Flying whales: Insights into the flipper of the humpback whale and its relatives using a novel tissue-illuminating...

Cetaceans primarily generate thrust through dorsoventral oscillations of the caudal flukes, while their flippers are generally associated with lift generation and torque production during manoeuvring. However, the humpback whale, which possesses the largest forelimbs among extant and extinct vertebrates, has been observed performing active flipper strokes that contribute to forward propulsion. This behaviour represents a unique combination of axial swimming and underwater flight among cetaceans. In this study we investigate the anatomical correlates of this rarely observed locomotor behaviour and examine how the distinctive flipper anatomy of the humpback whale compares to that of other cetaceans. We focus in particular on the musculoskeletal and tendinous organisation of the flipper. We compared the flipper musculoskeletal anatomy of the humpback whale with that of seven other cetacean species (two mysticetes and five odontocetes). A novel protocol to acquire images during dissections involving ultraviolet (UV) light was employed to enhance the contrast between skeletal elements, musculature and connective tissues, with a particular emphasis on tendons and entheses. Although all flippers are highly vascularised, antebrachial musculature is strongly reduced across cetaceans, a condition that is especially pronounced in humpback whales and several odontocete lineages that lack antebrachial muscles. These findings suggest that active control of the flipper is primarily achieved proximally at the shoulder joint between the scapula and humerus, where the musculature is markedly developed. This proximal control pattern appears to be shared among cetaceans despite substantial differences in flipper size and external morphology. Beyond its anatomical implications this study provides photographic documentation of key flipper structures across cetacean groups and demonstrates that UV-light imaging is an effective tool for revealing internal soft-tissue anatomy. This approach is likely to be valuable for the study of other anatomical systems.