Multilevel Societies of Female Sperm Whales (Physeter macrocephalus) in the Atlantic and Pacific: Why Are They So Different?

Killer whales (Orcinus orca) currently comprise a single, cosmopolitan species with a diverse diet. However, studies over the last 30 yr have revealed populations of sympatric "ecotypes" with discrete prey preferences, morphology, and behaviors. Although these ecotypes avoid social interactions and are not known to interbreed, genetic studies to date have found extremely low levels of diversity in the mitochondrial control region, and few clear phylogeographic patterns worldwide. This low level of diversity is likely due to low mitochondrial mutation rates that are common to cetaceans. Using killer whales as a case study, we have developed a method to readily sequence, assemble, and analyze complete mitochondrial genomes from large numbers of samples to more accurately assess phylogeography and estimate divergence times. This represents an important tool for wildlife management, not only for killer whales but for many marine taxa. We used high-throughput sequencing to survey whole mitochondrial genome variation of 139 samples from the North Pacific, North Atlantic, and southern oceans. Phylogenetic analysis indicated that each of the known ecotypes represents a strongly supported clade with divergence times ranging from approximately 150,000 to 700,000 yr ago. We recommend that three named ecotypes be elevated to full species, and that the remaining types be recognized as subspecies pending additional data. Establishing appropriate taxonomic designations will greatly aid in understanding the ecological impacts and conservation needs of these important marine predators. We predict that phylogeographic mitogenomics will become an important tool for improved statistical phylogeography and more precise estimates of divergence times.

Recent interest in animal cultures has been fuelled by high-profile reports of intra- and interpopulation differences in the behavioural repertoires of primates and cetaceans, consistent with the existence of socially learned traditions. Several studies have mapped spatial differences in behaviour, revealing a mosaic of behavioural phenotypes within species. The dominant current approach attempts to determine whether this is cultural variation by excluding asocial learning, ecological or genetic factors. However, claims of animal cultures remain controversial because such comparisons are subject to weaknesses; thus, new approaches to isolating the influence of culture on behaviour are required. Here we suggest that, rather than attributing behaviour to explanatory categories, researchers would often be better advised to partition variance in behaviour to alternative sources.