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Gone with the wind: short and long-term responses of leaf-cutting ants to the negative effects of wind on their foraging activity
Simposio; XXII Simpósio de Mirmecologia; 2015
The reproduction and survival of organisms ultimately depend on their feeding. Therefore, foraging behaviors should be selected to maximize benefits and reduce costs. Wind may negatively affect leaf-cutting ant foraging since it can reduce ants? speed or blow them off the trail by impacting on their loads. We quantified the effect of wind on ant foraging activity and evaluate whether this effect varies with (1) the arrangement of the foraging trail system, and (2) leaf characteristics such as area, mass and shape. Then, we tested whether ants respond to the negative effects of wind (3) at short-term, by selecting loads with characteristics that reduce wind interception; and (4) at long-term, by arranging the spatial design of the trail system in a way that reduces that effect. To evaluate this, we worked with Acromyrmex lobicornis, a leaf-cutting ant that inhabits regions that vary in wind conditions. We filmed trails parallel (low wind interception) and perpendicular (high wind interception) to the wind direction, and surrounded with vegetation (which stops the wind) or not so, from 19 nests in natural windy and windless conditions, and measured ants? speed and number of times that ants were blown off the trail. Experimentally, we generated wind in the trails of 13 nests, captured ants of similar size and their loads, and measured the variables mentioned above as well as the area, mass and shape of the loads. Finally, to evaluate the short and long-term responses of ants, we offered paper fragments with different size, mass and shape in windy and windless conditions, and measured the cardinal orientation of trails in windy and windless regions. We found that in windy conditions, loaded ants reduced their speed by 78% and were blown off the trail 39 times more than in windless conditions. The wind only affected loaded ants that were walking in trails perpendicular to the wind direction or upwind trails, both without vegetation. We found that wind effect increased with area, mass and shape of loads. At the short-term, the ants reduced the effect of wind by selecting smaller, lighter or more elongated loads in wind condition. However, we did not find a long-term response as the trails showed no particular spatial distribution in relation to wind direction. This is the first study that quantifies the negative effect of wind on the leaf-cutting ants? foraging, and that reports that workers can reduce this effect by selecting loads that intercept less the wind. Trail orientation seems to be modulated by other factors instead of responding to wind effect. These results show the negative effect on foraging of an ignored environmental factor, and also illustrate how short-term behavioral responses can mitigate these effects.