International Union for the Study of Social Insects (IUSSI2018), August 5-10, 2018 in Guarujá, Brazil.

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Does contact rate on foraging trails modulate recruitment in leaf-cutting ants?

Vincent Jean Louis Fourcassié, Vincent Fourcassié , Karla Vittori , Sofia Bouchebti
UMR CNRS5169, Research Center on Animal Cognition, Toulouse, France; UMR CNRS5169, Research Center on Animal Cognition, Toulouse, France ; Universidade Federal do ABC, Santo André, Brazil ; Department of Entomology, Hebrew University of Jerusalem, Israel
Contrary to what occurs in army ants, outbound and inbound traffic on leaf-cutting ant foraging trails are largely intermingled. As a consequence, laden workers returning to the nest experience a high rate of head-on collisions, which induces a delay in their progression and thus decreases the rate of food return to the nest. However, these collisions could also have a benefit for the colony if they promote information transfer between individuals. For ex ample, contact rate with inbound laden ants could be used by outbound ants as a cue to assess substrate availability at the end of the trail. A decrease in contact rate could thus downregulate recruitment by inducing outbound ants to make a U-turn or to move to another trail at a bifurcation while an increase could upregulate recruitment by enhancing their motivation to keep walking on the trail in order to collect and bring back leaf fragments to their nest. We explored the impact of head-on collisions on recruitment in the leaf-cutting ant Atta laevigata by artificially decreasing or increasing the rate of head-on collisions between individuals. Contrary to our prediction, we found that a decrease in the rate of head-on collisions did not influence the rate of recruitment, i.e. the number of ants exiting the nest per unit time, but induced a decrease in foraging efficiency, i.e. in the proportion of ants returning to the nest with a leaf fragment. Surprisingly, both unladen and laden ants returning to the nest contributed to the modulation of foraging efficiency, suggesting that outgoing ants are able to collect information from inbound ants, even when these latter do not carry a leaf fragment. On the other hand an increase in the rate of head-on collisions induced exactly the reverse phenomenon, i.e. a decrease in the rate of recruitment and no changes in foraging efficiency. Additional experiments allowed us to explore the mechanisms that could explain these results.