Foragers of leafcutter ant colonies respond to the speed and efficiency of other ants by varying leaf loads in size and weight.

Within ant colonies, each ant has a specific role. In the leaf-cutter species, foraging ants are tasked with collecting leaf fragments and bringing them back to the colony. One may think that a forager would collect the largest possible payload. However, high payloads are not shown to result in more efficient transport. Instead, foragers generally carry loads well below their maximum potential. Load size is influenced by two factors: a more manageable workload for processor ants, and the speed of other foragers. 
When foragers return to the colony, they pass their loads to the processor ants. Processors collect the material and distribute it among the colony. There are more foragers than processors. If every forager brought large of loads to the colony, the processors would be overwhelmed by the volume of leaves coming into the colony and fall behind. As a result, materials would not be distributed throughout the colony in a timely manner. 
Foragers also carry small loads in order to maintain a consistent speed in relation to other ants. Foragers travel to and from their colony in a single file line, also referred to as unilateral transport. This is because foraging ants travel by following the chemical scent of the forager directly in front of them. It’s as if ants travel down a one lane highway, where passing is illegal. Because smaller loads mean faster foraging and bigger loads mean slower foraging, if one ant chooses to take a larger than average load it will slow down the entire group. This can have a serious effect on the colony as a whole.

This summary was contributed by Allie Miller.


"When carrying resources from a collecting point to the nest, one would assume that animals would attempt to carry as much as possible to maximize their foraging efforts. However, among social insects that is not always the best strategy. Foragers carrying large loads might overwhelm the individuals processing the resources in the nest, causing a bottleneck. Additionally, a heavy load slows the carrier down. That may not be a significant cost for a solitary forager, but it may reduce the gains for a colony as a whole. Carrying loads well below maximum carrying performance actually reduces the burden on the resource processors and speeds up the forager, allowing for more foraging trips per unit time. But there is yet another factor whereby lighter loads can increase colony efficiency. Walking foragers, such as ants, often forage along well-defined trails and, depending on the number of foragers, this can potentially cause traffic problems. In that context, when one ant is slowed down by a heavy load it also slows down those ants following behind, regardless of load size. This phenomenon, called the 'truckdriver' effect – where a heavily laden truck slows down normally faster cars – describes another situation where carrying too much negatively impacts whole colony foraging efficiency." (Klok 2011:vi)

"At low traffic flow ants can cut and carry larger leaf fragments without the concern of slowing other ants, but when traffic flow increases they refrain from cutting larger fragments. It appears that individual ant foragers leaving the nest can estimate the outbound traffic flow and use this information to estimate the future flow of returning laden ants, thereby modulating the sizes of leaf cuttings made in order to avoid delays in overall colony foraging rates. This study shows remarkable flexibility in foraging behaviour and supports the idea that leaf-cutting ants make choices not only as individuals but also collectively." (Klok 2011:vi)

Journal article
HEAVY LOADS SLOW TRUCK DRIVERS AND ANTSJournal of Experimental BiologyAugust 6, 2011
C. J. Klok

Journal article
The ‘truck-driver’ effect in leaf-cutting ants: how individual load influences the walking speed of nest-matesPhysiological EntomologyOctober 25, 2016

Leaf-cutter AntsAtta cephalotesGenus