Hydrodynamic sensing does not facilitate active drag reduction in the golden shiner (Notemigonus crysoleucas).

The lateral line system detects water flow, which allows fish to orient their swimming with respect to hydrodynamic cues. However, it is unclear whether this sense plays a role in the control of propulsion. Hydrodynamic theory suggests that fish could reduce drag by coordinating the motion of the head relative to detected flow signals. To test this hypothesis, we performed measurements of undulatory kinematics during steady swimming in the golden shiner (Notemigonus crysoleucas) at three speeds (4.5, 11.0 and 22.0 cm s(-1)). We found that the phase shift between yaw angle and lateral velocity (20.5+/-13.1 deg., N=5) was significantly greater than the theoretical optimum (0 deg.) and the amplitude of these variables created a hydrodynamic index (H=0.05+/-0.03, N=6) that was less than an order of magnitude below the theoretical prediction. Furthermore, we repeated these measurements after pharmacologically ablating the lateral line hair cells and found that drag reduction was not adversely influenced by disabling the lateral line system. Therefore, flow sensing does not facilitate active drag reduction. However, we discovered that ablating the lateral line causes the envelope of lateral displacement to nearly double at the envelope's most narrow point for swimming at 4.5 cm s(-1). Therefore, fish may use hydrodynamic sensing to modulate the lateral amplitude of slow undulatory swimming, which could allow rapid responses to changes in environmental flow.

How swifts control their glide performance with morphing wings.

Gliding birds continually change the shape and size of their wings, presumably to exploit the profound effect of wing morphology on aerodynamic performance. That birds should adjust wing sweep to suit glide speed has been predicted qualitatively by analytical glide models, which extrapolated the wing's performance envelope from aerodynamic theory. Here we describe the aerodynamic and structural performance of actual swift wings, as measured in a wind tunnel, and on this basis build a semi-empirical glide model. By measuring inside and outside swifts' behavioural envelope, we show that choosing the most suitable sweep can halve sink speed or triple turning rate. Extended wings are superior for slow glides and turns; swept wings are superior for fast glides and turns. This superiority is due to better aerodynamic performance-with the exception of fast turns. Swept wings are less effective at generating lift while turning at high speeds, but can bear the extreme loads. Finally, our glide model predicts that cost-effective gliding occurs at speeds of 8-10 m s(-1), whereas agility-related figures of merit peak at 15-25 m s(-1). In fact, swifts spend the night ('roost') in flight at 8-10 m s(-1) (ref. 11), thus our model can explain this choice for a resting behaviour. Morphing not only adjusts birds' wing performance to the task at hand, but could also control the flight of future aircraft.

Syringeal muscles fit the trill in ring doves (Streptopelia risoria L.).

In contrast to human phonation, the virtuoso vocalizations of most birds are modulated at the level of the sound generator, the syrinx. We address the hypothesis that syringeal muscles are physiologically capable of controlling the sound-generating syringeal membranes in the ring dove (Streptopelia risoria) syrinx. We establish the role of the tracheolateralis muscle and propose a new function for the sternotrachealis muscle. The tracheolateralis and sternotrachealis muscles have an antagonistic mechanical effect on the syringeal aperture. Here, we show that both syringeal muscles can dynamically control the full syringeal aperture. The tracheolateralis muscle is thought to directly alter position and tension of the vibrating syringeal membranes that determine the gating and the frequency of sound elements. Our measurements of the muscle's contractile properties, combined with existing electromyographic and endoscopic evidence, establish its modulating role during the dove's trill. The muscle delivers the highest power output at cycle frequencies that closely match the repetition rates of the fastest sound elements in the coo. We show that the two syringeal muscles share nearly identical contraction characteristics, and that sternotrachealis activity does not clearly modulate during the rapid trill. We propose that the sternotrachealis muscle acts as a damper that stabilizes longitudinal movements of the sound-generating system induced by tracheolateralis muscle contraction. The extreme performance of both syringeal muscles implies that they play an important role in fine-tuning membrane position and tension, which determines the quality of the sound for a conspecific mate.

How the body contributes to the wake in undulatory fish swimming: flow fields of a swimming eel (Anguilla anguilla).

