In a social species of spider where individuals help each other in building a communal nest and rearing young, which task is taken on by which spider is down to personality and not physical size or attributes, researchers say.

In most social insects, like ants, size determines work assignments; large ants serve as soldiers to protect smaller workers, and an even larger queen is the source of new colony members.

However, researchers at the University of Pittsburgh have studied a species of spider, Anelosimus studiosus, where task assignments seem based on behavioral tendencies rather than physical traits.

Females of the species display two distinct and opposite personalities, the researchers found -- docile and aggressive -- with neither linked to any apparent physical differences.

The aggressive spiders were much more effective than docile ones when it came to capturing prey for food for the colony and three times more likely to respond if intruders attacked.

And the shared webs constructed by aggressive spiders were larger than ones created by docile individuals.

So are the docile spiders freeloaders, letting their aggressive compatriots to do most of the hard work of creating, running and protecting a colony?

If that were the case, the researchers asked, why are colonies with mixed personality types so successful in nature?

It's because the docile individuals are busy raising the next generation of young spiders, and they're very effective at it, they said.

"Discovering that docile females were daycare experts was the missing piece to the puzzle," researcher Colin Wright said.

"Now that we know that personality can organize colonies extremely well, to a level previously only thought possible in species with modified physical structures."

The study authors acknowledge that although they saw no apparent physical characteristics differentiating docile and aggressive spiders, it's possible there could be other differences underlying their distinct personalities, such as differences in their nervous systems.

They are also careful to note that they define the world "personality" in a way to differentiate it from its normal association with complicated human behaviors.

The "personality" in spiders, they say, is defined as "consistent variation in individual behavior across contexts." That means a spider will always respond in the same way for all its life and never demonstrate an ability to switch between passive behavior and aggression, as humans can.

In the spiders' case, the fixed difference between aggressive and docile personalities is a benefit, the researchers say; previous studies have found that colonies with a mix of the two personalities were more successful than those made up of individuals of just either single personality type.

The study, "Animal personality aligns task specialization and task proficiency in a spider society," was published in the journal PNAS, the Proceedings of the National Academy of the Sciences.

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