"Long before it's in the papers"
July 13, 2011

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“Personality” variation seen as vital to ants’ success

July 13, 2011
Courtesy of Gu­ten­berg Uni­vers­ity Mainz
and World Science staff

They at­tack oth­er col­o­nies, plun­der and rob, kill oth­er col­o­nies’ in­hab­i­tants or keep them as slaves. Ants, often seen as pro­to­types of so­cial be­ings pre­pared to sac­ri­fice their lives for their com­mun­ity, can al­so be cru­elly ag­gres­sive to­ward oth­er ant groups. 

But now, bi­ol­o­gists have found that ant col­o­nies are more suc­cess­ful at mul­ti­ply­ing when the work­ers vary strongly in ag­gres­siveness. This varia­t­ion may be part of their di­vi­sion of la­bor, re­search­ers say, a sys­tem con­sid­ered the key to so­cial in­sect so­ci­eties’ suc­cess.

A T. longispinosus work­er at­tacks in ag­gres­sion ex­per­i­ment (pho­to: An­dre­as Modl­meier)

“There are no fully ag­gres­sive [ant] col­o­nies. It seems that this is not ben­e­fi­cial in the nat­u­ral world and could rath­er be a dis­ad­van­tage,” said An­dre­as Modl­meier, who is stu­dy­ing ant “per­sonal­i­ties” for a doc­tor­al the­sis at Jo­han­nes Gu­ten­berg Uni­vers­ity Mainz in Ger­ma­ny. 

Some­one, in oth­er words, needs to hang back and look af­ter the ba­bies.

The con­cept of an­i­mal per­son­al­ity has gained pop­u­lar­ity among re­search­ers in re­cent years. Bi­ol­o­gists are now start­ing go be­yond just doc­u­ment­ing the ex­ist­ence of such in­di­vid­ual varia­t­ions in be­hav­ior­al ten­den­cies, in­to stu­dy­ing their role in the suc­cess or fail­ure of popula­t­ions.

“Ants have a col­o­ny char­ac­ter, but that there are al­so many in­di­vid­ual per­son­al­ity char­ac­ter­is­tics with­in an ant col­o­ny,” said Su­sanne Foitzik, a pro­fes­sor at the uni­vers­ity and head of a work group that in­cludes Modl­meier. One such char­ac­ter­is­tic is ag­gres­sion, she added; ag­gres­sive col­o­nies, for ex­am­ple, flee much less often than oth­ers do.

There are more than 15,000 ant spe­cies world­wide, ac­cord­ing to Foitzik’s group. About a third of the 150 Cen­tral Eu­ro­pe­an spe­cies are par­a­sit­ic, liv­ing at the ex­pense of oth­er ant spe­cies. These in­clude “slave-making ants,” which Foitzik’s group is stu­dy­ing with in­ter­est. But anoth­er spe­cies un­der study is Tem­notho­rax lon­gi­spi­no­sus, a vic­tim of slave mak­ers. En­slaved T. lon­gi­spi­no­sus work­ers search for food and care for the brood of the slave­maker. 

T. lon­gi­spi­no­sus lives in mixed oak forests in the north­east­ern U.S., where it builds nests in acorns, hick­o­ry nuts, and lit­tle twigs. They form col­o­nies av­er­ag­ing 35 work­ers and feed mainly on dead in­sects. The work­ers are just two to three mil­lime­ters long.

“Tem­notho­rax is par­tic­u­larly suit­a­ble for our ex­pe­ri­ments, as their col­o­nies are easy to keep in the la­bora­to­ry, and this makes it pos­si­ble to use large sam­ple sizes,” said Modl­meier. In ex­pe­ri­ments, he brought in­di­vid­ual ants to­geth­er with a dead work­er of anoth­er col­o­ny and watched how of­ten ag­gres­sive in­ter­ac­tions took place. He not­ed ac­tions such as open­ing of the jaws (a threat dis­play), bit­ing, pulling, and sting­ing. Ten work­er ants were se­lected from each of 39 dif­fer­ent col­o­nies to be clas­si­fied by their size, lev­el of ag­gres­sion, and explorato­ry be­hav­ior. Re­search­ers found that the repro­duc­tive suc­cess of ant col­o­nies in­creased with the varia­t­ion in the lev­el of ag­gres­sion with­in the col­o­ny. 

“Colonies might be more pro­duc­tive when tasks such as nest de­fense and brood care are dis­trib­ut­ed be­tween spe­cial­ized work­ers with dif­fer­ent ag­gres­sion lev­els,” Modl­meier said. An­i­mals with high ag­gres­sion lev­els could par­ti­ci­pate in com­pe­ti­tion and fights with oth­er col­o­nies, while less ag­gres­sive so­cial work­ers care for the off­spring, he added. The find­ings are pub­lished in the June 28 is­sue of the re­search jour­nal Be­hav­ior­al Ecol­o­gy.

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They attack other colonies, plunder and rob, kill other colonies’ inhabitants or keep them as slaves. Ants are usually seen as prototypes of social beings prepared to sacrifice their lives for their community, but they can also be cruelly aggressive toward other ant groups. Now, biologists have also found that ant colonies are more productive and raise more offspring when the workers vary strongly in aggressiveness. This variation may be part of their division of labor, researchers say, a system considered the key to social insect societies’ success. “There are no fully aggressive colonies. It seems that this is not beneficial in the natural world and could rather be a disadvantage,” said Andreas Modlmeier, who is studying ant “personalities” for a doctoral thesis at Johannes Gutenberg University Mainz in Germany. Someone, in other words, needs to hang back and look after the babies. The concept of animal personality has gained popularity among researchers in recent years. Biologists are now starting go beyond just documenting the existence of such individual variations in behavioral tendencies, into studying their role in the success or failure of populations. “Ants have a colony character, but that there are also many individual personality characteristics within an ant colony,” said Susanne Foitzik, a professor at the university and head of a work group that includes Modlmeier. “One such characteristic is aggression. Aggressive colonies, for example, flee much more rarely than others do. There are more than 15,000 ant species worldwide, according to Foitzik’s group. About a third of the 150 Central European species are parasitic, living at the expense of other ant species. These include “slave-making ants,” which Foitzik’s group is studying with particular interest. But another species under study is Temnothorax longispinosus, a victim of slave makers. Enslaved T. longispinosus worker ants search for food and care for the brood of the slavemaker. T. longispinosus lives in mixed oak forests in the northeastern U.S., where it builds nests in acorns, hickory nuts, and little twigs. They form colonies averaging 35 workers and feed mainly on dead insects. The workers are just two to three millimeters long. “Temnothorax is particularly suitable for our experiments, as their colonies are easy to keep in the laboratory, and this makes it possible to use large sample sizes,” said Modlmeier. In experiments, he brought individual ants together with a dead worker of another colony and watched how often aggressive interactions took place. He noted actions such as opening of the jaws (a threat display), biting, pulling, and stinging. Ten worker ants were selected from each of 39 different colonies to be classified by their size, level of aggression, and exploratory behavior. Researchers found that the reproductive success of ant colonies increased with the variation in the level of aggression within the colony. “Colonies might be more productive when tasks such as nest defense and brood care are distributed between specialized workers with different aggression levels,” Modlmeier said. Animals with high aggression levels could participate in competition and fights with other colonies, while less aggressive social workers care for the offspring, he added. The findings are published in the June 28 issue of the research journal Behavioral Ecology.