Kristen E. Crandell

Aerodynamics of tip-reversal upstroke in a revolving pigeon wing.

SUMMARY During slow flight, bird species vary in their upstroke kinematics using either a ‘flexed wing’ or a distally supinated ‘tip-reversal’ upstroke. Two hypotheses have been presented concerning the function of the tip-reversal upstroke. The first is that this behavior is aerodynamically inactive and serves to minimize drag. The second is that the tip-reversal upstroke is capable of producing significant aerodynamic forces. Here, we explored the aerodynamic capabilities of the tip-reversal upstroke using a well-established propeller method. Rock dove (Columba livia, N=3) wings were spread and dried in postures characteristic of either mid-upstroke or mid-downstroke and spun at in vivo Reynolds numbers to simulate forces experienced during slow flight. We compared 3D wing shape for the propeller and in vivo kinematics, and found reasonable kinematic agreement between methods (mean differences 6.4% of wing length). We found that the wing in the upstroke posture is capable of producing substantial aerodynamic forces. At in vivo angles of attack (66 deg at mid-upstroke, 46 deg at mid-downstroke), the upstroke wings averaged for three birds produced a lift-to-drag ratio of 0.91, and the downstroke wings produced a lift-to-drag ratio of 3.33. Peak lift-to-drag ratio was 2.5 for upstroke and 6.3 for downstroke. Our estimates of total force production during each half-stroke suggest that downstroke produces a force that supports 115% of bodyweight, and during upstroke a forward-directed force (thrust) is produced at 36% of body weight.

Transition from leg to wing forces during take-off in birds

SUMMARY Take-off mechanics are fundamental to the ecology and evolution of flying animals. Recent research has revealed that initial take-off velocity in birds is driven mostly by hindlimb forces. However, the contribution of the wings during the transition to air is unknown. To investigate this transition, we integrated measurements of both leg and wing forces during take-off and the first three wingbeats in zebra finch (Taeniopygia guttata, body mass 15 g, N=7) and diamond dove (Geopelia cuneata, body mass 50 g, N=3). We measured ground reaction forces produced by the hindlimbs using a perch mounted on a force plate, whole-body and wing kinematics using high-speed video, and aerodynamic forces using particle image velocimetry (PIV). Take-off performance was generally similar between species. When birds were perched, an acceleration peak produced by the legs contributed to 85±1% of the whole-body resultant acceleration in finch and 77±6% in dove. At lift-off, coincident with the start of the first downstroke, the percentage of hindlimb contribution to initial flight velocity was 93.6±0.6% in finch and 95.2±0.4% in dove. In finch, the first wingbeat produced 57.9±3.4% of the lift created during subsequent wingbeats compared with 62.5±2.2% in dove. Advance ratios were <0.5 in both species, even when taking self-convection of shed vortices into account, so it was likely that wing–wake interactions dominated aerodynamics during wingbeats 2 and 3. These results underscore the relatively low contribution of the wings to initial take-off, and reveal a novel transitional role for the first wingbeat in terms of force production.

Transition from wing to leg forces during landing in birds

Transitions to and from the air are critical for aerial locomotion and likely shaped the evolution of flying animals. Research on take-off demonstrates that legs generate greater body accelerations compared with wings, and thereby contribute more to initial flight velocity. Here, we explored coordination between wings and legs in two species with different wingbeat styles, and quantified force production of these modules during the final phase of landing. We used the same birds that we had previously studied during take-off: zebra finch (Taeniopygia guttata, N=4) and diamond dove (Geopelia cuneata, N=3). We measured kinematics using high-speed video, aerodynamics using particle image velocimetry, and ground-reaction forces using a perch mounted on a force plate. In contrast with the first three wingbeats of take-off, the final four wingbeats during landing featured ~2 times greater force production. Thus, wings contribute proportionally more to changes in velocity during the last phase of landing compared with the initial phase of take-off. The two species touched down at the same velocity (~1 m s−1), but they exhibited significant differences in the timing of their final wingbeat relative to touchdown. The ratio of average wing force to peak leg force was greater in diamond doves than in zebra finches. Peak ground reaction forces during landing were ~50% of those during take-off, consistent with the birds being motivated to control landing. Likewise, estimations of mechanical energy flux for both species indicate that wings produce 3–10 times more mechanical work within the final wingbeats of flight compared with the kinetic energy of the body absorbed by legs during ground contact.

