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June 04, 2013

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Old leaf-cutter ants “retire” to more manageable jobs

Dec. 13, 2010
Courtesy of the University of Oregon
and World Science staff

When their ra­zor-sharp jaws wear out, leaf-cutter ants change jobs as their more ef­fi­cient sis­ters take over slic­ing duty, say re­search­ers from two Or­e­gon uni­vers­i­ties.

Their stu­dy—ap­pear­ing in the ad­vance on­line is­sue of the jour­nal Be­hav­ior­al Ecol­o­gy and So­cio­bi­ol­ogy—offers a glimpse of na­ture’s way of pro­vid­ing for its dis­placed work­ers.

A leaf-cutter ant from Pa­na­ma at work. (Credit: Cour­tesy of Rob­ert Scho­field)


“This study demon­strates an ad­van­tage of so­cial liv­ing that we are fa­mil­iar with—hu­mans that can no long­er do cer­tain tasks can still make very worth­while con­tri­bu­tions to so­ci­ety,” said the pa­per’s lead au­thor, Rob­ert Scho­field of the Uni­vers­ity of Or­e­gon. 

“While di­vi­sion of la­bor is well doc­u­mented in so­cial in­sects, this is the first sug­ges­tion that some so­cial in­sects stop per­form­ing cer­tain tasks be­cause they are no long­er as good at them as they used to be. As so­cial or­gan­isms, these ants have the lux­u­ry of be­ing able to leave the cut­ting task to their more ef­fi­cient sis­ters.”

Leaf-cutter ants slice leaves, car­ry pieces back to the un­der­ground nest and, like ti­ny mush­room farm­ers, grow an ed­i­ble fun­gus on the plant bits. The ants do­ing the cut­ting are usu­ally mem­bers of the gen­er­al­ized for­ag­er caste, one of four size-based be­hav­ior­al castes of work­ers. The for­ag­ers are sec­ond in size to the ma­jors, the large work­ers that pro­tect the col­o­ny and do heavy clear­ing work on the trails con­structed to con­nect the nest to the leaf sources. The for­ag­ers al­so car­ry the cut­tings, scout for new re­sources and al­so help pro­tect the col­o­ny.

“Cut­ting leaves is hard work. Much of the cut­ting is done with a V-shaped blade be­tween teeth on their mandibles that they use like a tai­lor who holds a pair of scis­sors in a fixed V shape to slice through cloth,” Scho­field said. “This blade starts out as sharp as the sharpest ra­zor blade that hu­mans have de­vel­ope­d.”

Over time, though, their mandibles, or jaws, slowly dull. It takes long­er and re­quires more en­er­gy to get the job done. When it takes an ant about three times as much time and en­er­gy to cut out a leaf disc than it would have tak­en when her blades were sharp, be­hav­ior changes, the re­search­ers re­ported. The cut­ting ants rest their blades and join the delivery staff, car­rying the discs cut from the leaves in­to their nest.

“Imag­ine hav­ing only two ti­ny knives to use for your en­tire life, with no sharp­en­ing al­lowed,” Scho­field said. “You would want them to be made of the best ma­te­ri­al pos­si­ble. You would use them very care­ful­ly, but cut­ting would still get harder and harder as they dulled un­til you had to rely on oth­ers to cut for you. That’s what it is like to be a leaf-cutter an­t.”

The com­po­si­tion of the cut­ting blades is of par­tic­u­lar in­ter­est to re­search­ers. The find­ings sup­port the idea that wear and frac­ture are big prob­lems for smaller an­i­mals, Scho­field said. The re­search­ers es­ti­mate that, be­cause of we­ar, the col­o­ny spends twice as much en­er­gy cut­ting leaves as it would if all ants had sharp mandibles. This cost should have re­sulted in an ev­o­lu­tion­ary pres­sure to de­vel­op ma­te­ri­als that re­sist dulling, the re­search team not­ed. The cut­ting blades are in­deed made of a zinc-rich bioma­te­ri­al that the re­search­ers sus­pect is wear re­sistant.

