Conclusions

The mutualism between Macaranga trees and their specialized Crematogaster (Decacrema) ant partners illustrates how strongly insect-plant interactions can be determined and shaped by biomechanical factors. The slippery wax barriers on the stems of Macaranga myrmecophytes not only keep away generalist ants and thus protect the resident wax running ant partners against predation and competition, but they also act as an ecological isolation mechanism between different Crematogaster (Decacrema) ant associates. Numerous differences between associations of Crematogaster (Decacrema) wax runners and non-wax runners demonstrate that this mechanical factor has important ecological implications for Macaranga-ant mutualisms.

Our preliminary study into the biomechanics of wax running behavior surprisingly suggests that the difference in wax running capacity between Crematogaster (Decacrema) species is not caused by superior adhesion but by mechanical and locomotory adaptations. We found that Crematogaster (Decacrema) wax runners not only have longer legs but also spread them out more during climbing. The combination of long legs and a more sprawled posture is mechanically advantageous for climbing ants and may partly explain the exceptional wax running capacity of Macaranga ant partners.

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