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Arboreal ants must navigate variably sized and inclined linear structures across a range of substrate roughness when foraging tens of meters above the ground. To achieve this, arboreal ants use specialized adhesive pads and claws to maintain effective attachment to canopy substrates. Here, we explored the effect of substrate structure, including small and large-scale substrate roughness, substrate diameter, and substrate orientation (inclination), on adhesion and running speed of workers of one common, intermediately-sized, arboreal ant species. Normal (orthogonal) and shear (parallel) adhesive performance varied on sandpaper and natural leaf substrates, particularly at small size scales, but running speed on these substrates remained relatively constant. Running speed also varied minimally when running up and down inclined substrates, except when the substrate was positioned completely vertical. On vertical surfaces, ants ran significantly faster down than up. Ant running speed was slower on relatively narrow substrates. The results of this study show that variation in the physical properties of tree surfaces differentially affects arboreal ant adhesive and locomotor performance. Specifically, locomotor performance was much more robust to surface roughness than was adhesive performance. The results provide a basis for understanding how performance correlates of functional morphology contribute to determining local ant distributions and foraging decisions in the tropical rainforest canopy., (© The Author(s) 2020. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: journals.permissions@oup.com.)

The surface temperature of tree branches in the tropical rainforest canopy can reach up to 55°C. Ants and other small cursorial organisms must maintain adequate attachment in this extreme microenvironment to forage effectively and avoid falling. Ant adhesion depends on liquid secretions that should become less viscous at high temperatures, causing ants to slip. However, tropical arboreal ants have high thermal tolerance and actively forage on hot canopy surfaces, suggesting that these ants can maintain adhesion on hot substrates. We measured tarsal pad shear adhesion of 580 workers (representing 11 species and four subfamilies) of tropical arboreal ants at temperatures spanning the range observed in the field (23-55°C). Adhesive performance among species showed three general trends: (1) a linear decrease with increasing temperature, (2) a non-linear relationship with peak adhesive performance at ca. 30-40°C, and (3) no relationship with temperature. The mechanism responsible for these large interspecific differences remains to be determined, but likely reflects variation in the composition of the secreted adhesive fluid. Understanding such differences will reveal the diverse ways that ants cope with highly variable, and often unpredictable, thermal conditions in the forest canopy., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2018. Published by The Company of Biologists Ltd.)

Small, cursorial ectotherms like ants often are immersed in the superheated air layers that develop millimeters above exposed, insolated surfaces (i.e., the thermal boundary layer). We quantified the thermal microenvironments around tree branches in the tropical rainforest canopy, and explored the effects of substrate color on the internal body temperature and species composition of arboreal ants. Branch temperatures during the day (09:00-16:00) were hottest (often > 50°C) and most variable on the upper surface, while the lowest and least variable temperatures occurred on the underside. Temperatures on black substrates declined with increasing distance above the surface in both the field and the laboratory. By contrast, a micro-scale temperature inversion occurred above white substrates. Wind events (ca. 2ms -1 ) eliminated these patterns. Internal temperatures of bodies of Cephalotes atratus workers experimentally heated in the laboratory were 6°C warmer on white vs. black substrates, and 6°C cooler than ambient in windy conditions. The composition of ant species foraging at baits differed between black-painted and unpainted tree branches, with a tendency for smaller ants to avoid the significantly hotter black surfaces. Collectively, these outcomes show that ants traversing canopy branches experience very heterogeneous thermal microenvironments that are partly influenced in predictable ways by branch surface coloration and breezy conditions., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Small cursorial ectotherms risk overheating when foraging in the tropical forest canopy, where the surfaces of unshaded tree branches commonly exceed 50 °C. We quantified the heating and subsequent cooling rates of 11 common canopy ant species from Panama and tested the hypothesis that ant workers stop foraging at temperatures consistent with the prevention of overheating. We created hot experimental "sunflecks" on existing foraging trails of four ant species from different clades and spanning a broad range of body size, heating rate, and critical thermal maxima (CT max ). Different ant species exhibited very different heating rates in the lab, and these differences did not follow trends predicted by body size alone. Experiments with ant models showed that heating rates are strongly affected by color in addition to body size. Foraging workers of all species showed strong responses to heating and consistently abandoned focal sites between 36 and 44 °C. Atta colombica and Azteca trigona workers resumed foraging shortly after heat was removed, but Cephalotes atratus and Dolichoderus bispinosus workers continued to avoid the heated patch even after >5 min of cooling. Large foraging ants (C. atratus) responded slowly to developing thermal extremes, whereas small ants (A. trigona) evacuated sunflecks relatively quickly, and at lower estimated body temperatures than when revisiting previously heated patches. The results of this study provide the first field-based insight into how foraging ants respond behaviorally to the heterogeneous thermal landscape of the tropical forest canopy.

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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. [ABSTRACT FROM AUTHOR]

In our previous article, Shear adhesive performance of leaf‐cutting ant workers (Atta cephalotes), a technical error occurred when calculating shear adhesive performance divided by tarsal pad area of worker ants carrying leaves (Stark, A. Y., Davis, H. R., & Harrison, W. K. (2019). Biotropica, 51(4), 572–580). Shear adhesive performance of worker ants carrying leaves was mistakenly divided by the tarsal pad area of workers riding on leaves. This has slightly changed our results. After correcting for this error, there is now no statistically significant difference in shear adhesive performance divided by tarsal pad area of worker ants riding on leaves and those carrying leaves. Shear adhesive performance of these groups remains statistically different than soldiers, and our original results (shear adhesive performance of all three groups differ from one another) are consistent with the reported trend in the original article using true pad area for each individual (rather than estimated). We list below the pages and figures in the original article that this correction applies to. All authors agree with this change, apologize for their error, and appreciate the opportunity granted by the editors and publisher to make this correction. Our original conclusion, that there is variation in shear adhesive performance per unit area among worker roles of Atta cephalotes, is still supported by these corrected results. [ABSTRACT FROM AUTHOR]