Simple rules result in the adaptive turning of food items to reduce drag during cooperative food transport in the ant <Emphasis Type="Italic">Pheidole oxyops</Emphasis> |
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Authors: | T J Czaczkes F L W Ratnieks |
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Institution: | (1) Laboratory of Apiculture and Social Insects, School of Life Sciences, University of Sussex, Falmer, Brighton, Sussex, BN19QG, UK |
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Abstract: | Insect workers cooperate to carry out a variety of tasks. One example is cooperative transport of food items by two or more
ant workers, which is important in foraging in many species. We predicted that natural selection would result in strategies
that improve the performance of this task and tested this in Pheidole oxyops, a Neotropical ant in which ca. 70% of the biomass of dead insects brought back to the nest is transported cooperatively.
We specifically tested the hypothesis that groups would re-orient food items to reduce drag, given that legs, wings, and other
projections should affect the ease of dragging prey in different orientations. By presenting ants with artificial food items
and dead cockroaches, both of which required approximately twice as much force to drag backwards as forwards, and a control
which was equally easy to move in both orientations, we showed that natural groups of 3–20 food-transporting ants usually
turned items that were facing backwards (72 and 83% of trials for artificial food items or cockroaches, respectively), the
orientation requiring greater force, but not items facing forwards (10 and 12% of trials, respectively). Turning usually involved
a single ‘steering’ ant. The key role of the ‘steering’ ant was shown by removing either the current steering ant or a randomly
chosen ‘non-steering’ ant during turning. In 100% of the trials in which the steering ant was removed, turning stopped until
another ant took its place. Conversely, turning stopped in only 17% of trials in which a ‘non-steering’ ant was removed. Turning
is an emergent property of the system and may not have been directly selected for. Rather, turning seems to occur through
a combination of pre-existing retrieval behaviour and the underlying physics of large loads. Points where the food item catches
the ground can act as a fulcrum or pivot around which the item can rotate. Ants furthest from the fulcrum have more leverage
and so are more likely to play a key role in turning. A simple rule relevant to individual transport of food items such as
“grasp the food item and move towards the nest”, when used in the context of cooperative transport, has allowed the ants to
solve a seemingly complicated problem requiring coordination. |
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