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Title: Anatomical Patterns, Kinematics, and Propulsive Strategies of Foot-Based Swimming Birds
Abstract: Within the diversity of birds, numerous lineages have colonized aquatic environments. Birds that swim by paddling with their feet must generate propulsive forces underwater using legs originally adapted for walking. Despite this challenge, foot-propelled swimming has evolved independently in at least six avian lineages (including in cormorants, grebes, and loons) and to varying degrees of specialization. Understanding the extent of convergence among these groups reveals the functional demands of producing force underwater and the selective pressures that have shaped avian evolution. My dissertation focuses on examining anatomical, movement, and propulsive patterns underlying foot-based swimming in birds. In four chapters, I use comparative dissections, high-speed videography, and a new robotic technique to explore how paddling birds convergently adapt to a swimming lifestyle.
In the first chapter, I combine new qualitative dissection data with a review of historical studies on avian hindlimb myology to identify patterns in hindlimb muscle anatomy associated with swimming. I performed dissections of 36 hindlimb muscles in 35 specimens representing 8 bird species with varying degrees of swimming specialization. My findings show that birds specialized for diving streamline the body by integrating the proximal hindlimb into the skin covering the abdomen, allowing the attachments of several hindlimb muscles to shift distally along the leg. Diving birds also possess relatively enlarged ankle plantarflexors with loons demonstrating an extreme condition. Despite a similar lifestyle to loons, grebes reduce many distal limb muscles and instead possess long, calcified tendons.
To investigate a potential explanation for the anatomical divergence between grebes and loons observed in chapter one, I examine foot motion and force production during an impressive grebe mating behavior (Ch. 2) and during underwater swimming by common loons (Ch. 3, 4).
Chapter two focuses on grebe “rushing,” where breeding pairs run on water powered only by their legs. I spent a month filming wild rushing grebes in S. Oregon using high-speed cameras. My findings show that rushing grebes take up to 20 steps per second and their feet slap the water at 4 m/s. I reproduced water slaps in the lab using physical models of grebe feet, demonstrating that the slap alone contributes up to 30-55% of the required weight support and exerts forces approaching those associated with tendon calcification in birds.
In chapter three, I designed custom underwater camera cases to film wild loons in a rehabilitation pool. Loons are notoriously reclusive, precluding any previous, quantitative analysis of swimming. My high-speed footage shows that loons swim in a similar manner to grebes, synchronously paddling their feet lateral to the body and incorporating long-axis rotation at the knee. Loons use head-bobbing to increase visual acuity underwater and elicit tight but slow turns. To control turning, loons increase the speed of the outside foot, begin the power stroke of the outside foot before that of the inside foot, and bank the body away from the turn.
For my last, fourth, chapter, I design a novel bio-robotic approach to measure the fluid forces produced by wild swimming animals. I attached cadaveric loon feet to an industrial robot programmed to exactly replicate the swimming motions of a loon’s foot (based on data from Ch. 3). A force sensor attached to the robot measured swimming forces throughout the propulsive stroke. This robotic approach provides the most accurate exploration to date of how foot-propelled birds produce hydrodynamic forces. I find that swimming loons power swimming by generating more lift than drag, challenging traditional paradigms of aquatic locomotion.