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Updated: Aug 5, 2026
Behavioral Assessment of Manual Dexterity in Non-Human Primates
Published on: November 11, 2011
Functional Morphology of the Pedal Musculature and Grip Force Estimation in the Red‑and‑Green Macaw (Ara chloropterus
Sérgio Roberto Posso1, Vinicius Camargo Tavares1, Ana Caroline M Barbosa1
1LESCAN - Laboratory of Ecology, Systematics and Conservation of Neotropical Birds, Mato Grosso do Sul Federal University, Três Lagoas, MS, Brazil.
Abstract:
Psittaciformes possess zygodactyl feet that support perching, climbing, inverted postures, and foot-mediated food manipulation. Despite the functional importance of these behaviors, the myological and biomechanical bases of digital performance remain poorly documented in large-bodied psittacids, particularly macaws. Here, we investigate the digitized musculature and estimate the functional force output of the foot in the red-and-green macaw. We describe the intrinsic and extrinsic muscles acting on the digits, quantify muscle length and mass, and estimate digit-specific pinching forces based on physiological cross-sectional area and mechanical advantage. Digit IV is the longest digit, and its primary flexor, m. flexor perforatus digiti IV, is the longest and most massive digital flexor. Consistent with this morphology, digit IV generated the greatest pinching force (8.63 ± 1.52 N), indicating a dominant role in force production. The remaining digits are shorter and associated with more slender musculature, resulting in lower force outputs (digit II: 4.51 ± 0.92 N; digit III: 2.29 ± 0.30 N; digit I [hallux]: 1.06 ± 0.15 N). The combined force of the posterior digits (I + IV; 9.69 ± 1.74 N) approximates that of the anterior digits (II + III; 6.80 ± 1.22 N), indicating a near-balanced anterior-posterior force distribution that likely enhances stability during perching and climbing while facilitating precise manipulative behaviors. Apparently, the total pedal force (16.49 ± 3.22 N) scales with body size in Psittaciformes; however, this hypothesis requires testing by using explicit allometric and phylogenetic comparative approaches. Finally, this foot-gripping force is lower than that reported for large raptors (Accipitriformes and Falconiformes), reflecting functional specialization of the macaw pes for controlled arboreal locomotion, inverted postures, and fine motor manipulation rather than maximal force generation.
