Sandra Nauwelaerts

Take-off and landing forces in jumping frogs

SUMMARY Anurans use a saltatorial (jumping) mode of locomotion. A jumping cycle can be divided into four subphases: propulsion, flight, landing and recovery. We studied the landing phase during locomotion in Rana esculenta by measuring the ground reaction forces during propulsion and landing over a range of distances. Landing performance affects locomotor ability in jumping frogs. Landing and recovery together take up one third of the locomotor cycle. Peak landing forces are on average almost three times larger than propulsive forces. The forelimbs appear to be fully extended when they make contact with the substrate and absorb the first impact peak. The height of this peak varies depending on arm positioning and jumping distance. Since the stiffness of the arms stays constant over the full jumping range, it is possible that this is a limiting factor in the ability of the forelimbs to work as dampers. A spring-dashpot model is used to model the effect of arm angle at touch down. Damping during landing is performed by placing the forelimbs at an optimal angle to cancel frictional forces effectively.

Propulsive impulse as a covarying performance measure in the comparison of the kinematics of swimming and jumping in frogs

SUMMARY Animals have to modulate their locomotor behavior according to changes in external circumstances. The locomotor requirements are expected to be most extreme for species that move through different physical environments, such as water versus land. In this study, we examine the use of the propulsive impulse as a covariate in the comparison of the kinematics of locomotion of a semi-aquatic frog Rana esculenta, across land and through water. We focused on the propulsive phase because it is functionally the most significant phase of the locomotor cycle in both jumping and swimming, and it is also the most comparable. The frog alters the joint angles of its legs in order to adjust its performance (i.e. impulse) within both locomotor modes. The kinematics and this modulation of the propulsive phase differ between the two modes; however, we found that the impulse ranges of swimming and jumping do not fully overlap. Possible explanations for this include larger lateral forces during swimming, a reduced force transmission due to a lower external load during swimming and reduced muscle recruitment due to differences in coordination patterns.

Morphological correlates of aquatic and terrestrial locomotion in a semi-aquatic frog, Rana esculenta : no evidence for a design conflict

Semi‐aquatic frogs are faced with an unusual locomotory challenge. They have to swim and jump using the same apparatus, i.e. the hind limbs. Optimization of two tasks that require mutually incompatible morphologies or physiologies cannot occur simultaneously. In such cases, natural selection will result in some compromise, i.e. an intermediate phenotype that can perform both tasks reasonably well, but its performance will never match that of a specialized phenotype. We found no direct evidence for a trade‐off between jumping and swimming performance nor for a coupled optimization. This could be due to the importance of overall quality, as suggested by the fact that some frogs possess greater overall muscularity than others, irrespective of their body size. Another explanation could be that some morphological characteristics have a positive effect on both locomotor modes and others show a trade‐off effect. The net effect of these characteristics could result in an overall absence of correlation between the two locomotor performances. Size has a great influence on the morphological data and on jumping performance, but not if performance is expressed as velocity. The body shape of an anuran is conservative and scales mostly isometrically.

Propulsive force calculations in swimming frogs I. A momentum-impulse approach

SUMMARY Frogs are animals that are capable of locomotion in two physically different media, aquatic and terrestrial. A comparison of the kinematics of swimming frogs in a previous study revealed a difference in propulsive impulse between jumping and swimming. To explore this difference further, we determined the instantaneous forces during propulsion in swimming using an impulse–momentum approach based on DPIV flow data. The force profile obtained was compared with force profiles obtained from drag–thrust equilibrium of the centre of mass and with the force profiles generated during jumping. The new approach to quantifying the instantaneous forces during swimming was tested and proved to be a valid method for determining the external forces on the feet of swimming frogs. On the kinematic profiles of swimming, leg extension precedes propulsion. This means that it is not only the acceleration of water backwards that provides thrust, but also that the deceleration of water flowing towards the frog as a result of recovery accelerates the centre of mass prior to leg extension. The force profile obtained from the impulse–momentum approach exposed an overestimation of drag by 30% in the drag–thrust calculations. This means that the difference in impulse between jumping and swimming in frogs is even larger than previously stated. The difference between the force profiles, apart from a slightly higher peak force during jumping, lies mainly in a difference in shape. During swimming, maximal force is reached early in the extension phase, 20% into it, while during jumping, peak force is attained at 80% of the extension phase. This difference is caused by a difference in inter-limb coordination.

Fluoroscopic study of oral behaviours in response to the presence of a bit and the effects of rein tension

This study investigated intra-oral behaviours in horses wearing different bits with and without rein tension. Six riding horses wore a bridle and three bits: jointed snaffle, KK Ultra and Myler comfort snaffle. Lateral fluoroscopic images (30 Hz) were recorded for 20 s for each bit with loose reins and with 25 ^ 5 N bilateral rein tension. The videos were analysed to determine time spent in the following behaviours: mouth quiet, gently mouthing the bit, retracting the tongue, bulging the dorsum of the tongue over the bit, lifting the bit and other behaviours that were performed infrequently. Repeated-measures ANOVA indicated that behaviours did not differ between bits, so bit type was not predictive of behaviour, but there were significant effects of horse and rein tension. Horses spent less time quiet and more time mouthing the bit, retracting the tongue and bulging the tongue over the bit when tension was applied.

