UC San Diego

Ants are exceptionally strong for their size and can climb effortlessly on vertical smooth surfaces. Therefore, it is not a trivial question whether their climbing is notably influenced by gravity. We studied this question by comparing adhesive contact areas and kinematics of Argentine ant workers (L. humile) climbing in different directions on vertical surfaces. Both the real-time adhesive pad contact illuminated by a frustrated total internal reflection (FTIR) device, and the body and limb kinematics are simultaneously recorded by a high-speed camera. Comparing with ants walking on a horizontal surface, we first observed that the adhesive engagement primarily occurred during climbing bouts and was rarely present during walking on level ground. After analyzing over 600 videos, we found that speed differed between climbing upwards (v=22.40mm/s), and downward speed (v=18.96mm/s; p≪0.001, d=0.555) with decreased stride frequency and normalized stride length. Independent of climbing speed and climbing direction, the adhesive pad contact was primarily above the Center of Mass (CoM; 90%±10%). These findings indicate that gravity has a significant impact on ants climbing on smooth substrates.