Shape variation in the eye patch and dorsal fin of southern hemisphere killer whales (Orcinus orca)

Abstract

Morphological variation within and among species plays a critical role in evolutionary processes, influencing adaptation, survival, and reproductive success. Killer whale ( Orcinus orca ) morphology is known to vary on both an individual and population level with several ecotypes or forms documented worldwide. However, the extent of morphological variation among killer whales in Australian waters remains unclear, both among individuals within the region and in comparison to those in other parts of the southern hemisphere. This study assessed eye patch and dorsal fin shape variation in Australian and Antarctic killer whales to explore the evolutionary relationships among these groups. A large dataset of imagery was compiled and processed to achieve this, which provided representative sample sizes for five separate study groups: northwest Australia (NW), southwest Australia (SW), southeast Australia (SE), Antarctic type A (AA) and Antarctic type B1 (AB). Elliptical Fourier analysis was used to extract the feature outlines and enable multivariate data analyses. Principal component analysis and pairwise comparisons revealed significant morphological differences both within and between Australian and Antarctic killer whales. Eye patch shape variation was driven by the degree of taper and overall width whereas dorsal fin shape variation was driven by falcateness and broadness at its base. Hierarchical cluster analysis revealed considerable variation amongst these features, while linear discriminate analysis indicated that individuals could not be reliably classified into their respective study groups based on eye patch and dorsal fin shape alone. Nonetheless, these findings suggest the presence of both a tropical and temperate form of killer whale in Australia, with the latter resembling both the Antarctic Type A and B2s morphologically. To better understand their connectivity and divergence, dedicated research is needed to assess the evolutionary history of these populations. Such knowledge will be vital in defining global conservation management units for killer whales which are still considered a single, data deficient species by the International Union for Conservation of Nature (IUCN).

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