Flight Mechanics and Control of Escape Manoeuvres in Hummingbirds. I. Flight Kinematics

Hummingbirds are nature’s masters of aerobatic manoeuvres. Previous research shows that hummingbirds and insects converged evolutionarily upon similar aerodynamic mechanisms and kinematics in hovering. Herein, we use three-dimensional kinematic data to begin to test for similar convergence of kinematics used for escape flight and to explore the effects of body size upon manoeuvring. We studied four hummingbird species in North America including two large species (magnificent hummingbird, Eugenes fulgens, 7.8 g, and blue-throated hummingbird, Lampornis clemenciae, 8.0 g) and two smaller species (broad-billed hummingbird, Cynanthus latirostris, 3.4 g, and black-chinned hummingbirds Archilochus alexandri, 3.1 g). Starting from a steady hover, hummingbirds consistently manoeuvred away from perceived threats using a drastic escape response that featured body pitch and roll rotations coupled with a large linear acceleration. Hummingbirds changed their flapping frequency and wing trajectory in all three degrees of freedom on a stroke-by-stroke basis, likely causing rapid and significant alteration of the magnitude and direction of aerodynamic forces. Thus it appears that the flight control of hummingbirds does not obey the ‘helicopter model’ that is valid for similar escape manoeuvres in fruit flies. Except for broad-billed hummingbirds, the hummingbirds had faster reaction times than those reported for visual feedback control in insects. The two larger hummingbird species performed pitch rotations and global-yaw turns with considerably larger magnitude than the smaller species, but roll rates and cumulative roll angles were similar among the four species

Flight Mechanics and Control of Escape Manoeuvres in Hummingbirds. II. Aerodynamic Force Production, Flight Control and Performance Limitations

The superior manoeuvrability of hummingbirds emerges from complex interactions of specialized neural and physiological processes with the unique flight dynamics of flapping wings. Escape manoeuvring is an ecologically relevant, natural behaviour of hummingbirds, from which we can gain understanding into the functional limits of vertebrate locomotor capacity. Here, we extend our kinematic analysis of escape manoeuvres from a companion paper to assess two potential limiting factors of the manoeuvring performance of hummingbirds: (1) muscle mechanical power output and (2) delays in the neural sensing and control system. We focused on the magnificent hummingbird (Eugenes fulgens, 7.8 g) and the black-chinned hummingbird (Archilochus alexandri, 3.1 g), which represent large and small species, respectively. We first estimated the aerodynamic forces, moments and the mechanical power of escape manoeuvres using measured wing kinematics. Comparing active-manoeuvring and passive-damping aerodynamic moments, we found that pitch dynamics were lightly damped and dominated by the effect of inertia, while roll dynamics were highly damped. To achieve observed closed-loop performance, pitch manoeuvres required faster sensorimotor transduction, as hummingbirds can only tolerate half the delay allowed in roll manoeuvres. Accordingly, our results suggested that pitch control may require a more sophisticated control strategy, such as those based on prediction. For the magnificent hummingbird, we estimated that escape manoeuvres required muscle mass-specific power 4.5 times that during hovering. Therefore, in addition to the limitation imposed by sensorimotor delays, muscle power could also limit the performance of escape manoeuvres

Effects of Flight Speed upon Muscle Activity in Hummingbirds

Hummingbirds have the smallest body size and highest wingbeat frequencies of all flying vertebrates, so they represent one endpoint for evaluating the effects of body size on sustained muscle function and flight performance. Other bird species vary neuromuscular recruitment and contractile behavior to accomplish flight over a wide range of speeds, typically exhibiting a U-shaped curve with maxima at the slowest and fastest flight speeds. To test whether the high wingbeat frequencies and aerodynamically active upstroke of hummingbirds lead to different patterns, we flew rufous hummingbirds (Selasphorus rufus, 3 g body mass, 42 Hz wingbeat frequency) in a variable-speed wind tunnel $(\textrm{0–10 m s}^{-1})$. We measured neuromuscular activity in the pectoralis (PECT) and supracoracoideus (SUPRA) muscles using electromyography (EMG, $N=4$ birds), and we measured changes in PECT length using sonomicrometry ($N=1$). Differing markedly from the pattern in other birds, PECT deactivation occurred before the start of downstroke and the SUPRA was deactivated before the start of upstroke. The relative amplitude of EMG signal in the PECT and SUPRA varied according to a U-shaped curve with flight speed; additionally, the onset of SUPRA activity became relatively later in the wingbeat at intermediate flight speeds $(\textrm{4 and 6 m s}^{−1})$. Variation in the relative amplitude of EMG was comparable with that observed in other birds but the timing of muscle activity was different. These data indicate the high wingbeat frequency of hummingbirds limits the time available for flight muscle relaxation before the next half stroke of a wingbeat. Unlike in a previous study that reported single-twitch EMG signals in the PECT of hovering hummingbirds, across all flight speeds we observed 2.9±0.8 spikes per contraction in the PECT and 3.8±0.8 spikes per contraction in the SUPRA. Muscle strain in the PECT was 10.8±0.5%, the lowest reported for a flying bird, and average strain rate was 7.4±0.2 muscle lengths $s^{−1}$. Among species of birds, PECT strain scales proportional to body mass to the 0.2 power $(\infty M_b^{0.2})$ using species data and $\infty M_b^{0.3}$ using independent contrasts. This positive scaling is probably a physiological response to an adverse scaling of mass-specific power available for flight.Organismic and Evolutionary BiologyOther Research Uni

