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September 27, 2012

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Slave ant “rebellions” found to be common

Sept. 27, 2012
Courtesy of Jo­han­nes Gu­ten­berg Uni­vers­ity Mainz
and World Science staff

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.

A slave­maker pu­pa is killed by en­slaved host work­ers of the spe­cies T. lon­gi­spin­o­sus. (Pho­to: Al­ex­an­dra Achen­bach)

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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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% 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 Temnothorax 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% 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—Protomognathus 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% of the pupae survived, and in the New York colonies, only 49%, Foitzik said. In Ohio, the survival chances of the American slave-making ant was a bit higher at 58%. but this is still well below the survival rate of 85% 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 neighboring relatives—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 rebelling slaves.