Two distinct gait types in swimming frogs

During terrestrial locomotion, frogs use two distinct gaits: out-of-phase leg movements associated with slow crawling behaviour and in-phase leg movements during fast jumps. In Rana esculenta, crawling occurs during feeding, while jumping is used as an escape strategy. We examined whether a similar velocitydependent gait shift appears in swimming R. esculenta. Typically, swimming frogs propel themselves by kicking both hind limbs simultaneously. Observations of out-of-phase leg movements in swimming frogs have been reported, but were usually assumed to be associated only with directional changes. We demonstrate that alternating-leg swimming is used quite frequently and that it results in a significantly lower velocity to the one obtained by using in-phase leg movements. This difference is likely to be associated with energetic costs. Mathematical estimates of positive mechanical work required to move the centre of mass revealed that out-of-phase swimming is energetically less expensive than the in-phase gait at a comparable speed, but may not be efficient at high speeds. Possible explanations for this phenomenon are higher inertial energy losses for in-phase swimming, but at high speeds jet propulsion and an interaction effect may gain importance.

Prey handling using whole-body fluid dynamics in batoids.

Fluid flow generated by body movements is a foraging tactic that has been exploited by many benthic species. In this study, the kinematics and hydrodynamics of prey handling behavior in little skates, Leucoraja erinacea, and round stingrays, Urobatis halleri, are compared using kinematics and particle image velocimetry. Both species use the body to form a tent to constrain the prey with the pectoral fin edges pressed against the substrate. Stingrays then elevate the head, which increases the volume between the body and the substrate to generate suction, while maintaining pectoral fin contact with the substrate. Meanwhile, the tip of the rostrum is curled upwards to create an opening where fluid is drawn under the body, functionally analogous to suction-feeding fishes. Skates also rotate the rostrum upwards although with the open rostral sides and the smaller fin area weaker fluid flow is generated. However, skates also use a rostral strike behavior in which the rostrum is rapidly rotated downwards pushing fluid towards the substrate to potentially stun or uncover prey. Thus, both species use the anterior portion of the body to direct fluid flow to handle prey albeit in different ways, which may be explained by differences in morphology. Rostral stiffness and pectoral fin insertion onto the rostrum differ between skates and rays and this corresponds to behavioral differences in prey handling resulting in distinct fluid flow patterns. The flexible muscular rostrum and greater fin area of stingrays allow more extensive use of suction to handle prey while the stiff cartilaginous rostrum of skates lacking extensive fin insertion is used as a paddle to strike prey as well as to clear away sand cover.

Speed Modulation in Swimming Frogs

Abstract Swimming movements of 7 European green frogs (Rana esculenta) were studied, starting from the detailed analysis of the speed and timing of the propulsive, glide, and recovery phases of their intermittent swimming behavior. First, the authors identified the spatiotemporal factors used by the frogs to modulate their swimming behavior. None of the gait variables correlated strongly with average swimming speed, and no significant correlations were found between variables belonging to different phases. There did not seem to be an obvious control strategy. Instantaneous speeds at the transition of the different phases all increased significantly with average speed, however. The strong correlation between maximal speed at the end of propulsion and the speed averaged over a cycle might reflect the dominance of the propulsive phase in the determination of the overall swimming speed. The modulation of swimming speed thus seemed largely comparable with the regulation of jumping distance. That finding was confirmed in a mathematical model, in which the positive correlations between both glide and recovery speeds, on the one hand, and average speed, on the other, were shown to be only mathematical consequences of the strong impact of the propulsive phase on overall swimming performance. That finding suggests that the correlations did not result from an active control strategy.

Inertial properties of equine limb segments

Quantifying the dynamics of limb movements requires knowledge of the mass distribution between and within limb segments. We measured segment masses, positions of segmental center of mass and moments of inertia of the fore and hind limb segments for 38 horses of different breeds and sizes. After disarticulation by dissections, segments were weighed and the position of the center of mass was determined by suspension. Moment of inertia was measured using a trifilar pendulum. We found that mass distribution does not change with size for animals under 600 kg and report ratios of segmental masses to total body mass. For all segments, the scaling relationship between segmental mass and moment of inertia was predicted equally well or better by a 5/3 power fit than by the more classic mass multiplied by segmental length squared fit. Average values taken from previous studies generally confirmed our data but scaling relationships often needed to be revised. We did not detect an effect of morphotype on segment inertial properties. Differences in segmental inertial properties between published studies may depend more on segmental segmentation techniques than on size or body type of the horse.

Comparison of the head and neck position of elite dressage horses during top-level competitions in 1992 versus 2008.

Among veterinary surgeons, interest has recently increased in the role of the horses neck as a causative factor in complex locomotor disturbances. Specifically, controversy surrounds the trend for the head to be carried behind the vertical (BHV) in contravention of Fédération Equestre Internationale (FEI) rules. The aim of this study was to determine whether the head angulation of elite dressage horses has changed over the last 25 years, and whether head angulation correlates with the competition score awarded. Head angle was measured from videos recorded during the Grand Prix test at the 1992 Olympic Games and the 2008 World Cup Final, during collected canter (CC), collected trot (CT), passage (Pa), and piaffe (Pi). Head angulations were BHV in CC and CT in both 1992 and 2008. The likelihood of being BHV during Pa or Pi was significantly greater in 2008 than in 1992 (P <0.05). Higher scores correlated significantly with head positions that were further BHV during Pi in 2008 (P <0.05). Head angulations were orientated BHV in all paces in 2008, whereas in 1992 this was only the case for CT and CC. These findings support the hypothesis that, in recent years, FEI dressage judges have not penalised horses for a head position BHV. The findings also support the need for further studies of the effects of head and neck position on the health of horses.