Surface Roughness Affects the Running Speed of Tropical Canopy Ants

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Cursorial central-place foragers like ants are expected to minimize travel costs by choosing the least resistive pathways to food resources. Tropical arboreal and semi-arboreal ants locomote over a variety of plant surfaces, and their choice of pathways is selective. We measured the roughness of tree trunk and liana stem surfaces using laser scanning technology, and explored its consequences for running speed in various ant taxa. The average amplitude of tree trunk surface roughness differed interspecifically, and ranged from 1.4–2.2 mm among three common tree species (Anacardium excelsum, Alseis blackiana, and Dipteryx panamensis). The roughness of liana stems also varied interspecifically (among Tontelea ovalifolia, Bauhinia sp. and Paullinia sp.) and was an order of magnitude lower than tree surface roughness (mean amplitude ranged 0.09–0.19 mm). Field observations of various ant species foraging on tree trunks and liana stems, and on dowels covered with sandpaper, showed that their running speed declined with increasing amplitude of roughness. The effect of roughness on running speed was strongest for mid-sized ants (Azteca trigona and Dolichoderus bispinosus). The accumulation rate of ants at baits did not vary with tree surface roughness, but was significantly lower on moss-covered versus moss-free bark. Collectively, these results indicate that the quality of plant substrates can influence the foraging patterns of arboreal ants, but likely is more important for resource discovery than for dominance on bare tree surfaces.