Shear adhesive performance of leaf‐cutting ant workers (Atta cephalotes)

Abstract

Wingless arboreal ants must resist the force of gravity while traversing substrates in their environment. For leaf‐cutting ants like Atta cephalotes, foraging may also include a ca. 30 m vertical descent while carrying a load 1–6 times their body mass. We hypothesized that heavier and larger ants would carry heavier and larger loads and that adhesive performance would positively correlate with load mass. We found no relationship between ant mass, body length, head width, or adhesive performance, and the load size an ant carried. In addition to workers carrying vegetative loads (most often leaves), workers in an active foraging trail also include smaller workers riding on the leaves carried by larger workers, and large major workers, providing protection from aerial and ground attacks (Soldiers), respectively. Despite varying functional roles, all foraging ants require secure attachment to the substrate. We measured shear adhesive performance of each foraging role and found that Soldiers produced the highest shear adhesive forces. However, when controlling for tarsal pad area, we found that ants carrying loads have higher shear adhesive performance per unit area than those riding on leaves, and that Soldiers have the lowest shear adhesive performance per unit area. This suggests that while leaf choice does not appear to be dictated by size, mass, or shear adhesive performance of individual ants, overall, ants who carry leaves adhere more strongly given their pad size than those who do not. Abstract in Spanish is available with online material.

How Gradient Affects the Foraging Decisions of Leaf Cutting Ants at the Food Source

“…majors, leaf carrying workers and "riders", who were smaller than leaf-carrying workers who climb leaves being carried by other workers to deter predation), not specifically on the size of individual workers. This resulted in proportionally less adhesion against sheering forces for the majors compared to regular workers and although riders had the greatest proportional tarsal pad size, carrying workers had the greatest adhesive strength, suggesting that other factors beyond tarsal pad size are at play when determining their grip strength, against shearing forces (Stark et al, 2019). However, in terms of raw adhesive strength, the majors produced the greatest adhesive strength.…”

“…Examinations of the tarsal pads found that the size of these tarsal pads did not scale proportionately with ant size (Stark et al, 2019). However, this study was focused on the roles that the ants played (i.e.…”

“…The tarsal pads are smooth, lubricated areas on the ant's feet that can be used to create an adhesive force through surface tension and fluid viscosity under perpendicular loadings and through viscosity and friction under parallel loadings (Stark et al, 2019). This means that while travelling on a flat surface (0° in this study), surface tension is a factor, but while travelling vertically up or down, friction replaces it.…”

“…Atta cephalotes has two main structures on their feet: The claws and the tarsal pads (Stark et al, 2019). The claws are curved and extend away from the tarsal pad located in the centre of each foot, giving each leg three points of contact with the ground when walking on a flat surface and hook into asperities on vertical surfaces (Pattrick et al, 2018).…”

“…Presumably, these two factors would rise and diminish in importance and impact across the intermediate angles. Non-arboreal ants either lack these pads or have much smaller pads than arboreal ants like Atta cephalotes so their potential importance in gripping onto surfaces with a rugosity typical of tree barks is hard to ignore (Stark et al, 2019).…”