Resource specialisation and the divergence of killer whale populations

Abstract

Individual resource specialisation is common in natural populations associated with competition and ecological opportunity (see Aroujo et al., 2011), and well known for the killer whale (where social groups specialise) and other delphinid cetaceans (see Hoelzel, 2002). Prey choice affects a predator’s temporal and spatial pattern of habitat use. For the killer whale, social groups (pods) learn where prey resources are seasonally abundant, and the techniques required to exploit different resources efficiently. Some fish prey, especially anadromous species such as salmon, may provide predictable seasonally rich concentrations, whereas marine mammal prey may be more patchily distributed and show a different pattern of temporal abundance (and accessibility). However, these resources are found within the same waters, though the timing and technique for capture may differ. Foote and Morin (2015) suggest that the co-occurrence of populations in the same ocean doesn't necessarily imply that they differentiated in sympatry, which is clearly true. However, as Moura et al. (2015) and others (for example, Hoelzel et al., 1998, 2007) have discussed, it is the life history and behaviour of killer whales that suggest the potential for differentiation in sympatry. Although the proximity of resources brings killer whale pods into sympatry, the differential pattern of spatial and temporal habitat use, as well as fidelity to pods that forage by similar learned methods, could serve to isolate resource specialist communities reproductively

Killer whales differentiating in geographic sympatry facilitated by divergent behavioural traditions

“…However, each bootstrapped comparison is based on just five SNPs, so power is low and significance is based on z-scores, which assume normal distributions, unlikely for these datasets. Foote and Morin (2016) emphasise the importance of the North Atlantic population in support of their conclusions, however, our ABC modelling analyses that included the North Atlantic (Hoelzel and Moura, 2015) supported the topology presented in our nuclear consensus phylogeny (Moura et al, 2015).…”

“…We note that the broader implications from the reticulate gene flow we have each described may be inconsistent with earlier proposals for multiple killer whale species (for example, Morin et al, 2010), though this can also occur among established species. Further, neither our nuclear tree nor the consensus phylogeny generated in Foote and Morin (2016) support the same topology or inference as the mtDNA tree (Foote et al, 2011;Moura et al, 2015;Hoelzel and Moura, 2015), suggesting that the topology of the single-gene tree represented by mtDNA is unlikely to represent the true species history.…”

Genome-wide SNP data suggest complex ancestry of sympatric North Pacific killer whale ecotypes

“…We observe discordance between the observed (nonsignificant) and the expected (significant) f 4 -statistics for this combination of populations whether we assume the consensus topology reconstructed from SNAPP or the topology inferred from concatenated RAD sequences by Moura et al (2015). This combination of populations was not compared by Hoelzel and Moura (2015) in their tests of different topologies using DIY-ABC (Cornuet et al, 2014) presented as support for their earlier inference of sympatric evolution of North Pacific ecotypes (Moura et al, 2015), as they excluded the Atlantic population.…”

“…Accordingly, the number of fixed differences between the sampled killer whale populations (reported in Table 2 of Moura et al, 2014a) ranges from just 0 to 15. By sampling 1.7 Mb of the nuclear genome representing many genes, Hoelzel and Moura (2015) claim they have produced a reliable topology that accurately reconstructs the history of these populations. However, by concatenating the data they have ignored coalescent variance and assumed that all loci share the same genealogy (Knowles and Carstens, 2007;Kubatko and Degnan, 2007).…”

Killer whales redistribute white shark foraging pressure on seals

“…NEP killer whale populations form distinct and stable social groups (pods), which differ in specialization of prey choice (ecotypes). Three recognized NEP ecotypes, ‘resident,’ ‘transient,’ and ‘offshore’, exhibit genetic and phenotypic differentiation 45–48 . Transient pods typically feed on marine mammals including elephant seals and sea lions, whereas resident pods target teleosts, mainly salmonids 45,49 .…”