Traffic restrictions for heavy vehicles: Leaf-cutting ants avoid extra-large loads when the foraging flow is high

A better knowledge of the behaviors that reduce traffic congestions is essential to understand the success of the trail system despite of costs. Leaf-cutting ants use a trunk-trail system to transport leaf fragments into their nests. Some ants carry extra-large leaf fragments and walk slower than the rest of laden workers, thus slowing the ant column behind them. Here we experimentally address whether fragment size selection by leaf-cutting ants depends on the foraging ant flow. If ant behavior aims at minimizing delays associated with carrying extra-large loads, we expect that extra-large loads will be selected mostly under low ant flow conditions. In 38 foraging trails from 18 nests of Acromyrmex crassipinus located in Chaco Serrano woodland, Argentina, we recorded the removal of medium and extra-large baits under variable ant flow conditions. Ants selected extra-large loads mainly under low flow conditions; the increment of ant flow caused an exponential decrease in the proportion and in the preference to carry extra-large fragments. Restriction of heavy vehicles during peak hours is a common traffic rule that prevents traffic jams in transport networks. Our results suggest that this rule may also apply in ant societies that use foraging trails. Avoiding delays generated by carrying large loads appear to be another reason to transport leaf fragments below the individual load capacity, which might help to better understand the high variation in load sizes carried by leaf-cutting ants. This work might help to explain how by following simple traffic rules the trail system can be successful despite its costs, and also illustrate how individual ant behavior can be influenced by nestmates, thereby improving resource harvest in the colony as a whole.