales when hunting seals around the Scottish coast was 5 individuals, similar to the Pacific mammal-eating killer whales i,iii .The mean group size of Atlantic killer whales when hunting fish off Iceland was 15 individuals. The differences in sociality between Atlantic groups suggest that phylogenetic constraints may influence the formation of cohesive social groups at the primary social tier, however, the second social tier appears to be more plastic and influenced to some extent by ecological selection.

A multilevel society of herring-eating killer whales indicates adaptation to prey characteristics

“…In the North Atlantic, the only published study addressing sociality found greater similarities 114 between the Scottish mammal-eating population and Northeast Pacific mammal-eating 115 population relative to residents, despite greater phylogenetic distance, suggesting that ecology 116 drives sociality more than phylogenetic inertia does (Beck et al 2012). The study included a 117 limited dataset from Icelandic herring-eating killer whales and their social structure was not 118 explored in detail.…”

Seasonal movements of killer whales between Iceland and Scotland

“…Four other whales originally seen in Iceland and then photographed in the nearshore waters off Scotland (T-38/12, 997/19, IF-4/21 and 993/62; Beck et al 2012) were re-identified in Iceland. Each of those made at least one intra-annual movement between Iceland and Scotland ( Table 1).…”

“…Jakobsson & Stefánsson 1999); thus, specializing on a specific stock may not be an effective strategy. However, this would require the ability to shift between specialized feeding strategies adapted to different prey resources (Similä & Ugarte 1993, Deecke et al 2011, Beck et al 2012. Our understanding of the extent of behavioral plasticity in foraging strategies and to what extent killer whales may switch prey types requires future study, particularly combining seasonal behavioral observations and diet assessment in different feeding contexts.…”

Killer whale call frequency is similar across the oceans, but varies across sympatric ecotypes

“…In the eastern North Atlantic, there is evidence for two partially sympatric populations that may specialize on different prey types . Feeding on either fish or marine mammals is observed in the area, but it is not clear if any groups specialize on specific prey or switch between prey types (Beck et al, 2012).…”

“…However, behavioral observations suggest that while hunting on seals, North Atlantic killer whales demonstrate behavioral features, such as group size (Beck et al, 2012) and vocal activity (Deecke et al, 2011), which are similar to North Pacific transients, despite being genetically closer to North Pacific residents. The similarity in behavior and vocal activity between the North Pacific transients and North Atlantic killer whales may be due to convergent foraging strategies, but it is not known whether ecological specialization might also lead to convergence in call structure.…”