Grasping behavior in the white-tailed sea eagle (Accipitridae, Aves) explained by muscle architecture

Abstract

Hindlimbs of raptors play an essential role in feeding due to the grasping abilities that allow them to hunt and kill their prey. This study explores and quantifies for the first time the structural and mechanical features of the digit flexor muscles in one of the largest piscivorous European raptors, the white-tailed sea eagle. We studied the myological architecture of nine muscles of five females and five males of Haliaeetus albicilla. We found that (1) reversed sexual dimorphism was not reflected in any of these structural or mechanical variables; (2) when the physiological cross-sectional area values of muscles were scaled altogether against body mass, they had a positive allometric tendency, implying that larger muscles have proportionally higher physiological cross-sectional area values than smaller ones. Conversely, different scaling patterns were obtained when each flexor was analyzed individually: Only three muscles were positively allometric, and four muscles were isometric. (3) Three major groups can be identified in relation to the different features here analyzed: (1) flexor digitorum longus, flexor hallucis longus and tibialis cranialis: large muscles, with high physiological cross-sectional area and tendon cross-sectional area, low tendon/belly ratio and isometric; (2) flexor hallucis brevis: small muscle, with intermediate physiological cross-sectional area and tendon cross-sectional area, low tendon/belly ratio, short fiber length and isometric; (3) musculi perforans and perforans et perforatus: intermediate muscles, with low physiological cross-sectional area and tendon cross-sectional area, high tendon/belly ratio and allometric. Haliaeetus albicilla hunts mainly fish, an elusive prey, and their muscles have an anatomical design related to their common purpose, to grip prey. However, different patterns that characterize each muscle might respond to different requirements related to this particular prey item and hunting behavior and to the role each digit can perform.