Terrestrial locomotion of the Svalbard rock ptarmigan: comparing field and laboratory treadmill studies

Abstract

Research into the terrestrial locomotion of birds is often based upon laboratory treadmill experiments. However, it is unclear how transposable these results are for birds moving in the wild. Here, using video recordings, we compared the kinematics of locomotion (stride frequency, stride length, stance phase, swing phase, duty factor) and speed range of Svalbard rock ptarmigan ( Lagopus muta hyperborea ) under field and laboratory treadmill conditions. Our findings indicate that the kinematics of walking and aerial running are conserved when moving on the treadmill and in the field. Differences, however, were found when grounded running under the two conditions, linked to substrate. Substrate effects were confirmed by analysing trials only moving over very hard snow. In line with laboratory treadmill energetic predictions, wild ptarmigan have a preferred speed during walking and to a lesser extent when aerial running but not when moving with a grounded running gait. The birds were also capable of a higher top speed in the field than that observed during treadmill studies. Our findings demonstrate that laboratory treadmill research provides meaningful information relevant to wild birds while highlighting the importance of understanding the substrate the animals are moving over.

Examining the accuracy of trackways for predicting gait selection and speed of locomotion

“…All terrestrial locomotion involves interaction with a substrate. The slow walking speeds, however, are thought to negate the influence of substrate on locomotion kinematics as they provide greater resistance to external ground perturbations acting on the centre of mass [54,55] whilst fast running speeds where only possibly over hard substrates anyway [1]. The reason there is 7 much more variability in U during the grounded running gait which reduces the ability to make accurate predictions is that a grounded running gait is used as a mechanism to move faster while also maintaining stability, by cancelling the effects of uneven ground and a changing substrate through increased times of contact of the foot to the ground and a more compliant leg [11,45,56].…”

“…The reason there is 7 much more variability in U during the grounded running gait which reduces the ability to make accurate predictions is that a grounded running gait is used as a mechanism to move faster while also maintaining stability, by cancelling the effects of uneven ground and a changing substrate through increased times of contact of the foot to the ground and a more compliant leg [11,45,56]. For ptarmigan moving over snow, modifications in l stride during grounded running are required to prevent falls [1] since shorter steps improve locomotion over slippery surfaces by keeping the centre of mass above the supporting limbs [57,58].…”

“…Therefore, here, walking, grounded running and aerial running gaits were analysed separately. In a previous study [1] the relationship between l stride and U was determined for free ranging male Svalbard ptarmigan. Here the same data were used, but this time U became the dependent variable and l stride the independent variable to produce a gait specific predictive relationship (figure 1a).…”

“…Studies of terrestrial animal locomotion, however, are overwhelmingly conducted under laboratory conditions using treadmills [1]. Treadmill studies have facilitated great insight into the biomechanics of locomotion and using this approach the correlation between kinematic parameters like stride length (l stride ), stride frequency (f stride ) stance (t stance ) and swing (t swing ) time with speed (U) has been widely reported in the literature across a range of species.…”

“…Archipelago of Svalbard meaning they spend approximately half a year locomoting over snow and they are also one of the few species in which a comprehensive laboratory treadmill dataset exists which can be used for comparison. Recently one of the first comparisons of the kinematics of locomotion under field and laboratory treadmill conditions was undertaken in the Svalbard rock ptarmigan [1]. The kinematics of locomotion were conserved for ptarmigan moving in the field and during laboratory treadmills studies but only for walking and aerial running gaits.…”