Neuromuscular anatomy and evolution of the cetacean forelimb

The forelimb of cetaceans (whales, dolphins, and porpoises) has been radically modified during the limb-to-flipper transition. Extant cetaceans have a soft tissue flipper encasing the manus and acting as a hydrofoil to generate lift. The neuromuscular anatomy that controls flipper movement, however, is poorly understood. This study documents flipper neuromuscular anatomy and tests the hypothesis that antebrachial muscle robustness is related to body size. Data were gathered during dissections of 22 flippers, representing 15 species (7 odontocetes, 15 mysticetes). Results were compared with published descriptions of both artiodactyls and secondarily aquatic vertebrates. Results indicate muscle robustness is best predicted by taxonomic distribution and is not a function of body size. All cetaceans have atrophied triceps muscles, an immobile cubital joint, and lack most connective tissue structures and manus muscles. Forelimbs retain only three muscle groups: triceps (only the scapular head is functional as the humeral heads are vestigal), and antebrachial extensors and flexors. Well-developed flexor and extensor muscles were found in mysticetes and basal odontocetes (i.e., physeterids, kogiids, and ziphiids), whereas later diverging odontocetes (i.e., monodontids, phocoenids, and delphinids) lack or reduce these muscles. Balaenopterid mysticetes (e.g., fin and minke whales) may actively change flipper curvature, while basal odontocetes (e.g., sperm and beaked whales) probably stiffen the flipper through isometric contraction. Later diverging odontocetes lack musculature supporting digital movements and are unable to manipulate flipper curvature. Cetacean forelimbs are unique in that they have lost agility and several soft tissue structures, but retain sensory innervations.