Scho­field was the lead au­thor of a study pub­lished in 2001 that iden­ti­fied a family of bioma­te­ri­als in man­dib­u­lar teeth, tar­sal claws, stings and oth­er such tools of small or­gan­isms. In 2009, a team led by Scho­field re­ported that a si­m­i­lar type of sub­stance em­pow­ers the claw tips of striped shore crab and is pre­s­ent on the walk­ing legs of Dun­ge­ness crabs.

“Hu­mans are just start­ing to try to en­gi­neer ti­ny machines and tools, and we have a lot still to learn from or­gan­isms that have coped with be­ing small for mil­lions of years,” Scho­field said. “In ad­di­tion, it’s good to know how im­por­tant wear is to these ants, be­cause they are ag­ri­cul­tur­ pests, and this re­search hints that crops that pro­duce high lev­els of wear might dis­cour­age them.”


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When their razor-sharp jaws wear out, leaf-cutter ants change jobs, remaining productive while letting their more efficient sisters take over cutting, say researchers from two Oregon universities. Their study—appearing in the advance online issue of the journal Behavioral Ecology and Sociobiology—provides a glimpse of nature’s way of providing for its displaced workers. “This study demonstrates an advantage of social living that we are familiar with—humans that can no longer do certain tasks can still make very worthwhile contributions to society,” said the paper’s lead author Robert Schofield of the University of Oregon. “While division of labor is well documented in social insects, this is the first suggestion that some social insects stop performing certain tasks because they are no longer as good at them as they used to be. As social organisms, these ants have the luxury of being able to leave the cutting task to their more efficient sisters.” Leaf-cutter ants slice leaves, carry pieces back to the underground nest and, like tiny mushroom farmers, grow an edible fungus on the plant bits. The ants doing the cutting are usually members of the generalized forager caste, one of four size-based behavioral castes of workers. The foragers are second in size to the majors, the large workers that protect the colony and do heavy clearing work on the trails constructed to connect the nest to the leaf sources. The foragers also carry the cuttings, scout for new resources and also help protect the colony. “Cutting leaves is hard work. Much of the cutting is done with a V-shaped blade between teeth on their mandibles that they use like a tailor who holds a pair of scissors in a fixed V shape to slice through cloth,” Schofield said. “This blade starts out as sharp as the sharpest razor blade that humans have developed.” Over time, though, their mandibles slowly dull. It takes longer and requires more energy to get the job done. When it takes an ant about three times as much time and energy to cut out a leaf disc than it would have taken when her blades were sharp, behavior changes, the researchers reported. The cutting ants rest their blades and join the delivery staff, carrying the discs cut from the leaves into their nest. “Imagine having only two tiny knives to use for your entire life, with no sharpening allowed,” Schofield said. “You would want them to be made of the best material possible. You would use them very carefully, but cutting would still get harder and harder as they dulled until you had to rely on others to cut for you. That’s what it is like to be a leaf-cutter ant.” The composition of the cutting blades is of particular interest to researchers. The findings support the idea that wear and fracture are big problems for smaller animals. The researchers estimate that, because of wear, the colony spends twice as much energy cutting leaves as it would if all ants had sharp mandibles. This cost should have resulted in an evolutionary pressure to develop materials that resist dulling, the research team noted. The cutting blades are indeed made of a zinc-rich biomaterial that the researchers suspect is wear resistant. Schofield was lead author of a study published in 2001 that had identified a family of biomaterials present in mandibular teeth, tarsal claws, stings and other such tools of small organisms. In 2009, a team led by Schofield reported that a similar type of substance empowers the claw tips of striped shore crab and is present on the walking legs of Dungeness crabs. “Humans are just starting to try to engineer tiny machines and tools, and we have a lot still to learn from organisms that have coped with being small for millions of years,” Schofield said. “And in addition, it’s good to know how important wear is to these ants, because they are agricultural pests, and this research hints that crops that produce high levels of wear might discourage them.”