Attachment Force vs Climbing Performance Is Wax Running Capacity Based on Greater Attachment or Superior Locomotion

To investigate whether the wax running capacity of Crematogaster (Decacrema) ants is based on special adaptations of the attachment system to waxy substrates, we compared the ants' climbing performance with actual attachment forces generated on waxy Macaranga stems. Climbing performance was quantified by placing workers of Crematogaster (Decacrema) msp.2 (wax runners) and Crematogaster (Decacrema) msp.4 (non-wax runners) on vertical waxy M. hypoleuca stems (n = 34 workers from 2 colonies of each morphospecies; 2 stems of 11 mm diameter) and determining the proportion of ants capable of walking to one of two finishing lines 5 cm above and below the release point within 10 min [17]. Attachment was measured in the same ants as the centrifugal shear force required to detach them from waxy M. hypoleuca stems (13 stems with diameters between 8 and 13 mm). Surprisingly, we found that attachment forces per body weight were not significantly different between species, with a trend toward greater forces in the non-wax runners (two-tailed U-test, U = 483.5, P = 0.092) (Figure 8.5B). This is in striking contrast to the clearly superior climbing performance of wax runners (Fisher's exact test, P < 0.001) (Figure 8.5A). To test whether this unexpected result was caused by artifacts due to ill-defined surfaces of Macaranga trees kept in the greenhouse, we repeated the same set of experiments using a standardized, artificial substrate and found exactly the same effect (unpublished results). Our finding thus indicates that the difference in wax running capacity between Crematogaster (Decacrema) mor-phospecies is not the result of superior adhesion but rather due to behavioral and/or locomotory adaptations. These locomotory adaptations could be based both on

140 120 100

40 20 0

msp.2 msp.4

Crematogaster (Decacrema)

msp.2 msp.4

Crematogaster (Decacrema)

msp.2 msp.4

Crematogaster (Decacrema)

FIGURE 8.5 Climbing performance vs. attachment force of Crematogaster (Decacrema) wax runners (msp.2) and non-wax runners (msp.4) on waxy stems of M. hypoleuca. (A) Climbing capacity; ants that walked a stretch of 5 cm upward or downward along the stem were classified as "successful." (B) Maximum in-plane attachment force measured on the same substrate, using the centrifuge method (Figure 8.3A).

morphological characters and on kinematic variables, which will be considered in the following sections. It should be noted that our result does not indicate that adhesion is irrelevant for wax running. The much poorer wax running performance of many generalist ants [17] may also be caused by weaker attachment forces to the wax crystal substrates.

0 0

Post a comment