Development of a Humid Climate Definition

The role of humidity in indoor air quality has become of increasing concern in recent years. High indoor humidities can result in microbial growth on building surfaces, resulting in poor indoor air quality, as well as damage to the building and its contents. In addition to the IAQ impacts, high indoor humidity can cause occupant discomfort. The public review draft of ASHRAE Standard 62-1989R included requirements for installation of dehumidification controls in buildings with mechanical cooling located in humid climates. The draft standard included a definition of humid climate: where, during the warmest six consecutive months of a typical year, the wetbulb temperature is 19°C (67°F) or higher for 3500 hours or more, or 23°C (73°F) or higher for 1750 hours or more. This definition is that used in the 1993 ASHRAE Handbook of Fundamentals to define the humid climate region. The only areas in the continental United States which meet these criteria are close to the Gulf coast, all of Florida, and along the Atlantic coast as far north as southern North Carolina While it is clear that buildings in this humid climate region need to be carefully designed with regard to humidity control, it is also clear that buildings in other areas have an equal need for humidity control. The work described in this paper examines a number of potential indicators of "humid climate" and correlates them with the prevalence of indoor humidity problems in three building types. The FSEC 2.3 energy simulation computer program (Kerestecioglu et al. 1989) was used to simulate the three building types, using weather from 10 cities in the southeastern U.S. The FSEC software was selected because it is capable of accurately modeling moisture transfer within the building space and the dehumidification performance of cooling coils at part-load conditions, and predicting resulting humidity levels. The buildings modeled were a retail store (similar to a K-Mart or Wal-Mart), a large office building, and a fast food restaurant. Existing building models were employed for this study with ventilation rates in accordance with ASHRAE Standard 62-1989. The HVAC systems used were typical for these building types, without any special humidity control measures. The selected indicators of humidity problems are the number of hours per year with space humidity above 60% RH and the number of occupied hours with space humidity above 60% RH. TMY2 weather data (NREL 1995) for 10 cities was used for the annual building energy simulations. TMY2 data was also used to calculate a number of potential humid climate parameters for the same 10 cities. These included: the number of hours and the wetbulb-degree hours above 3 different wetbulb temperatures, the number of hours and grain-hours above 4 different humidity ratios, and the sensible, latent and total Ventilation Load Index (VLI). The VLI is the load (latent, sensible or total) generated by bringing one cfm of outdoor air to space neutral conditions over the course of one year (Hamman, et al. 1997). The ability of each climate parameter to predict indoor humidity problems was analyzed and compared. Implications of using the selected parameters to define a humid climate will be discusse

Ricci flows, wormholes and critical phenomena

We study the evolution of wormhole geometries under Ricci flow using numerical methods. Depending on values of initial data parameters, wormhole throats either pinch off or evolve to a monotonically growing state. The transition between these two behaviors exhibits a from of critical phenomena reminiscent of that observed in gravitational collapse. Similar results are obtained for initial data that describe space bubbles attached to asymptotically flat regions. Our numerical methods are applicable to "matter-coupled" Ricci flows derived from conformal invariance in string theory.Comment: 8 pages, 5 figures. References added and minor changes to match version accepted by CQG as a fast track communicatio

Low speed maneuvering flight of the rose-breasted cockatoo (Eolophus roseicapillus). II. Inertial and aerodynamic reorientation

