# Underwater paddling kinematics and hydrodynamics in a surface swimming duck versus a diving duck

## TL;DR

Diving ducks use more efficient paddling techniques underwater to counter buoyancy compared to surface-swimming ducks.

## Contribution

The study reveals how subtle paddling differences in ducks lead to significant hydrodynamic effects for underwater swimming.

## Key findings

- Mandarin ducks use alternate foot paddling exclusively, while pochards switch between alternate and simultaneous paddling.
- Pochards swim with a tilted body posture that correlates with swimming speed and limits foot motion.
- Foot lift in pochards generates downthrust to counter buoyancy, unlike mandarins whose lift interferes with buoyancy.

## Abstract

Some duck species mostly swim on the water surface while others frequently dive underwater. We compared the paddling kinematics of mandarin ducks (Axis galericulata) that feed on the surface and diving ferruginous pochards (Aythya nyroca) that feed underwater. Both species were trained to perform the same horizontal, submerged swimming at 1 m depth in a controlled set-up. Mandarins used alternate foot paddling exclusively, while pochards varied their gait between alternate foot paddling and simultaneous paddling with both feet. Unlike mandarins, pochards swam with their body tilted at an angle that was negatively correlated with the swimming speed and limited their foot motion to a smaller arc. Hydrodynamic modeling revealed that lift generated by the webbed foot provided thrust to propel both duck species forward. However, mandarins' feet generated lift-based upthrust that interfered with the need to counter their buoyancy, while pochards directed the foot lift to provide vertical downthrust against their buoyancy. The relatively subtle differences in foot motion between the two species result in a substantial hydrodynamic effect that may hint at the kinematic changes required when transitioning from surface to submerged swimming in the evolution of foot-propelled diving waterfowl.

Summary: Diving ducks use paddling kinematics underwater that is more efficient for countering buoyancy compared with that used by dabbling ducks.