The biomechanical origin of extreme wing allometry in hummingbirds

Dimitri A Skandalis,Christopher C Witt,Robert Dudley,Jimmy A Mcguire,Kenneth C Welch,David Lentink,Paolo S Segre,Derrick J E Groom,Joseph W Bahlman,Douglas L Altshuler

doi:10.1038/s41467-017-01223-x

Abstract

Flying animals of different masses vary widely in body proportions, but the functional implications of this variation are often unclear. We address this ambiguity by developing an integrative allometric approach, which we apply here to hummingbirds to examine how the physical environment, wing morphology and stroke kinematics have contributed to the evolution of their highly specialised flight. Surprisingly, hummingbirds maintain constant wing velocity despite an order of magnitude variation in body weight; increased weight is supported solely through disproportionate increases in wing area. Conversely, wing velocity increases with body weight within species, compensating for lower relative wing area in larger individuals. By comparing inter- and intraspecific allometries, we find that the extreme wing area allometry of hummingbirds is likely an adaptation to maintain constant burst flight capacity and induced power requirements with increasing weight. Selection for relatively large wings simultaneously maximises aerial performance and minimises flight costs, which are essential elements of humming bird life history.

Highlights

Flying animals of different masses vary widely in body proportions, but the functional implications of this variation are often unclear
The exponent of the allometric relationship of hummingbird wing area to body weight has been estimated between 1.1 and 1.3, compared to about 0.7 across all other birds[10, 11]. This large exponent indicates that larger species have very large wings for their body weight, even though larger wings are predicted to be negatively associated with many aspects of aerial agility[11] and so could compromise flight performance
We find that the allometry of force production among and within hummingbird species is solely a function of changes in the allometries of wing area and wing velocity

Summary

Introduction

Flying animals of different masses vary widely in body proportions, but the functional implications of this variation are often unclear We address this ambiguity by developing an integrative allometric approach, which we apply here to hummingbirds to examine how the physical environment, wing morphology and stroke kinematics have contributed to the evolution of their highly specialised flight. Understanding the origin of this wing area allometry and how it influences flight performance has the potential to explain how hummingbirds have diversified into their specialised ecological niche, and explain the biomechanical evolution of flying animals more generally. An integrative perspective on this problem must be able to explain not just the presence or absence of an allometry, and explain its magnitude We approach this general problem by considering the mechanisms that contribute to the generation and cost of aerodynamic force in flight, and develop a framework to unify many aspects of hummingbird flight physiology. According to a blade element model (developed in ‘Methods’), the time-averaged equation for vertical, weight-supporting aerodynamic force during hummingbird hovering is FV

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Methods

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Conclusion