Rock ant decisions swayed by six-legged social media

Collective choice changes when scouts say ‘follow me’

rock ants

SOCIAL SHARING  Collective decisions can change when rock ants start leading nest mates one by one to check out a possible new home.

Norasmah Basari and N. Franks

Rock ants don’t tweet, but they do recruit followers. And that social input can change the outcome of a group decision.  

Colonies of Temnothorax albipennis ants decide as a group which small craggy crevice to move into. They can even compare averages of a sort when choosing between nests that stay comfy for different proportions of time, an earlier study found. Yet those choices based on comparing averages turn out differently if ants start leading nest mates over to check out appealing sites, researchers say in an upcoming issue of Behavioral Ecology.

“A small amount of social information can massively influence the outcome of a collective decision,” reports paper coauthor Dominic Burns of the University of Bristol in England.

The Bristol lab of Nigel Franks has studied these small ants as examples of how social animals without language, formal mathematics or candidate debates collectively make choices. In the basics of the process, “there are a lot of comparisons that can be made between collective decision making in humans and ants,” Burns says. Analyzing decision making that evolution has honed in ants might spark insights into the human version, he says.

In picking nests, ants favor a dark crevice over one with light shining in. And narrow entrances appeal more than wide ones. Researchers have mixed those qualities to create an ant nest with stable but “mediocre” conditions: unpleasantly constant light but an attractively narrow entrance. As an alternative to this stable site, researchers offered the colonies a changeable site with a not-great entrance and repeating 10-minute periods with some darkness and then bright light.

An earlier experiment with this setup, published last year, found that ant colonies typically picked the stable site (unrelenting light, but great entrance) if the changeable site had only 2.5 minutes of darkness alternating with 7.5 minutes of light. But most colonies no longer preferred that stable site when the alternative was a better changeable site, with 7.5 pleasantly dark minutes out of each 10-minute period.

For the most part, colonies reached their decision as individual ants happened upon possible nesting sites without any guidance from nest mates and lingered longer in the more favorable nest. The decision to move into one site instead of the other solidified when a certain number of ants, a quorum, sensed each other poking around the same nest.

In the new experiment, Burns and his colleagues set up the alternative nests far enough apart so ants didn’t discover them rapidly without help. This slowdown evoked what’s called tandem running: An ant that found a possible new nest site and judged it favorably led a nest mate over to take a look. In this scenario, ant colonies made different collective decisions.

Out of 41 colony decisions, 40 favored the changeable nest even if it had only 2.5 minutes of darkness. The reason, Burns says, is that ants finding the changeable site when it was good and dark were motivated to go recruit a nest mate to come see. Even though on average only five tandem runs occurred per test, the influx eventually brought up ant numbers to the quorum threshold inside the changeable nests. In the nests with constant light, though, ants hardly ever even started to recruit a nest mate and the small number of explorers who discovered the nest on their own hardly ever reached the quorum threshold.

Whether the ants’ decisions about changeable nests would prove wise choices in the real world is unknown, Burns cautions. In some other experiments, animals relying on information from neighbors have made clearly counterproductive choices, notes Guillaume Rieucau of Florida International University of North Miami, who has studied collective decisions in fish and birds. Yet he knows of no experiments showing that blindly copying choices leads to full-blown cascades of dumb decisions among animals. Or not among animals other than humans.

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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