Global taxonomy of short-finned pilot whales:
In the 1980s, two different morphological types of short-finned pilot whales living off the coast of Japan were described by Japanese scientists (Kasuya et al. 1988). These types had been known for generations to local hunters and fishermen, known as Naisa goto and Shiho goto (goto means pilot whale). They were confirmed to be different in body size, skull morphology, melon shape, and color patterns. Since the 1980s, we’ve learned through continued research that these animals are also genetically and acoustically different (Oremus et al. 2009, Van Cise et al. 2017), and that they are found throughout the Pacific
Ocean, with non-overlapping distributions in the eastern and western Pacific (Van Cise et al. 2016).
These scientific discoveries have led to several questions, such as: Why and how did these two types diverge? How long ago did they diverge, and do they mix? And finally, are these two types different enough that we should consider them, and manage them, as separate sub-species or species?
To answer these questions, we’ve sequenced complete mitochondrial genomes and targeted locations in the nuclear genome, called single nucleotide polymorphisms (SNPs), in about 200 individuals sampled throughout the global range of this species. We will use this data to estimate divergence time, rate of dispersal and admixture, and differentiation between the two types. We’ll compare significant events in the genetic history of this species with the historical environmental record, and compare current spatial patterns with current environmental conditions, to better understand what historical events may have led to the divergence of these types, and what current environmental conditions are helping to maintain their separation.
Socio-genetic divergence in short-finned pilot whales:
Within the expanse of the pelagic ocean, there are relatively few environmental barriers to dispersal, especially within the local range of a species or individual. In some species of whale, social behavior may be an important driver of population structure among groups inhabiting the same geographical space.
Short-finned pilot whales in the Main Hawaiian Islands live for decades or more in stable social units (Mahaffy et al. 2015). Individual social units are selective in how they associate with other social units; groups of social units are called clusters. Several tens of clusters sympatrically use the habitat around a specific island or island groups; these groups of clusters are called an island community (Baird 2016).
This well-documented heirarachical social structure provides a unique opportunity to examine links between sociality, ecological behavior, relatedness, and mate choice within populations, and examine how these decisions affect genetic structure. Using a combination of acoustic recordings and sequences from both nuclear and mitochondrial DNA, we look for patterns of similarity and differentiation in the vocal repertoire and genetic code of short-finned pilot whales throughout the Hawaiian Islands, and compare these patterns with potential geographic/ecological barriers as well as with social barriers.
From this research, we have learned that the social behavior of short-finned pilot whale is inextricably linked with its genetic and cultural heritage, and both affects and is affected by its genetic structure. We learned that stable social units defined by Mahaffy et al. are in fact family units. Clusters are loose networks of related individuals, similar to an extended family network, and clusters that were more genetically similar were more likely to associate. We found that clusters may have distinct vocal dialects, which may help them distinguish among themselves when they come across each other. And we also saw evidence that mate selection may be restricted among clusters, which will perpetuate and increase the genetic divergence we detected among clusters.
This research expands the small but growing body of evidence suggesting that gene-culture coevolutionary theory, or the concept that culture evolves alongside genetics and can affect and be affected by the genetics of populations and species, is applicable to non-human social species. The number of species that we know to have links between social behavior and genetic structure is small, including species like killer whales, elephants, and non-human primates. However, highly-evolved social structure has been documented in species across all major taxa; it is possible that continued research will show that social behavior is linked with the evolutionary ecology of many more species than previously thought.