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Ants on the assembly line

Ant study deep-sixes favorite theory!

Social insects are astonishingly successful -- in some habitats the ants, wasps, bees and termites outweigh animals with backbones. This is an insult to the top vertebrates (aren't we totally important?), but biologists have another concern, summed up in a single word: Why?

The standard answer is division of labor through specialization. Ant colonies typically contain workers, who forage for food or care for the young, soldiers, who protect the colony or attack others, and of course, the queen, who sits around and lays eggs 24/7. According to conventional wisdom, specialists are better at their jobs, and that allows the whole colony to thrive.

6 model paint jars next to brushes and microscope
Photo courtesy Megan Levardo, UA NASA Space Grant Intern
It looks like an art studio, but here's how you paint labels on your ant colony -- never know when this skill will come in handy!

And now comes a study showing almost no support whatsoever for this well-accepted rationale. Anna Dornhaus, an assistant professor in the department of ecology and evolutionary biology at the University of Arizona, established some colonies of an ant species that's widespread in Europe.

She painted the ants to identify them, and timed them doing four tasks:

little ant

Carrying immature ants to move the colony to a new location,

little ant

foraging for honey,

little ant

foraging for protein (dead flies), and

little ant

collecting sand grains to build the nest (the sand is about one-third as big as a worker).

When Dornhaus compared ants that specialized in one task to those that worked on several, her data refuted the bedrock assumption that specialization raises individual productivity. "There is no pattern at all," she says, "no significant correlation between how specialized I am and how good I am at a task. I was surprised. I absolutely expected to find that individuals were better at the tasks they were doing, either because they picked the work they were good at, or they learned to get better at certain tasks as they specialized."

Among ants, the queen is always larger than the workers. The study may not apply to the 15 percent of ants that have specialized bodies, Dornhaus cautions. Among some army ants, for example, the soldiers are much brawnier than the workers.

But among 85 percent of ant varieties, including the rock ants she studied, all non-queen females are alike in body size and shape.

If specialization does not increase efficiency, what is going on? "Maybe the system is more complicated than I thought, or maybe it is not as optimized," Dornhaus says. Biologists assume that the continual struggle to survive that drives evolution will refine and perfect behaviors, she says. "In many cases, that is true, but not always. Sometimes, animals will stick to a behavior that has become useless."

click photo for movie
Painted ants bustle around hundreds of white eggs
Courtesy Anna Dornhaus
One minute in the life of an ant colony. The queen (she's big and not painted) lays all the eggs and is the mother of the entire colony. The eggs, larvae, and pupae (the immature stages of ants), are round with a black spot. Watch adult ants inspect, feed, and clean the brood. At 15 seconds into the video, notice an ant trucking a larva from the old nest. Ants that are face-to-face for a few seconds may be feeding each other.

Something new!

Specialization has long intrigued social-insect researchers, but they have concentrated on a different question: Why Ant Annie would tend the young while Ant Annette became a soldier. (Among social insects, males are only involved in mating; females do all the heavy lifting -- and everything else). "Social insect people have almost entirely focused on the sensory stimuli that provoke an individual to engage in a task," says Lars Chittka, of the School of Biological and Chemical Sciences at the University of London. The question of whether the individuals that are stimulated to perform a specific task are also the ones that do it most efficiently "has not been addressed empirically so far," he says. "Dornhaus shows that there is no link between the animal's propensity to perform a task, and their efficiency at that task."

If the result are broadly applicable to social insects, ecologists owe us some explanations. Chittka, who has worked with bees for many years, wrote via email "that just like in human societies, efficiency at job specialty is only in small part a result of 'talent,' or innate tendency to engage in a job: it is much more a result of perfecting skills with experience."

The Dornhaus results may have been different, he suggests, if the individuals had time to hone their skills. "In almost all tasks that social insects engage in, there's improvement with experience," he says.

Switch hitters

Dornhaus, however, found that skill did not seem to increase over time. "Specialists had opportunities for learning, but that did not seem to make them more efficient; they had done tasks repeatedly in the past but were not doing them faster."

One possible explanation involves the effort needed to change from one task to another, she says. "It may be that switching tasks is very expensive ... in terms of time and cognitive effort." If so, it could still be more productive to specialize, even if the specialists do not do their jobs faster than non-specialists.

We mentioned to Dornhaus that specialization and division of labor on assembly lines have proven to be excellent (if boring) ways to improve factory productivity, and Dornhaus returned to the switching issue. "Most of these tasks do not require a lot of learning, but they are efficient because you save the switching costs, so everybody works faster. It's not that a person from another part of the assembly line could not work on your part."

WWII-era pic of bomber body mass storage hanger
Photo: Marie Hansen for Life, 1943
During World War II, assembly lines built hundreds of thousands of airplanes. People work faster on assembly lines, but apparently ants do not.

If switching does not explain the superior performance of social insects, however, Dornhaus says a major explanation for their success "falls apart. Then we do not know why ants have a division of labor, why they specialize. We always thought specialization and division of labor is a great innovation of social insects. It's surprising to find it is not as beneficial as we thought."

- David Tenenbaum

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• Anna Dornhaus, 2008, Specialization does not predict individual efficiency in an ant. PLoS Biol 6(11): e285. doi:10.1371/journal.pbio.0060285.

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