Climbing, falling, and jamming during ant locomotion in confined environments

Abstract

Locomotion emerges from effective interactions of an individual with its environment. Principles of biological terrestrial locomotion have been discovered on unconfined vertical and horizontal substrates. However, a diversity of organisms construct, inhabit, and move within confined spaces. Such animals are faced with locomotor challenges including limited limb range of motion, crowding, and visual sensory deprivation. Little is known about how these organisms accomplish their locomotor tasks, and such environments challenge human-made devices. To gain insight into how animals move within confined spaces, we study the locomotion of the fire ant Solenopsis invicta, which constructs subterranean tunnel networks (nests). Laboratory experiments reveal that ants construct tunnels with diameter, D, comparable to body length, L = 3.5 ± 0.5 mm. Ants can move rapidly (> 9 bodylengths per s) within these environments; their tunnels allow for effective limb, body, and antennae interaction with walls, which facilitate rapid slip-recovery during ascending and descending climbs. To examine the limits of slip-recovery in artificial tunnels, we perform perturbations consisting of rapid downward accelerations of the tunnels, which induce falls. Below a critical tunnel diameter, D s = 1.31 ± 0.02 L, falls are always arrested through rapid interaction of appendages and antennae with tunnel walls to jam the falls. D s is comparable to the size of incipient nest tunnels (D = 1.06 ± 0.23 L), supporting our hypothesis that fire ants construct environments that simplify their control task when moving through the nest, likely without need for rapid nervous system intervention.animal locomotion | extended phenotype | locomotion control | social insect | stability T errestrial animals and increasingly robots must move in diverse and complex environments, including running across flat landscapes (1), swimming in sand (2), climbing rough or smooth vertical surfaces (3), and squirming through cracks (4). The bulk of discoveries of locomotor behaviors and control strategies have been made by challenging animals in the laboratory in simplified environments that are typically featureless, flat, and unconfined (5). Such simplifications have allowed discovery of general principles in locomotor modes of walking, running, and climbing (6-9). Recent studies have generated appreciation for the importance of mechanical interactions with the environment, and through biological experiment (10) and robot modeling (11,12) have demonstrated that stable and robust movement can emerge as a result of appropriately tuned dynamics of limb-ground interaction (13,14). For example, rapid perturbations to locomotion may be corrected by so-called "preflexes" (15) in which mechanical design of the limb and appropriate kinematics enable rapid recovery from perturbation (6,8,10). However, typical substrates that legged locomotors contend with differ in orientation, can deform in response to foot/body contact (1, 11), and are rough on multiple size scales (16, 17); litt...

Glass-like dynamics in confined and congested ant traffic

“…From measurements of the cross-sectional area of fire ant foraging tunnels 59 , we estimate the effective diameter of horizontal foraging tunnels in nature to be, D = 7.8 ± 1.9 mm (tunnels are elliptical in cross-section with eccentricity of 2, See 35 ). Vertical nest entrance tunnels in natural nests were reported in the range of 3-4 mm in diameter and a laboratory x-ray study found that vertical tunnels were D = 3.7 ± 0.8, See 60 . ∆τ * is a diverging function with decreasing diameter and predicts that bi-directional traffic should be impossible in tunnels smaller than D c = 1.47 mm.…”

Cockroaches traverse crevices, crawl rapidly in confined spaces, and inspire a soft, legged robot

“…The study of climbing has led to undiscovered templates (11) that define physical interactions through frictional van der Waals adhesion (12,13) and interlocking with claws (14) and spines (5). Burrowing (15,16), sand swimming (17), and locomotion in tunnels (18) have yielded new findings determining the interaction of bodies, appendages, and substrata.…”

“…Locomotion in confined environments offers several challenges for animals (18) that include limitations due to body shape changes (19,20), restricted limb mobility (21), increased body drag, and reduced thrust development (22). Examining the motion repertoire of soft-bodied animals (23), such as annelids (19), insect larvae (24), and molluscs (25), has offered insight into a range of strategies used to move in confined spaces.…”

Fire ants actively control spacing and orientation within self-assemblages

“…As we are unable to distinguish antennae from legs, and antennae do not contribute to the tensile integrity of the structure, they are excluded from our analysis (see Materials and methods, 'Calculation of measurements'). However, while antennae have no role in supporting tensile forces, they may be important in providing resistance to compressive stresses, as shown by the ability of ants to arrest falling in tunnels by extending their antennae (Gravish et al, 2013).…”