Uneven substrates constrain walking speed in ants through modulation of stride frequency more than stride length

Abstract

Natural terrain is rarely flat. Substrate irregularities challenge walking animals to maintain stability, yet we lack quantitative assessments of walking performance and limb kinematics on naturally uneven ground. We measured how continually uneven 3D-printed substrates influence walking performance of Argentine ants by measuring walking speeds of workers from laboratory colonies and by testing colony-wide substrate preference in field experiments. Tracking limb motion in over 8000 videos, we used statistical models that associate walking speed with limb kinematic parameters to compare movement over flat versus uneven ground of controlled dimensions. We found that uneven substrates reduced preferred and peak walking speeds by up to 42% and that ants actively avoided uneven terrain in the field. Observed speed reductions were modulated primarily by shifts in stride frequency instead of stride length (flat R 2 : 0.91 versus 0.50), a pattern consistent across flat and uneven substrates. Mixed effect modelling revealed that walking speeds on uneven substrates were accurately predicted based on flat walking data for over 89% of strides. Those strides that were not well modelled primarily involved limb perturbations, including missteps, active foot repositioning and slipping. Together these findings relate kinematic mechanisms underlying walking performance on uneven terrain to ecologically relevant measures under field conditions.

Tardigrades exhibit robust inter-limb coordination across walking speeds

“…Changes in limb kinematics and ICPs with substrate stiffness. Natural terrain is rarely homogeneous, and legged animals often need to cope with environmental inconsistencies such as changes in substrate roughness [5,34] or stiffness [21]. These irregularities often constrain organismal performance, and can result in constraints on walking speed as well as changes in limb kinematics.…”

Vision does not impact walking performance in Argentine ants

“…Argentine ants walk at 5-15 strides per second, corresponding to a minimum stride duration of 66 ms (Clifton et al, 2020). Therefore it is likely that tactile sensation from the antennae is sufficiently fast to enable decelerations within one stride of encountering an obstacle.…”

“…Ants identified in each frame were associated across frames using a Kalman filter generating "trackways" (Straw et al, 2011). For details see the supplementary materials in (Clifton et al, 2020).…”

“…Comparatively, neural conduction speeds are relatively fast at 0.5-3.7 m/s in cockroaches (Pearson et al, 1970). Ants walk at an average of 10-12 strides per second (Clifton et al, 2020;Reinhardt and Blickhan, 2014), limiting intra-stride response times to within 100 ms. Given the relatively slow transduction speeds of walking insect photoreceptors (Frolov et al, 2017), fast movements of the limbs during walking may preclude visual feedback (Full and Koditschek, 1999).…”

Side-Impact Collision: Mechanics of Obstacle Negotiation in Sidewinding Snakes

“…Additionally, the most common locomotor mode in snakes, lateral undulation, uses the same discrete environmental structures, which are obstacles to limbed animals, as push points for generating propulsive force . As a consequence, while obstacles typically slow down limbed animals (Clifton et al, 2020;, the presence of these structures allows snakes to switch locomotor modes from concertina (generally slow ) to lateral undulation (the fastest mode ) ). As such, snake speed during lateral undulation increases with increasing obstacle density (except at extreme densities in which snakes no longer have room to use lateral undulation effectively) .…”