Ant Enforcers: To call in punishment, top ant smears rival

In Brazilian ant colonies where a female has to fight her way to the top, she stays in power through some judicious gang violence, say researchers.

KIDS TODAY. Young Dinoponera quadriceps strike aggressive poses. Monnin

When a Dinoponera quadriceps alpha ant smears a chemical on a challenger, low-ranked workers–probably the alpha’s daughters–rush in and subdue the rival, report Thibaud Monnin of the Université Pierre et Marie Curie in Paris and his colleagues in the Sept. 5 Nature. Observers had seen bits of the behavior before but hadn’t put it together as collusion between the top ant and a gang, according to Monnin.

In most ant colonies, treating a larva royally makes it into a queen. However, in about 1 percent of known ant species, there’s no such physiological split between the reproductive queen and the nonreproductive workers. All females look alike and have the capacity to become reproductive if they achieve top rank.

D. quadriceps is among the largest known ants, reaching 3 centimeters not counting their antennae. The 80 or so ants in a nest fight to determine a linear hierarchy. The alpha ant mates once in her lifetime and rules for perhaps a year or two until she’s displaced, says Monnin.

Females emerge from their pupae fighting among themselves and with their older nest mates. They posture, slap their antennae against rivals’, and bite, although deaths rarely result. The feisty youngsters can win a place high in the hierarchy, and the feistiest may displace the current alpha ant should her power be waning.

Monnin is interested in how ant communities manage conflicts, so he and his colleagues studied D. quadriceps in laboratory colonies. The researchers found that the secretory Dufour’s gland looks larger and darker in mated females than in unmated ones. Analysis showed chemical differences, too.

The researchers dabbed glandular extracts from females of alpha, beta, and low-rank ants onto the beta female in another colony. Extracts from lowly workers elicited no interest. Extract from a beta ant inspired nest mates to groom the marked ant. An alpha’s extracts, though, brought five or six workers to attack and hold down the targeted ant.

Monnin reports that he’s seen gangs of D. quadriceps workers immobilize a victim for several days. During one of these marathons, the alpha female may come by periodically and rub her abdomen against the former rival, as if renewing the chemical mark, he says. When the rival finally is released, her status plunges to that of a lowly nursemaid and never recovers.

The researchers calculate that it’s in the gang members’ interest to prolong their mother’s reign. As long as she’s laying the eggs, the young will be sisters to the gang. If a sister takes over, the young will be less closely related–just nieces.

This is “extremely interesting,” says Joan M. Herbers of Ohio State University in Columbus. Slave-making ants sometimes mark victims for violence during a takeover, yet “this is the first study of which I am aware that shows this is going on inside a single colony.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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