Sexually dimorphic characteristics of short‐finned pilot whales, false killer whales, pygmy killer whales, and melon‐headed whales assessed using fin and body morphometrics from photographs taken at sea

Abstract

Adults of many free-ranging delphinid species cannot be reliably sexed at sea. Sexually mature, known-sex adult short-finned pilot, pygmy killer, melon-headed, and false killer whales were profiled from at-sea photos to assess proportional fin and body dimorphism. Males of all four species had larger dorsal fins proportional to anterior body length. False killer whales showed no further dimorphism, while melon-headed whales showed subtle dimorphism in dorsal fin overhang. Pygmy killer whales showed subtle dimorphism in lateral positioning of the top-most point and overhang. Lastly, short-finned pilot whales showed strong dimorphism in the leading edge, lateral positioning of the top-most point, and height of the overhang. Generalized linear models showed strong predictive accuracy for pilot and false killer whales, and moderate accuracy for pygmy killer and melon-headed whales. Results indicate that adult shortfinned pilot and false killer whales can be accurately sexed in the field or via morphometrics. Adult pygmy killer and melon-headed whales have subtle sexual dimorphism that can be detected using morphometrics. The presence of dimorphism gives insight into potential reproductive strategies and social structure. Further analysis is needed to explore morphometric differences between inshore and

Errata Note: Common Cetacean Species Misidentifications

“…Distinguishing species from aerial still images is not necessarily straightforward, however, as distortion created by the water can change the shape (e.g., making flippers with rounded tips appear pointed, or vice versa, or making a rounded head appear pointed). Body proportions, in particular the relative length of the back from the anterior insertion of the dorsal fin to the blowhole in comparison to the length of the dorsal fin base, can be used to discriminate false killer whales and short-finned pilot whales from each other and from the other two species (Figure 1), although it is important to note that body proportions can vary with age and sex (Yahn et al, 2023). Even when coloration patterns are obscured, body proportions can be used to separate false killer whales and short-finned pilot whales from each other and from the other two species, with false killer whales having the largest distance (relative to length of dorsal fin base) between the blowhole and the anterior insertion of the dorsal fin, and short-finned pilot whales having the smallest distance.…”