Kinematics and aerodynamics of avian upstrokes during slow flight

ABSTRACT Slow flight is extremely energetically costly per unit time, yet highly important for takeoff and survival. However, at slow speeds it is presently thought that most birds do not produce beneficial aerodynamic forces during the entire wingbeat: instead they fold or flex their wings during upstroke, prompting the long-standing prediction that the upstroke produces trivial forces. There is increasing evidence that the upstroke contributes to force production, but the aerodynamic and kinematic mechanisms remain unknown. Here, we examined the wingbeat cycle of two species: the diamond dove (Geopelia cuneata) and zebra finch (Taeniopygia guttata), which exhibit different upstroke styles – a wingtip-reversal and flexed-wing upstroke, respectively. We used a combination of particle image velocimetry and near-wake streamline measures alongside detailed 3D kinematics. We show that during the middle of the wingtip-reversal upstroke, the hand-wing has a high angular velocity (15.3±0.8 deg ms−1) and translational speed (8.4±0.6 m s−1). The flexed-wing upstroke, in contrast, has low wingtip speed during mid-upstroke. Instead, later in the stroke cycle, during the transition from upstroke to downstroke, it exhibits higher angular velocities (45.5±13.8 deg ms−1) and translational speeds (11.0±1.9 m s−1). Aerodynamically, the wingtip-reversal upstroke imparts momentum to the wake, with entrained air shed backward (visible as circulation of 14.4±0.09 m2 s−1). In contrast, the flexed-wing upstroke imparts minimal momentum. Clap and peel in the dove enhances the time course for circulation production on the wings, and provides new evidence of convergent evolution on time-varying aerodynamic mechanisms during flapping in insects and birds. Summary: Some birds exhibit an upstroke style that enhances aerodynamic force production during energetically expensive slow flight. This aerodynamic signature is closely linked with their wing motion.

Stick or grip? Co-evolution of adhesive toepads and claws in Anolis lizards.

Exploring the relationship between phenotype and performance in an ecological and evolutionary context is crucial to understanding the adaptive nature of phenotypic traits. Despite their ubiquity in vertebrates, few studies have examined the functional and ecological significance of claw morphologies. Here we examine the adhesive toepad and claw system of Anolis lizards. Claw characters are significantly different between lizards classified as arboreal (perch height≥1m) and non-arboreal (perch height<1m). Arboreal species possess significantly higher and longer claws, and show trends toward decreased claw curvature and wider claw tip angles. Toepad size and claw length and height are tightly correlated with each other and with perch height, suggesting that the adhesive toepad and gripping claw have co-evolved to accommodate different habitats. The functional morphology and evolution of claws are ripe areas for future investigation.

Notes on the Natural History of the Little-Known Ecuadorian Horned Anole, Anolis proboscis

Abstract Until very recently, the horned anole of Ecuador, Anolis proboscis, was known from only six male specimens, the last collected in 1966. The species was rediscovered in 2005 and in subsequent years additional specimens have been collected in the general vicinity of the type locality. These include several females, which lack the conspicuous rostral appendage seen in males. Despite rediscovery, the natural history of this enigmatic anole remains almost completely unknown. We conducted an ecological and behavioral study of this species near Mindo, Ecuador. Anolis proboscis is an extremely slow-moving and cryptic species that often occurs high in the trees. The rostral horn notwithstanding, A. proboscis is indistinguishable from Greater Antillean anoles of the “twig” ecomorph class in morphology, ecology, and behavior. The possession of a horn only by males suggests a role in sexual selection. We found the horn in life to be soft and highly flexible and thus unsuitable for use as a weapon in male–male combat; hence, the horn most likely serves as a signal and may be involved in mate choice or territorial displays. However, we did not observe any social encounters during our observations and are thus unable to test these hypotheses. Given its cryptic morphology and behavior, it is not surprising that A. proboscis is so rarely observed.

