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of the Azov-Black Sea Ornithological Station

Language of the article: Russian Cite: Broshko, Y. O. (2016). Some cases of morphofunctional adaptations of the limb skeleton in birds. Branta: Transactions of the Azov-Black Sea Ornithological Station, 19, 112-125 Keywords: birds, bipedalism, limbs, bones, cross-section of the mid-shaft, mechanical loads Views: 1074 Branta copyright

Branta Issues > Issue №19 (2016)

Branta: Transactions of the Azov-Black Sea Ornithological Station, 112-125

Some cases of morphofunctional adaptations of the limb skeleton in birds

Y. O. Broshko

Bipedalism in birds has significant effect on their limb morphology that is very conservative. Overspecialization of limbs leads to more significant adaptations of the skeleton.
In this work, the limb bones of seven bird species were investigated. They are humerus, ulna, radius, femur, tibiotarsus and tarsometatarsus (Table 1). The following parameters were evaluated: bone mass (m, g), total bone length (l, mm), frontal (df, mm) and sagittal (ds, mm) diameters of the mid-shaft; parameters of cross-sectional geometry of the mid-shaft: cortical area (А, mm²), second moments of area (Imax, Imin, mm4), polar moment of area (J, mm⁴) (Table 2). The following indices were also calculated: ratio of mid-shaft diameters (df/ds), cross-sectional index (ik), and ratio of second moments of area (I_max/I_min) (Table 3). Cortical area, second moments of area and polar moment of intertia represent the bone resistance to the various mechanical loads: compression, bending and torsion, respectively. The cross-section shape of the mid-shaft is directly associated with these parameters. Interspecific allometry of bone characteristics was investigated as well (Table 4 and 5).
It was established that wing bones are mainly characterized by elliptical cross-section shape (Fig. 1). However, in representatives of the genus Anas this shape is circular. It is explained by significant torsional loads caused by the intensity of flight. The most typical cross-section shape of hindlimb bones (especially femur) is circular (Fig. 2). This indicates the predominance of torsional loads in hindlimbs, caused by the most types of terrestrial locomotion. Swimming is accompanied by a significant increase of bending loads in the sagittal plane thereby resulting in an elliptical cross-section shape of the Anas femur. However, the latter characteristic is not the single adaptation to swimming.
Mechanical characteristics increase more intensively to adapt bird limb bones to the growth of relative mechanical loads induced by increasing of their body mass. It is demonstrated by positive allometry of these characteristics (b > 0.67 – for cortical area, b > 1.33 – for second moments of area). Linear dimensions of bones are mainly isometric to the body mass. Thus, in bipedalism, limb bone properties undergo qualitative changes (increase in strength and resistance to loads) rather than quantitative ones (relative enlargement).

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