Abstract
The grasping capability of birds' feet is a hallmark of their evolution, but the mechanics of avian foot function are not well understood. Two evolutionary trends that contribute to the mechanical complexity of the avian foot are the variation in the relative lengths of the phalanges and the subdivision and variation of the digital flexor musculature observed among taxa. We modelled the grasping behaviour of a simplified bird foot in response to the downward and upward forces imparted by carrying and perching tasks, respectively. Specifically, we compared the performance of various foot geometries performing these tasks when actuated by distally inserted flexors only, versus by both distally inserted and proximally inserted flexors. Our analysis demonstrates that most species possess relative phalanx lengths that are conducive to grasps actuated only by a single distally inserted tendon per digit. Furthermore, proximally inserted flexors are often required during perching, but the distally inserted flexors are sufficient when grasping and carrying objects. These results are reflected in differences in the relative development of proximally and distally inserted digital flexor musculature among ‘perching’ and ‘grasping’ taxa. Thus, our results shed light on the relative roles of variation in phalanx length and digit flexor muscle distribution in an integrative, mechanical context.
Highlights
To flight, a foot capable of grasping is a quintessential, but comparatively less appreciated and understudied, aspect of avian evolution [1,2,3,4,5]
The vast majority of bird species appear to have relatively less musculoskeletal mass allocated to the 2 distal hind limbs compared with the grasping appendages of sized mammals, yet many retain extremely dexterous multi-digit grasping and manipulation capabilities [6]
We believe that the multiarticular digit flexor muscle–tendon units in many bird species may be adaptive for allowing powerful grasping, with relatively few muscles and potentially lower musculoskeletal mass requirements
Summary
A foot capable of grasping is a quintessential, but comparatively less appreciated and understudied, aspect of avian evolution [1,2,3,4,5]. The behavioural and functional performance attributes of grasping have primarily been investigated in raptorial birds, such as hawks, falcons and owls [10,11,12,13,14], and to some extent in parrots [15,16] These groups are highly specialized for powerful (raptors) and dexterous (parrots) manipulation with their feet, and these attributes are reflected in various aspects of their foot anatomy. On the other hand, demonstrate additional, and/or relatively greater development of, tarsal and digital extensor muscles that may afford them greater pedal dexterity [20,21] Despite their taxonomic moniker, the ‘perching birds’ (Passeriformes) are in many respects comparatively less specialized for generating powerful or dexterous grasps. Passeriforms have evolved a diversity of foot musculoskeletal configurations presumably adapted to various locomotor modes and substrates, with comparatively fewer muscles [22]
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