A leaf-cutter ant colony can only get so big, new research suggests.
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Atta colombica workers transporting leaves.
Credit: Wikimedia
LONDON: Leaf-cutting ants are less productive as their colonies expand, new research suggests, indicating that there is a limit to how big a colony can get.
A paper published in the Proceedings of the Royal Society B today has shown that the amount of food brought in by foraging worker ants doesn't increase at the same rate as the colony size, even if the colony has more workers to cover more ground. Researchers suggest that this finding could be used to improve and understand transport infrastructure in human cities.
"We measured the foraging rate by counting the number of leaf fragments brought into the nest by unit time and estimated the total number of ants working out on the trail system at any one time," said lead author Martin Burd, professor of evolutionary ecology from Monash University in Melbourne. "We found that productivity increased less than proportionately with the number of foraging workers as colonies get bigger."
Why the reduction in productivity?
Leaf-cutting ants are famous for their ability to chew and transport leaf fragments back to their nest where they are used to cultivate a fungus that can provide food for the entire colony. These ants have larger colonies than most social insects, housing up to 8 million workers whose main role involves forming trails and foraging for fresh plant material.
The research team observed the trails of 18 colonies of ant species Atta colombica and Atta cachalot as they left and re-entered their nest. They also measured the networks of trails formed as the ants foraged for food and counted the number of leaf fragments brought back to the nest. They found that foraging rates and productivity were affected by the overall size of a colony, and put it down to the longer trails that were being built.
"Foraging rate scales inversely with length of trail the ants have to travel," said Burd. "If they have to travel twice the distance, they travel for twice as long and therefore the delivery rate [is] cut in half. We saw this effect scaled up to whole colony, because more workers are sent out in response."
Reaching the limit
The researchers also found that as the colonies grow in size, worker ants are recruited to larger trail networks to exploit the larger distances. But the number of ants recruited didn't appear to meet the demand.
"Given that these colonies have millions of workers, at any one time a maximum of a few tens of thousands of workers [are] out foraging. So these colonies have excess reserves of foragers that they're not sending out," said Burd. "Therefore, colonies become less efficient at exploiting resources when trying to reach larger areas and we don't understand the regulating mechanism [behind this]. If they were recruiting more ants in proportion to the increased distances, they would suffer no drop in productivity. But they don't do that and as a result they do suffer a drop in productivity."
The findings suggest that colonies will eventually reach a limit where the returns, in the form of leaf fragments, simply aren't profitable to the colony as a whole. This sets an upper limit on the size a colony can reach and the size of its foraging territory.
