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.

Reversed sexual dimorphism in the leg muscle architecture of the Eurasian sparrowhawk

“…Unlike previous studies (e.g., Mosto et al, 2021; Sustaita, 2008; Ward et al, 2002), we considered more individual muscles for allometric analysis and found 13 out of 14 were positively allometric, which implies that, with increasing body size, these specific muscles become much greater in PCSA and thus more force‐generation capacity in large‐bodied individuals. For the Eurasian sparrowhawk that shows significant RSD in body mass, the positive allometry indicated a potential for increasing force generation in females than expected for their body mass.…”

“…The ratio is greater in mammalivores species: 73% in the red‐tailed hawk ( Buteo jamaicensis calurus ) (Donohue & Dufty, 2006) and 78% in the North American golden eagle ( Aquila chrysaetos canadensis ) (Harmata & Montopoli, 2013). Among piscivorous species, the ratio of male to female body mass is 80% in the white‐tailed sea eagle ( Haliaeetus albicilla ) and 88% in the osprey ( Pandion haliaetus ) (Mosto et al, 2021; Muriel et al, 2010). The intersexual difference in body mass is least in carrion‐eating raptors, with a value of 99% in the griffon vulture ( Gyps fulvus ) and 89% in the bearded vulture ( Gypaetus barbatus ) (Lόpez‐Lόpez et al, 2011; Xirouchakis & Poulakakis, 2008).…”

“…In accipiters, they are used not only to catch, hold, and carry prey, but also as the main killing weapon by the firm grasp of their toes and talons (a specific term referring only to the claws of birds of prey), which results in suffocation via thoracic compression and/or penetration of vital organs of the prey (Goslow, 1972; Sustaita, 2008). These capacities are exerted mainly by the powerful flexor muscles along with the strong, curved, sharp talons (Mosto et al, 2021). The force‐generating capacity of the feet may reflect functional demands for foraging on different types and sizes of prey.…”

“…Average prey size was suggested to be an important selective pressure driving the morphological evolution of raptor feet (Fowler et al, 2009; Snyder & Wiley, 1976). Unfortunately, within this context, there is only one study dealing with leg musculature, which found no RSD in any of the architectural variables in the piscivorous white‐tailed sea eagle Haliaeetus albicilla (Mosto et al, 2021).…”

Form and function in the avian pelvis

“…The raptor pelvis is characterized by a wide and flaring iliac wing in the preacetabular pelvis and a craniocaudally abbreviated postacetabular region. Although the vast majority of work on the raptor hindlimb is focused on the distal limb (e.g., Einoder & Richardson, 2007; Goslow, 1972; Mosto et al, 2021; Ward et al, 2002), it has been observed that raptors commonly extend their entire legs when striking prey (Goslow, 1971). Hip flexors may also play an important role in lifting prey after the strike.…”