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

Abstract

16Non-social factors can influence animal social structure. In killer whales (Orcinus orca), fish-17 vs. mammal-eating ecological differences are regarded as key ecological drivers of their 18 multilevel society, including group size, but the potential importance of specific target prey

Killer whales (Orcinus orca) in Iceland show weak genetic structure among diverse isotopic signatures and observed movement patterns

“…Association data were available from Tavares et al () for sampled individuals with a minimum sighting total of five days ( N = 47). Tavares et al () considered individuals associated in a day if photographed by the same camera/photographer within 20 s and used the half‐weight index (HWI, ranging from 0 to 1) to quantify associations between pairs of individuals. The existence and strength of social associations between individuals from different putative genetic units was used to evaluate the social isolation of genetic units.…”

“…Individuals seen in both seasons had lower nitrogen stable isotope ratios ( 15 N/ 14 N, represented as δ 15 N), consistent with a diet predominantly composed of herring, while killer whales only seen seasonally (including sampled individuals that travel between Iceland and Scotland) exhibited larger variation in δ 15 N, suggesting that some individuals have a diet including other prey (Samarra, Vighi, et al, ). However, there is no social isolation between individuals with different observed movement patterns and isotopic signatures, that is, putative herring‐specialists remaining year‐round in Iceland have been photographed in close proximity with Icelandic‐Scottish killer whales (Tavares, Samarra, & Miller, ). It is unknown whether the apparent absence of social isolation in the Icelandic population corresponds to an absence of genetic divergence among individuals with different isotopic values and observed movement patterns.…”

Movements and site fidelity of killer whales (Orcinus orca) relative to seasonal and long-term shifts in herring (Clupea harengus) distribution

“…In combination with studies on 632 stable isotopes, these differences in individual movement patterns suggest a mosaic of 633 foraging preferences (Samarra et al 2017b). Importantly, such differences in 634 movement patterns and feeding preferences are not reflected in social isolation 635 (Tavares et al 2017), thus suggesting that these differences may occur in the absence 636 of reproductive isolation. Future research using genetic markers will be crucial to 637 further assess the degree of gene flow and potential structuring within this population.…”

“…Due to 193 the observed fluidity in the social structure of this population (Tavares et al 2017), all 194 analyses were conducted treating each individual independently of its potential social 195 group. However, it is likely that if whales form stable, long-term groups, the 196 movements of sets of individuals identified in this study will be correlated.…”

Simple foraging rules in competitive environments can generate socially structured populations

“…There is growing empirical evidence that food availability and clumpiness can drive flexibility in social and foraging ties of marine predators (e.g., Gazda, Iyer, Killingback, Connor, & Brault, ), even in the stable social tiers of killer whales. Type, biomass, predictability, and density of prey influence not only foraging strategies but also social strength and connectivity, suggesting that ecological conditions can bend even phylogenetic inertia (Beck et al., ; Foster et al., ; Tavares, Samarra, & Miller, ). Our model captures that initial period—all individuals were the same, but from these emergent cliques there could arise increasing behavioral differences through time.…”