Undulatory swimmers generate thrust by passing a transverse wave down their body. Thrust is generated not just at the tail, but also to a varying degree by the body, depending on the fish's morphology and swimming movements. To examine the mechanisms by which the body in particular contributes to thrust production, we chose eels, which have no pronounced tail fin and hence are thought to generate all their thrust with their body. We investigated the interaction between body movements and the flow around swimming eels using two-dimensional particle image velocimetry. Maximum flow velocities adjacent to the eel's body increase almost linearly from head to tail, suggesting that eels generate thrust continuously along their body. The wake behind eels swimming at 1.5 Ls(-1), where L is body length, consisted of a double row of double vortices with little backward momentum. The eel sheds two vortices per half tail-beat, which can be identified by their shedding dynamics as a start-stop vortex of the tail and a vortex shed when the body-generated flows reach the 'trailing edge' and cause separation. Two consecutively shed ipsilateral body and tail vortices combine to form a vortex pair that moves away from the mean path of motion. This wake shape resembles flow patterns described previously for a propulsive mode in which neither swimming efficiency nor thrust is maximised but sideways forces are high. This swimming mode is suited to high manoeuvrability. Earlier recordings show that eels also generate a wake reflective of maximum swimming efficiency. The combined findings suggest that eels can modify their body wave to generate wakes that reflect their propulsive mode.

Hydrodynamics of unsteady fish swimming and the effects of body size: comparing the flow fields of fish larvae and adults.

Zebra danios (Brachydanio rerio) swim in a burst-and-coast mode. Most swimming bouts consist of a single tail flick and a coasting phase, during which the fish keeps its body straight. When visualising the flow in a horizontal section through the wake, the effects of the flow regime become apparent in the structure of the wake. In a two-dimensional, medio-frontal view of the flow, larvae and adults shed two vortices at the tail during the burst phase. These vortices resemble a cross section through a large-core vortex ring: two vortex cores packed close together with the central flow directed away from the fish. This flow pattern can be observed in larvae (body length approximately 4 mm) at Reynolds numbers below 100 as well as in adult fish (body length approximately 35 mm) at Reynolds numbers above 1000. Larval vortices differ from those of adult zebra danios mainly in their relatively wider vortex cores (higher ratio of core radius to ring radius) and their lower vortex circulation. Both effects result from the increased importance of viscosity on larval flows. During the coasting phase, larval and adult flows again differ because of the changing importance of viscosity. The high viscosity of the water causes large vortical flows adjacent to the larva's body. These regions of high vorticity represent the huge body of water dragged along by the larva, and they cause the larva to stop almost immediately after thrust generation ceases. No such areas of high vorticity are visible adjacent to adult zebra danios performing a comparable swimming manoeuvre. The rapid decrease in vortex circulation and the severe reduction in the coasting distance due to viscous drag contribute to the high cost that larvae - unlike adult fish - face when using a burst-and-coast swimming style.

Aquatic vertebrate locomotion: wakes from body waves.

Vertebrates swimming with undulations of the body and tail have inflection points where the curvature of the body changes from concave to convex or vice versa. These inflection points travel down the body at the speed of the running wave of bending. In movements with increasing amplitudes, the body rotates around the inflection points, inducing semicircular flows in the adjacent water on both sides of the body that together form proto-vortices. Like the inflection points, the proto-vortices travel towards the end of the tail. In the experiments described here, the phase relationship between the tailbeat cycle and the inflection point cycle can be used as a first approximation of the phase between the proto-vortex and the tailbeat cycle. Proto-vortices are shed at the tail as body vortices at roughly the same time as the inflection points reach the tail tip. Thus, the phase between proto-vortex shedding and tailbeat cycle determines the interaction between body and tail vortices, which are shed when the tail changes direction. The shape of the body wave is under the control of the fish and determines the position of vortex shedding relative to the mean path of motion. This, in turn, determines whether and how the body vortex interacts with the tail vortex. The shape of the wake and the contribution of the body to thrust depend on this interaction between body vortex and tail vortex. So far, we have been able to describe two types of wake. One has two vortices per tailbeat where each vortex consists of a tail vortex enhanced by a body vortex. A second, more variable, type of wake has four vortices per tailbeat: two tail vortices and two body vortices shed from the tail tip while it is moving from one extreme position to the next. The function of the second type is still enigmatic.