AQUATIC LOCOMOTION IN BIRDS – BIOMECHANICS, MORPHOMETRICS, AND EVOLUTION

The entire diversity of life on earth exists in air or water. Whether an organism lives in air or water provides the most fundamental description of its physical world and establishes an organism’s ecological niche on the most essential level. Because these two fluids are vastly different from one another, they also dictate, via the process of natural selection, the morphology and physiology of the organisms which call them home. By studying how organisms interact with these fluids – to locomote or obtain food, for example – we have the ability to not only link organism form and function, but also to study the process of evolution itself. These two goals have been the focus of my dissertation, using diving birds as a model system. Of the 40 extant orders of birds, 16 orders contain aquatic or semi-aquatic members – species which regularly locomote on or in water as part of their life-history. Birds constitute just over 30% of all terrestrial vertebrates; thus, the bird species which move in water are both substantial and diverse. Only 1 of 16 orders have lost the ability to fly. Species in the remaining 15 orders face simultaneous selection for effective and efficient locomotion in both air and water, despite the vast differences between these two fluids. In Chapter 1 of this dissertation, I review the biomechanics of aquatic locomotion in birds and test existing hypotheses surrounding their morphologies. In Chapter 2, I use geometric morphometrics to determine how the multifunctionality of semi-aquatic birds – specifically, the wings of wing-propelled diving birds – has constrained or facilitated their morphological diversity. In Chapters 3 and 4, I use kinematic analysis to test whether the pressures of retaining aerial flight mean that species which use their wings for locomotion in both air and water are less effective and less efficient in water than lineages which have lost aerial flight. Finally, in Chapter 5, I document submerged aquatic locomotion in non-aquatic birds, despite a lack of selection or experience for this behavior, altering current understanding of the evolution of aquatic lifestyles in vertebrates