Sept. 10, 2008 -- They are capable of carrying up to seven times their body weight, but leaf cutter ants are slacking off for the greater good, according to new research.
In a paper published in today's Biology Letters, Martin Burd of Australia's Monash University details how a lower level of productivity by foraging leaf cutter ants improves productivity within the colony.
"What looks inefficient is actually efficiency," Burd, who is attached to the School of Biological Sciences, said.
Burd measured the work done by worker ants tasked with collecting and harvesting leaf fragments in colonies of Atta colombica. He measured the load the ants carried, the time it took to cut leaf fragments and the rate at which fragments were delivered to the colony.
Burd and co-author Jerome Howard at the Department of Biological Sciences at the University of New Orleans found the ants were carrying about half the maximum load they could manage.
Even when the faster delivery time due to the lighter load was taken into account, they were still 35 percent less productive than their optimal performance.
Burd said it was "pretty clear they were underperforming." He added that the inefficiency was explicable only if it improved transportation and processing of the leaf matter inside the nest.
Once the leaf fragment is delivered to the colony, its tissue is processed for use to cultivate fungal gardens that provide feed for the colony's larvae, Burd said.
It is then transported by workers to one of the colony's fungal gardens where it is cleaned and dissected into tiny particles that are then implanted in the gardens.
Burd found there is a strong size effect on the internal processing, which can take more than 60 minutes to complete. To show this the team supplied four laboratory colonies with known sizes of leaf fragments.
They then measured the time it took from the introduction of the leaf discs to the colony until the dissection of the last disc on any of the colony's gardens.
They found the optimal leaf fragment size for peak productivity in the laboratory colonies was about 96 millimeters square.This was similar to the size of leaf fragments harvested in the field by the ants.
"The inefficiency of the outside work contributed to the efficiency of the whole process," Burd said.
The research adds to the 30-year-old Spandrel debate in biology about how to analyze adaptations. These recent findings support the view of the late Stephen Gould and Richard Lewontin, who argued that individual adaptations may not be understood outside the context of the whole organism.
"But their insistence that suboptimality of component parts does not contribute to optimality of the whole seems incorrect," Burd said.
"We have shown that Atta colonies can operate at or near an ergonomic optimum ... that is not necessarily apparent when the component tasks are examined in isolation."
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