SUMMARY The reconfigurable, flapping wings of birds allow for both inertial and aerodynamic modes of reorientation. We found evidence that both these modes play important roles in the low speed turning flight of the rose-breasted cockatoo Eolophus roseicapillus. Using three-dimensional kinematics recorded from six cockatoos making a 90° turn in a flight corridor, we developed predictions of inertial and aerodynamic reorientation from estimates of wing moments of inertia and flapping arcs, and a blade-element aerodynamic model. The blade-element model successfully predicted weight support (predicted was 88±17% of observed, N=6) and centripetal force (predicted was 79±29% of observed, N=6) for the maneuvering cockatoos and provided a reasonable estimate of mechanical power. The estimated torque from the model was a significant predictor of roll acceleration (r2=0.55, P<0.00001), but greatly overestimated roll magnitude when applied with no roll damping. Non-dimensional roll damping coefficients of approximately –1.5, 2–6 times greater than those typical of airplane flight dynamics (approximately –0.45), were required to bring our estimates of reorientation due to aerodynamic torque back into conjunction with the measured changes in orientation. Our estimates of inertial reorientation were statistically significant predictors of the measured reorientation within wingbeats (r2 from 0.2 to 0.37, P<0.0005). Components of both our inertial reorientation and aerodynamic torque estimates correlated, significantly, with asymmetries in the activation profile of four flight muscles: the pectoralis, supracoracoideus, biceps brachii and extensor metacarpi radialis (r2 from 0.27 to 0.45, P<0.005). Thus, avian flight maneuvers rely on production of asymmetries throughout the flight apparatus rather than in a specific set of control or turning muscles

Designed Guanidinium-Rich Amphipathic Oligocarbonate Molecular Transporters Complex, Deliver and Release siRNA in Cells

The polyanionic nature of oligonucleotides and their enzymatic degradation present challenges for the use of siRNA in research and therapy; among the most notable of these is clinically relevant delivery into cells. To address this problem, we designed and synthesized the first members of a new class of guanidinium-rich amphipathic oligocarbonates that noncovalently complex, deliver, and release siRNA in cells, resulting in robust knockdown of target protein synthesis in vitro as determined using a dual-reporter system. The organocatalytic oligomerization used to synthesize these co-oligomers is step-economical and broadly tunable, affording an exceptionally quick strategy to explore chemical space for optimal siRNA delivery in varied applications. The speed and versatility of this approach and the biodegradability of the designed agents make this an attractive strategy for biological tool development, imaging, diagnostics, and therapeutic applications

Oligocarbonate Molecular Transporters: Oligomerization-Based Syntheses and Cell-Penetrating Studies

A new family of guanidinium-rich molecular transporters featuring a novel oligocarbonate backbone with 1,7-side chain spacing is described. Conjugates can be rapidly assembled irrespective of length in a one-step oligomerization strategy that can proceed with concomitant introduction of probes (or by analogy drugs). The new transporters exhibit excellent cellular entry as determined by flow cytometry and fluorescence microscopy, and the functionality of their drug delivery capabilities was confirmed by the delivery of the bioluminescent small molecule probe luciferin and turnover by its intracellular target enzyme

A New Era of Morphological Investigations: Reviewing Methods for Comparative Anatomical Studies

The increased use of imaging technology in biological research has drastically altered morphological studies in recent decades and allowed for the preservation of important collection specimens alongside detailed visualization of bony and soft-tissue structures. Despite the benefits associated with these newer imaging techniques, there remains a need for more “tra- ditional”methods of morphological examination in many comparative studies. In this paper, we describe the costs and benefits of the various methods of visualizing, examining, and comparing morphological structures. There are significant differences not only in the costs associated with these different methods (monetary, time, equipment, and software), but also in the degree to which specimens are destroyed. We argue not for any one particular method over another in morphological studies, but instead suggest a combination of methods is useful not only for breadth of visualization, but also for the financial and time constraints often imposed on early-career research scientists

The Acute Effects of the Atypical Dissociative Hallucinogen Salvinorin A on Functional Connectivity in the Human Brain

Salvinorin A (SA) is a κ-opioid receptor agonist and atypical dissociative hallucinogen found in Salvia divinorum. Despite the resurgence of hallucinogen studies, the effects of κ-opioid agonists on human brain function are not well-understood. This placebo-controlled, within-subject study used functional magnetic resonance imaging for the first time to explore the effects of inhaled SA on strength, variability, and entropy of functional connectivity (static, dynamic, and entropic functional connectivity, respectively, or sFC, dFC, and eFC). SA tended to decrease within-network sFC but increase between-network sFC, with the most prominent effect being attenuation of the default mode network (DMN) during the first half of a 20-min scan (i.e., during peak effects). SA reduced brainwide dFC but increased brainwide eFC, though only the former effect survived multiple comparison corrections. Finally, using connectome-based classification, most models trained on dFC network interactions could accurately classify the first half of SA scans. In contrast, few models trained on within- or between-network sFC and eFC performed above chance. Notably, models trained on within-DMN sFC and eFC performed better than models trained on other network interactions. This pattern of SA effects on human brain function is strikingly similar to that of other hallucinogens, necessitating studies of direct comparisons