Light level impacts locomotor biomechanics in a secondarily diurnal gecko,Rhoptropus afer

Abstract

Locomotion through complex habitats relies on the continuous feedback from a number of sensory systems, including vision. Animals face a visual trade-off between acuity and light sensitivity that depends on light levels, which will dramatically impact the ability to process information and move quickly through a habitat, making ambient illumination an incredibly important ecological factor. Despite this, there is a paucity of data examining ambient light in the context of locomotor dynamics. There have been several independent transitions from the nocturnal ancestor to a diurnal activity pattern among geckos. We examined how ambient light level impacted the locomotor performance and high-speed three-dimensional kinematics of a secondarily diurnal, and cursorial, gecko (Rhoptropus afer) from Namibia. This species is active under foggy and sunny conditions, indicating that a range of ambient light conditions is experienced naturally. Locomotor speed was lowest in the 'no-light' condition compared with all other light intensities, occurring via a combination of shorter stride length and lower stride frequency. Additionally, the centre of mass was significantly lower, and the geckos were more sprawled, in the no-light condition relative to all of the higher light intensities. Locomotor behaviour is clearly sub-optimal under lower light conditions, suggesting that ecological conditions, such as very dense fog, might preclude the ability to run quickly during predator-prey interactions. The impact of ambient light on fitness should be explored further, especially in those groups that exhibit multiple transitions between diel activity patterns.

Non-uniform evolutionary response of gecko eye size to changes in diel activity patterns

“…Precise habitat clutter data for geckos are unavailable, hence a formal test of this hypothesis is still wanted, but it is possible that the ground-dwelling/climbing axis of eye diversity [ 6 ] is related to habitat clutter, as ground-dwelling specialists tend to be more common in open habitats [ 22 ]. We propose that habitat and DAP jointly drive visual range, locomotor speed [ 24 , 25 ], and, ultimately, eye size evolution, offering a new perspective on Leuckart's Law [ 26 ]. However, even with full understanding of the photic environment, eye size can follow different evolutionary trajectories, as size is not the only determinant of visual performance.…”