Quasi-static force requirements are not sufficient to explain arolium engagement in climbing Argentine ants

Argentine ants (Linepithema humile) utilize adhesive pads (arolia) to climb smooth surfaces. Previous research found that ants can adjust their individual arolium engagement according to their locomotion mode. However, it remains unclear how they distribute arolium engagement across multiple limbs to climb effectively, and how arolium engagement varies within a climbing step. As the arolium is a well-known adhesive organ, we hypothesized that engagement across different legs is distributed according to the normal forces required for balancing the body during climbing. To test this, we measured Argentine ants' arolium engagement on a vertical glass surface using a frustrated total internal reflection (FTIR) sensor and compared it with the required normal forces from a quasi-static model. Contrary to the required normal force, the measured arolium engagement was asymmetric between upward and downward climbing, and changed over time. Our results indicate that quasi-static force requirements are not sufficient to explain arolium engagement in climbing Argentine ants, and suggest that other factors, such as body dynamics, ant anatomy and behavioral preferences, should be included.