Multiple paths to aquatic specialisation in four species of Central American Anolis lizards

Aquatic anoles present an interesting ecomorphological puzzle. On the one hand, the link between habitat use and morphology is well established as convergent within the Caribbean anole radiation. On the other hand, aquatic anoles do not appear to form an ecomorphological group – rather, it appears that there may be several ways to adapt to aquatic habitats. We explore this issue by examining the ecology, morphology and performance of four species of Central American aquatic anoles belonging to two different lineages. Overall, we find that aquatic anoles overlap in multiple ecological and morphological dimensions. However, we do find some differences in substrate use, claw and limb morphology, and bite force that distinguish Anolis aquaticus from the other three species (A. lionotus, A. oxylophus and A. poecilopus). Our results suggest that A. aquaticus is adapted to climb on boulders, whereas the other species utilise vegetation in streamside habitats.

Modeling observed animal performance using the Weibull distribution

ABSTRACT To understand how organisms adapt, researchers must link performance and microhabitat. However, measuring performance, especially maximum performance, can sometimes be difficult. Here, we describe an improvement over previous techniques that only consider the largest observed values as maxima. Instead, we model expected performance observations via the Weibull distribution, a statistical approach that reduces the impact of rare observations. After calculating group-level weighted averages and variances by treating individuals separately to reduce pseudoreplication, our approach resulted in high statistical power despite small sample sizes. We fitted lizard adhesive performance and bite force data to the Weibull distribution and found that it closely estimated maximum performance in both cases, illustrating the generality of our approach. Using the Weibull distribution to estimate observed performance greatly improves upon previous techniques by facilitating power analyses and error estimations around robustly estimated maximum values. Summary: Using the Weibull distribution to model observed animal performance data allows for more robust statistical analyses and estimates of maximum performance.

Tempo and mode of performance evolution across multiple independent origins of adhesive toe pads in lizards

Understanding macroevolutionary dynamics of trait evolution is an important endeavor in evolutionary biology. Ecological opportunity can liberate a trait as it diversifies through trait space, while genetic and selective constraints can limit diversification. While many studies have examined the dynamics of morphological traits, diverse morphological traits may yield the same or similar performance and as performance is often more proximately the target of selection, examining only morphology may give an incomplete understanding of evolutionary dynamics. Here, we ask whether convergent evolution of pad‐bearing lizards has followed similar evolutionary dynamics, or whether independent origins are accompanied by unique constraints and selective pressures over macroevolutionary time. We hypothesized that geckos and anoles each have unique evolutionary tempos and modes. Using performance data from 59 species, we modified Brownian motion (BM) and Ornstein–Uhlenbeck (OU) models to account for repeated origins estimated using Bayesian ancestral state reconstructions. We discovered that adhesive performance in geckos evolved in a fashion consistent with Brownian motion with a trend, whereas anoles evolved in bounded performance space consistent with more constrained evolution (an Ornstein–Uhlenbeck model). Our results suggest that convergent phenotypes can have quite distinctive evolutionary patterns, likely as a result of idiosyncratic constraints or ecological opportunities.

Flight performance in the altricial zebra finch: developmental effects and reproductive consequences

Abstract The environmental conditions animals experience during development can have sustained effects on morphology, physiology, and behavior. Exposure to elevated levels of stress hormones (glucocorticoids, GCs) during development is one such condition that can have long‐term effects on animal phenotype. Many of the phenotypic effects of GC exposure during development (developmental stress) appear negative. However, there is increasing evidence that developmental stress can induce adaptive phenotypic changes. This hypothesis can be tested by examining the effect of developmental stress on fitness‐related traits. In birds, flight performance is an ideal metric to assess the fitness consequences of developmental stress. As fledglings, mastering takeoff is crucial to avoid bodily damage and escape predation. As adults, takeoff can contribute to mating and foraging success as well as escape and, thus, can affect both reproductive success and survival. We examined the effects of developmental stress on flight performance across life‐history stages in zebra finches (Taeniopygia guttata). Specifically, we examined the effects of oral administration of corticosterone (CORT, the dominant avian glucocorticoid) during development on ground‐reaction forces and velocity during takeoff. Additionally, we tested for associations between flight performance and reproductive success in adult male zebra finches. Developmental stress had no effect on flight performance at all ages. In contrast, brood size (an unmanipulated variable) had sustained, negative effects on takeoff performance across life‐history stages with birds from small broods performing better than birds from large broods. Flight performance at 100 days posthatching predicted future reproductive success in males; the best fliers had significantly higher reproductive success. Our results demonstrate that some environmental factors experienced during development (e.g. clutch size) have stronger, more sustained effects than others (e.g. GC exposure). Additionally, our data provide the first link between flight performance and a direct measure of reproductive success.