Ant slave ‘rebellions’

This video from the USA says about itself:

Acorn ant (Temnothorax longispinosus) larva close-up and a worker feeding a larva.

Courtesy of Jo­han­nes Gu­ten­berg Uni­vers­ity Mainz in Germany and World Science staff:

Slave ant “rebellions” found to be common

Sept. 27, 2012

Ants held as slaves in nests of oth­er ant spe­cies of­ten dam­age their op­pres­sors through acts of sab­o­tage, ac­cord­ing to new re­search.

Ant re­searcher Su­sanne Foit­zik of Jo­han­nes Gu­ten­berg Uni­vers­ity Mainz in Ger­ma­ny said she in­i­tially not­ed the “re­bel­lion” be­hav­ior three years ago, in find­ings re­ported in the April 2009 is­sue of the jour­nal Ev­o­lu­tion. More re­cent re­search, she said, has re­vealed that the phe­nomenon—seen among ants that are en­slaved in or­der to raise their mas­ters’ off­spring—is wide­spread.

In three ant popula­t­ions in West Vir­gin­ia, New York, and Ohio, Foit­zik ex­plained, en­slaved work­ers of the ant spe­cies Tem­notho­rax long­i­spin­os­us have been ob­served ne­glect­ing and kill­ing the off­spring of their slave­mak­ers rath­er than car­ing for them. As a re­sult, only 45 per­cent of the slave­mak­ers’ off­spring sur­vived on av­er­age—lit­tle over half the sur­viv­al rate of the slave spe­cies’ brood in its own free-liv­ing nests.

The Amer­i­can slave-making ant Pro­to­mog­nathus amer­i­canus is a “so­cial par­a­site” of an an­cient line­age that de­pends en­tirely on oth­er ant spe­cies, called the host spe­cies, to sur­vive. Slave work­ers care for the brood in par­a­site nests, br­ing food to their mas­ters and feed them, and even de­fend the nest.

The ants be­come slaves when work­ers from the slave-making ant col­o­ny at­tack the nests of the spe­cies T. long­i­spin­os­us, kill the adults, and steal the brood. Back in the mas­ters’ nest, which can be in hol­low acorns, nut­shells, or twigs, the slave­mak­ers ex­ploit the nat­u­ral brood care be­hav­ior of the emerg­ing slave work­ers. The slaves feed and clean the lar­vae, the maggot-like off­spring of their mas­ters.

“Probably at first the slaves can­not tell that the lar­vae be­long to anoth­er spe­cies,” said Foit­zik. As a re­sult, 95 per­cent of the brood sur­vives the lar­val stage. But the situa­t­ion changes when the lar­vae be­come pu­pae, or un­dergo their met­amor­phosis in­to the adult stage. “The pu­pae, which al­ready look like ants, bear chem­i­cal cues on their cu­ti­cles [shell-like skele­tons] that can ap­par­ently be de­tected. We have been able to show that a high frac­tion of the slave­maker pu­pae are killed by slave work­ers.”

The pu­pae are ei­ther ne­glected or ac­tively killed by be­ing at­tacked and torn apart, the re­search­ers found. Sev­er­al slaves at once may as­sault a pu­pa, which can­not move or de­fend it­self dur­ing the pu­pal stage and is al­so un­pro­tected by a cocoon—P. amer­i­can­us be­ing one of a num­ber of ant spe­cies which, for un­clear rea­sons, don’t make co­coons.

In par­a­site nests in West Vir­gin­ia, only 27 per­cent of the pu­pae sur­vived, and in the New York col­o­nies, only 49 per­cent, Foit­zik said. In Ohio, the sur­viv­al chances of the Amer­i­can slave-making ant was a bit high­er at 58 per­cent—but this was still well be­low the sur­viv­al rate of 85 per­cent for pu­pae of the “slave” spe­cies when in their own free-liv­ing nests.

A ques­tion is pre­cisely what mem­bers of the “slave” or host spe­cies achieve by re­belling.

“The en­slaved work­ers do not di­rectly ben­e­fit from the kill­ings be­cause they do not re­pro­duce,” said Foitzik. But their free rel­a­tives in the sur­round­ing area—which might very well be their sisters—indi­rectly ben­e­fit, she not­ed, as slave­maker col­o­nies weak­ened by re­bel­lions are less capa­ble of suc­cess­fully launch­ing new raids.

In­ter­est­ingly, Foit­zik added, ge­o­graph­ic dif­fer­ences in the slave spe­cies’ re­sponses fit pre­dic­tions of ev­o­lu­tion­ary the­o­ry that popula­t­ions will evolve dif­fer­ent traits in re­sponse to dif­fer­ent pres­sures from their lo­cal en­vi­ron­ment. An ex­am­ple: while host ants in New York are very ag­gres­sive and of­ten suc­cess­fully thwart slave raids, West Vir­gin­ian hosts prof­it more from the slave re­bel­lion be­cause, as ge­net­ic anal­y­ses in­di­cate, the neigh­bor­ing col­o­nies are more of­ten close rel­a­tives to the “re­bels.”

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