Locomotor Ontogeny and the Evolution of Avian Flight

Ashley Margaret Heers, The University of Montana

Abstract

Understanding the origins of organismal diversity is one of biology's most enduring quests. Many authors have posited that modularity, or the developmental decoupling of body parts into discrete functional units, facilitates adaptive radiation by allowing body parts to become independently and functionally specialized without compromising one another. This may be particularly true in birds, which have compartmentalized the single locomotor unit of their ancestors into discrete wing and leg locomotor modules, and exploited terrestrial and aerial environments. Specializing one set of limbs for terrestrial locomotion and one set for flight presumably facilitates navigation of aerial environments without compromising terrestrial locomotion, and vice versa. Therefore understanding modularity may be key to understanding bird locomotion, and some of the most striking architectural and behavioral diversity in the history of animals. Traditionally, wings and legs have been viewed as discrete and independent body parts with distinct and autonomous functions: wings for aerial locomotion, legs for terrestrial locomotion. This paradigm, however, may be misleading. First, recent work demonstrates that birds often engage their wings and legs cooperatively. Second, the degree of wing- leg autonomy may be constrained by unexplored tradeoffs, between (i) allocating energy to wings versus legs during development, or between (ii) wing versus leg investment and performance (since legs must be carried as baggage by wings during flight and vice versa). Thus, to fully appreciate how locomotor modularity influences locomotor behavior in flying animals, we must explore how wing and leg modules cooperatively interact and potentially tradeoff during ontogeny and evolution. Using birds as a focal group, I have thus pursued two questions: (1) How does cooperative use of wings and legs help to bridge the developmental transition from an obligately-bipedal juvenile to a flight-capable adult? (2) Do tradeoffs between wing versus leg investment and performance influence locomotor ontogeny and evolution? This work offers important and novel insight into ontogenetic and evolutionary construction of the avian bauplan.