Spiders have varied personalities

Pitt researcher calls traits matter of life, death

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Take a look in any hedgerow around Pittsburgh and you will probably find a tiny predator, a grass spider sitting in a web. According to biologist Jonathan Pruitt, that spider has a personality.

It might be bolder than other grass spiders, or more timid, more aggressive, or more prone to sexual cannibalism.

Like people, not all individuals of the same species act alike, said Mr. Pruitt, an energetic 27-year-old assistant professor at the University of Pittsburgh. In May, he published the results of a six-year study showing that the personality of a spider doesn't just affect its own success, it can shape the fortunes of its entire spider society.

Spider societies are rare because most arachnid species live solitary lives, unable to tolerate other spiders venturing into their webs. But a few species live in cooperative groups, where females share tasks like maintaining the communal web, raising offspring and fighting off intruders.

In the social species that Mr. Pruitt studied, personality is a simple affair. Individuals are considered either aggressive or docile and colonies are usually a mix of the two types.

To test how the mix of personalities affects the group's success, he conducted a field study in Tennessee where he established colonies of three types: a pair of docile spiders, a pair of aggressive spiders or a pair of one docile and one aggressive. He then left each pair to multiply in the wild, coming back to observe the colonies every summer for six years.

At first, the colonies founded by docile pairs seemed to do the best, reproducing faster and spreading out to start new colonies nearby.

But as the population exploded, the webs were invaded by other species of spiders. These intruders act as parasites, stealing food, raising their eggs in the colony and even eating their unwilling hosts. By the end of the study, the groups of colonies founded by docile pairs were all dead, overrun by intruders. "A whole lineage blinks out in a wave of extinction," Mr. Pruitt said.

Meanwhile, the colonies founded by aggressive spiders or a mix of personalities avoided the boom and bust because they were better at defending their webs against other spiders, even if they didn't reproduce and spread as fast.

Crucially, personality didn't matter in the absence of a common enemy. When Mr. Pruitt removed invading species of spider from the webs, all the different types of colonies survived about as well as each other.

Mr. Pruitt compares aggressive individuals to a social immune system. "They keep this social disease, these parasites, at a low abundance," he said.

Personality is important for human societies too, Mr. Pruitt suggests. It influences what vocations people choose and how well groups perform.

"I would be as interested in working with people as with spiders. It's just harder," he said.

Mr. Pruitt and his colleagues are also researching personality in other species, including sexual cannibalism in grass spiders, boldness in social spiders in Africa and India, and slave rebellions in ants.

Tiny slavemaking ants, which can live in colonies inside acorns, kidnap and enslave the brood of other ant species. The slaves do all the colony's work, including foraging for food and raising the slavemakers' offspring. Mr. Pruitt's team is looking at the personalities of both masters and their slaves. "Sometimes they'll have a revolt and kill the slavemakers, but it's pretty infrequent," he said. "I bet you it has something to do with their personality types."

neigh_city - science

Cristy Gelling: cgelling@post-gazette.com. First Published October 13, 2013 8:00 PM


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