Bilateral directional asymmetry of the appendicular skeleton of the harbor porpoise (Phocoena phocoena)

Directional asymmetry (DA) of the lengths, diameters, and masses of the scapula, humerus, radius, and ulna were analyzed on a sample of 213 harbor porpoises from Denmark and West Greenland. The levels of DA were consistent across yearlings and older animals, mature and immature animals, sexes, and populations. All investigated variables showed significant DA favoring the right side. For the pooled sample, DA of lengths ranged from 0.2% SE 0.1 (ulna) to 1.2% SE 0.1 (scapula). DAs of diameters were 1.5% SE 0.2 for the humerus, 0.6% SE 0.1 for the radius, and 1.3% SE 0.2 for the ulna. DA of mass ranged from 2.8% SE 0.5 (humerus) to 4.3% SE 0.7 (ulna). The humerus and ulna had significantly larger mean diameter/length ratios on the right side than the left, making them more robust. The large DA of scapula length indicates larger muscle mass associated with the right flipper, while the generally more robust right humerii and ulnae may be designed for higher levels of mechanical stress. These DAs and the examples of lateralized behavior recorded in cetaceans, point to the existence of lateralized use of the flippers at the population level in harbor porpoises and possibly other cetacean species.