Anelosimus studiosus. Credit: Joe Lapp CC BY 2.0
Anelosimus studiosus. Credit: Joe Lapp CC BY 2.0

Social Spiders Pick The Best Careers For Their Personalities

I was a terrible scientist; I’m much better at being a writer. I lack the creative spark for designing clever experiments, but I can fashion a decent metaphor. My intellectual skittishness stops me from narrowing in on a single field, but it’s a boon when it comes to dealing with a broad spectrum of topics. It took me two unsuccessful years as a PhD student to work this out, and to find a job that’s better suited to my skills and temperament. Because of that, I envy the spider Anelosimus studiosus. It naturally falls into the right career.

While most spiders hunt alone, there are a few hundred species of social spiders that live in colonies. A.studiosus is one of them. Up to 50 individuals gather together to spin large collective webs, which ensnare larger prey than each spider could trap on its own.

All the colony members look the same, but they don’t all behave in the same way. The females can be aggressive or docile. It’s surprisingly easy to suss out their personalities: just put two of them in a small box overnight and check on them the next morning. If they’re both docile, they will have built a joint web in one corner of the box. If one of them is aggressive, the pair will be at opposite corners (you can then pair each individual with a known docile spider to confirm its personality).

Colin Wright from the University of Pittsburgh has now found that these personality types do different jobs within the web, creating a natural division of labour. They’re a little like ants and termites, where small workers clean and forage, and big soldiers guard and defend. But unlike these social insects, the social spiders don’t have distinctive castes with different physiques. Instead, their roles are defined by their personalities.

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Social spider web. Joe Lapp

When Wright’s team, led by Jonathan Pruitt, first started studying A.studiosus, they couldn’t work out what the docile spiders did. They didn’t seem to repair webs, repel invaders, or catch prey. “The prevailing theory was that they were probably social parasites, a sort of selfish variant that freeloaded on the success of aggressive spiders,” says Wright.

But when the researchers checked the fates of colonies in the wild, they found that those with a mix of docile and aggressive members were more likely to survive than those with just a single type. The docile members were clearly doing something important.

It turns out that they act as the colony’s babysitters. They spend most of their time standing watch over the eggs, sitting amid clusters of spiderlings, or directly feeding the youngsters by regurgitating food—just like a mother bird might. Meanwhile, the aggressive spiders generally avoid these tasks; instead, they spend most of their time building the communal web, catching prey, and defending their colonies. (Although aggressive spiders repel docile ones in a simple container, the two types happily share webs—their different tasks mean that they rarely interact.)

The team also found that across the spiders show a perfect match between aptitude and career. That is, they tend to do the jobs that they’re best at.

Compared to the docile spiders, the aggressive ones are better at repelling a rival species of spider from their webs, at building webs that restrained crickets for longer, and at successfully subduing crickets that landed in their webs. This is probably because the docile females rarely respond to intruders—even prey—and when they do respond, they do so slowly.

By contrast, the docile spiders were better at looking after the colony’s young. When challenged with large broods, they raised twice as many to the point when the youngsters no longer needed care. That’s probably because  they’re less likely to fight with their youngsters over food. They’re also less likely to eat the spiderlings—one of the more important signs of a good parent.

“Rarely are results of a study so clear and interpretable,” says Wright. “We thank our spiders for being such a pleasure to work with.”

The results clearly show that something as subtle and hidden as a spider’s personality can help to organise its society. “Most people are more concerned with the physical characteristics of a species, and the idea that spiders have personality sounds somewhat like the topic of a children’s book,” he says. “But spider personalities are very real, and if you overlook them, at least in this species, you miss so much of the story!

For now, it’s not clear why the spiders naturally fall into their respective careers, or even what drives their different personalities in the first place. They seem to be largely heritable, but no one knows whether the spiders also learn to behave in a certain way, or if they can switch their personalities in certain situations.

And there’s another crucial piece of missing information, says Trine Bilde From Aarhus University, who has also studied social spiders: it’s not clear if the two types of spider are actually helping each other. “We don’t know whether docile females are fostering offspring of aggressive females, or if the aggressive females are sharing their catch with docile females,” she says.

The team are now trying to answer these questions. In the meantime, Wright suggests that biologists should pay more attention to personality types, when trying to understanding how animal societies work. For example, division of labour, of the kind that Wright studied, has been linked to the evolutionary origins of physically distinct castes in ants, bees and termites. But while physical castes only exist in a narrow range of animal groups, “individual differences in personality have been detected in almost every animal system imaginable,” says Wright.

“We feel that, in just about every instance, one could simply replace the word caste with personality,” says Wright. As with the spiders, this approach could help scientists to discover hidden variations in the behaviour of animals that otherwise look exactly the same—variations that could shape the structure of their societies in important ways.

Reference:  Wright, Holbrook & Pruitt. 2014. Animal personality aligns task specialization and task proficiency in a spider society. PNAS http://dx.doi.org/10.1073/pnas.1400850111

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