Directional differences in head stabilisation in Acanthodactylus pardalis lizards

Running inevitably causes the animal trunk to undulate. The consequential head rotations have to be stabilised in space for a steady gaze and an accurate sense of self-motion for balance. The ecology and anatomy of the species determine the necessity to stabilise the head in yaw, pitch, and roll direction. Terrestrial lizards, running with a sprawled body posture, are especially prone to undulations in the horizontal (yaw) plane. Measurements on an experimental oscillation platform show that Acanthodactylus pardalis lizards stabilise their head less in pitch direction (54% stabilisation) than in yaw and roll direction (66% and 64% stabilisation, respectively). Because we performed these experiments in darkness, the lizards based their head stabilisation on vestibular information. Hence, we hypothesised that their vestibular system is less sensitive in pitch direction than in yaw and roll direction. Yet, this was not confirmed by a detailed Fluid-Structure Interaction model of the membranous labyrinth, which showed that not pitch sensitivity (88% of yaw sensitivity), but roll sensitivity (73% of yaw sensitivity) is the lowest. So why is the head stabilisation in darkness almost as good in roll direction as in yaw direction? While this may be due to neurological nonlinearities, it seems worth noticing that the moment of inertia is lowest in roll direction due to the elongated head shape. Hence, less torque is needed to stabilise a head rotation in roll direction than in the other two directions. (c) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Running causes undulations in animal trunk and head rotations need to be stabilised for balance. Experimental measurements on Acanthodactylus pardalis lizards show that head stabilisation in pitch direction is lower compared to yaw and roll direction. Despite hypotheses about vestibular system sensitivity, a Fluid-Structure Interaction model suggests that roll sensitivity is lowest, possibly due to neurological nonlinearities. However, the elongated head shape reduces the moment of inertia in roll direction, making head stabilisation easier in this direction.