Adhesion and running speed of a tropical arboreal ant (Cephalotes atratus) on wet substrates

Abstract

In the tropical forest canopy, wingless worker ants must cling to and run along diverse vegetative surfaces with little protection from sun, wind and rain. Ants rely in part on their tiny adhesive tarsal pads to maintain sufficient contact with substrates to prevent falls under these varied conditions. Here, we examined the effects of substrate wettability and surface water on the tarsal pad adhesive performance of a common tropical arboreal ant. Ant adhesion was consistently higher on an intermediately wetting substrate (static water contact angle ca 90°) when resisting both perpendicular (normal) force and parallel (shear) force. Normal adhesion was maintained on intermediately wetting and hydrophobic substrates following the addition of rain-mimicking water droplets, whereas shear adhesion declined on all substrate types tested after wetting. Ant running speed was slower on wet substrates. On wood substrates, normal and shear adhesion declined with increasing wetness from dry, to misted, to water-soaked. These differences probably contributed to lower ant running speed on wet wood. The results of this study provide the first quantitative assessment of tropical arboreal ant adhesive performance under substrate conditions that are commonly encountered in the rainforest canopy.

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

“…The maximum load resisted by the adhesive tarsal pads during this shear slide was recorded. Although ants primarily experience frictional forces while sliding, the fluid nature of the adhesive system also contains a normal (perpendicular) adhesive component during a “lubricated slide” (Stark & Yanoviak, ). To represent both normal adhesion and frictional forces ants experience when sliding, we define the resistance we measure in this experiment as “shear adhesion.” Shear adhesion of each ant was tested three times; however, only the maximum load resisted of the three trials was used for statistical analyses.…”

“…However, we did not find a difference in trial order and shear adhesive performance (ANOVA F 2,447 = 0.08, P = 0.9196 ) , suggesting if there were contributions from the pad, they were consistent across all three trials. We did not test normal adhesive performance (i.e., force required to pull an ant perpendicular to the surface) due to the expected low values (Federle et al., ; Stark & Yanoviak, ) and experimental limitations in the field. Given previous reports, we expect the same trend in normal and shear adhesive performance.…”

“…The glass surface was cleaned with ethanol, followed by water, and then dried between ants to remove any residue left behind on the surface. More details on ant adhesion tests in the field can be found elsewhere (Stark & Yanoviak, ; Stark et al., ).…”

“…This is particularly true in the tropics, where environmental hazards can be extreme. To circumvent these hazards, ants use behavioral (e.g., conspecific protection for foraging workers; Wilson, ), physiological (e.g., elevated thermal tolerance for foraging in sun flecks; Bujan, Yanoviak & Kaspari, ; Spicer et al., ; Stark, Adams, Fredley & Yanoviak, ), and physicochemical (e.g., secure attachment to hot, wet, and rough substrates; Federle, Baumgartner & Hölldobler, ; Scholz et al., ; Stark, Arstingstall & Yanoviak, ; Stark & Yanoviak, ) modifications to avoid abiotic and biotic hazards in their environment.…”

The effect of variable temperature, humidity, and substrate wettability on Gecko (Gekko gecko) locomotor performance and behavior

“…The ability of geckos to run equally fast on wet and dry substrates (hydrophilic and intermediately wetting) demonstrates that geckos can run in environmentally relevant conditions, like rain‐wetted surfaces, many of which they cannot statically cling to (Stark et al, 2012, 2013, 2015). Moreover, these results suggest that there is a complex and nonintuitive relationship between adhesive and locomotor performance in temporarily adhesive organisms, particularly in variable environmental conditions (e.g., surface water, substrate roughness; Federle & Endlein, 2004; Federle et al, 2000; Stark & Yanoviak, 2018, 2020; Stark et al, 2012, 2015).…”