Quantitative Morphometric and Scanning Electron Microscopic Analysis of Scale Dimensions and Epidermal Thickness in Scaly Shank Skin: Habitat-Correlated Adaptations in Aquatic and Terrestrial Birds.

The integumentary system of avians is represented by the scaly shank skin, which plays an essential role in adaptation to environmental conditions. In this paper, we provide the first set of quantitative morphometric data on the dimensions of scales, sulcus width (SuW), thickness of layers of epidermis, and dermal adipose tissue in two species of different habitats, namely, the Egyptian Balady Duck (EBD; Anas boschas domesticus, aquatic) and the Broad-Breasted White Turkey (BBWT; Meleagris gallopavo domesticus, terrestrial). Two scale types (scute and scutella) were identified and quantified using calibrated ImageJ software based on gross, SEM, and histological photographs (n = 20 scales/species). Scale length (SL), scale width (SW), aspect ratio (AR; SL/SW), and scale area (SA) were quantified in gross photographs; SuW in SEM images. Furthermore, total epidermal thickness (TET), stratum corneum thickness (SCT), stratum germinativum thickness (SGT), and adipose tissue thickness were quantified in histological sections (n ≥ 10 fields/aspect/bird). The dorsal scute scales of BBWT were significantly larger than those of EBD in terms of SL (5.46 ± 1.36 vs. 2.57 ± 0.29 mm); SW (7.53 ± 0.83 vs. 2.59 ± 0.29 mm); SA (26.18 ± 1.03 vs. 7.86 ± 2.16 mm2), all p < 0.05. SuW was significantly wider in BBWT compared to EBD across all aspects assessed (p < 0.05). TET and SCT of BBWT were significantly greater than those of EBD on dorsal (TET: 210.98 ± 19.45 vs. 164.42 ± 17.03 µm; p < 0.05; SCT: 155.21 ± 18.27 vs. 79.15 ± 25.42 µm; p < 0.05) and medial aspects (TET: 169.31 ± 18.8 vs. 88.83 ± 13.22 µm, p < 0.05). In contrast, the EBD had much thicker dermal fat tissue (1028.73 ± 27.65 vs. 647.77 ± 23.37 µm, respectively; p < 0.05). This is consistent with their energy requirements to swim in an aquatic environment. From these data, one can conclude that biomechanical requirements due to habitat lead to a specific architectural design of bird shank scales.