Research

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Hummingbird flight mechanics and maneuvers
To elucidate both physical and physiological limits to animal flight capacity,  we are continuing to study hummingbirds exposed to various kinds of aerodynamic challenge, including flight in turbulent airstreams, heavy rain, transient perturbations, and through vegetational clutter. Miller Postdoctoral Fellow Alejandro Rico-Guevara is also studying the interaction between hummingbird breathing and feeding mechanics relative to experimentally imposed variability in aerodynamic and metabolic demand.  For a comparative perspective on limits to flight performance, Michael Dillon (University of Wyoming) and I are analyzing the allometry of maximum mechanical power output in hymenopterans.  Using eleven species of hovering orchid bee in Panama, we are quantifying the relative decline in maximum force and power output with increasing body size, trends which characterizes all volant taxa.  Graduate students in the laboratory are also pursuing studies of hovering evolution in the Old World sunbirds (Leeann Louis) and of rotational and translational accelerations for a diversity of Andean hummingbirds (Ashley Smiley).

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Evolutionary origins of insect and bird flight
The origin of flight in both insects and volant vertebrates remains a topic of great interest among biologists.  Work with Steve Yanoviak (University of Louisville) suggests that controlled aerial behavior in insects preceded the origins of wings proper, and that flight initially began via stages of gliding and maneuvering in terrestrial hexapods.  Collaborative work with Professor Xing Xu (Institute of Vertebrate Paleontology & Paleoanthropology, Beijing) on the origins of bird flight is linking the now abundant fossil record of four-winged avian precursors with biomechanical assessment of flight capacity.  Ph.D. student Erik Sathe is investigating incipient aerial behaviors in extant arboreal lizards, as well as the consequences of wing flapping for a robotic glider.

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Biomechanical and physiological adaptation to altitude
Hovering at higher elevations requires simultaneous compensation for the effects of reduced air density on force production, and for the consequences of lower oxygen partial pressure on the aerobically challenging requirements of flight.  Jim McGuire of the Museum of Vertebrate Zoology, Chris Witt at the University of New Mexico, and I have been examining morphological parameters, resistance to hypoxia, and flight performance for more than sixty hummingbird species across a 4000 meter gradient in the Peruvian Andes.  This analysis is embedded within a molecular phylogenetic framework for about three hundred trochilid species, and incorporates both metabolic and aerodynamic analyses of hovering flight.  In a similar vein, I and Michael Dillon (University of Wyoming) are evaluating flight-related morphological and physiological traits for bumblebees across steep elevational gradients in southwest China.  Comparison of maximal flight performance for numerous bumblebee species covering an altitudinal range of 800 – 4000 meters elucidates intra- and interspecific patterns of adaptation in morphometrics, kinematics, and the aerodynamics of hovering flight.

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Ecophysiology of Neotropical butterfly migrations
Since 1987, I and collaborators (Bob Srygley, USDA; Phil DeVries, University of New Orleans) have been studying natural flight performance of long-distance migratory butterflies (and the diurnal moth Urania fulgens) during the early rainy season in central Panama.  This work has included assessment of airspeeds, thermoregulatory capacity, lipid reserves, use of geomagnetic and sun compasses, and sensory mechanisms of flight control.  Ongoing studies include relationships between migratory population dynamics and climate, and the use of the antennae in flight control and orientation (in collaboration with Sanjay Sane, NCBS, Bangalore).

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Ethanol and the ripening sequence of tropical fruits
A fundamental prediction of the “drunken monkey” hypothesis is that ethanol is a low-level but widespread constituent within the pulp of tropical fruits consumed by primates. Working with postdoctoral fellow Matt Medeiros, I am quantitatively assessing the ripening sequence of palm and fig fruits on Barro Colorado Island, Panama, including such parameters as color, texture, sugar content, and ethanol concentration. Comparable work on the fruits of Spondias mombin (Anacardiaceae), as eaten by spider monkeys, is being done collaboratively with Chrissy Campbell at Cal State Northridge. Ph.D. student Aleksey Maro is currently investigating the ripening sequence of and ethanol content within fruits consumed by chimpanzees in forest of Kibale, Uganda.