Effects of particle size and thickness of quartz sand on the webbed foot kinematics of mallard (Anas platyrhynchos)

The webbed foot structure of mallards (Anas platyrhynchos) exhibits effective anti-subsidence properties when walking on soft ground. To investigate the effects of quartz sand particle size and thickness on joint angles and the movement patterns of webbed feet, we created a testing substrate with quartz sand and utilized high-speed cameras and kinematic analysis tools for data acquisition. Mallards mainly adjusted the tarsometatarso-phalangeal joint (TMTPJ) during touchdown and lift-off stages in response to increasing particle size or enhanced ground roughness. Conversely, adjustments to the intertarsal joint (ITJ) predominantly took place during mid-stance. Conversely, mallards predominantly adjusted the ITJ during touchdown and lift-off when coping with increased quartz sand thickness, with TMTPJ adjustments mainly occurring at touch-down. As quartz sand particle size increased, the TMTPJ angle increased, the ITJ angle decreased, toe closure advanced, and the duty factor decreased throughout the entire stride cycle. In contrast, increasing quartz sand thickness led to more delayed TMTPJ adjustments, slower webbed foot closure, and an increased duty factor throughout the stride cycle. Mallards modify their leg posture to notably decrease the touch-down foot angle upon encountering sandy terrain. This action subsequently forms a depression beneath their feet, contributing to sand consolidation and limiting flow. During the stance phase, the mallard's weight is distributed across the webbed foot, generating minimal pressure and preventing significant subsidence while walking on sandy ground.