Spiders divide labor by personality, say scientists

The social spider Anelosimus studiosus performs jobs within the colony based on its distinct personality, according to researchers at the University of Pittsburgh.

Ever wondered if your job really fits your personality? If you were a spider, there might be no doubt.

Researchers at the University of Pittsburgh recently published a study showing that a species of social spider, Anelosimus studiosus, organizes colony duties by personality.

The spiders have two distinct personality types: aggressive and docile. Docile spiders used to be considered relatively unhelpful to a colony, as scientists rarely witnessed them participating in tasks such as web repair, prey capture, or colony defense. This new study explored the role of the docile spider, revealing that, compared with their more belligerent counterparts, they make better parents.

"They're actually integral in the system," says study coauthor Colin Wright, a graduate student in Pitt’s behavioral ecology program. "They can't really live without them.” The aggressive individuals will get so carried away that they will sometimes even kill their own young. The docile spiders serve as a motherly buffer.

The study shows that spiders with a certain personality perform certain tasks better than the other types and do those tasks more frequently, creating a division of labor within the colony. According to Mr. Wright, this balance is important for the species to survive. An entirely aggressive colony would kill off their young too often, while an all-docile colony would be ineffective at defending their colony.

If you were to graph the personalities of these spiders in the colony, you’d see two humps. "There's a bimodal distribution, where we have individuals that are docile and individuals that are aggressive and there's not much in between," explains Wright. This distinct division allowed the researchers to sort the spiders easily.

The researchers collected the spiders from their natural habitat in eastern Tennessee. When the female spiders reached maturity, they were put through a "spider personality test," as Wright called it. Two individuals were set in a clear container for 24 hours. Where they built their nests told the researchers which category the spiders fell into.

"If you have two docile individuals, they'll build a nest together and we'll see them in the same corner of the box the next day with a joint web."

The aggressive spiders, however, would be on opposite ends of the box. "They're pretty antagonistic towards each other," says Wright. But the aggressive spiders will even repel a docile one, so the researchers had to repeat the test with a known docile spider. Once they were sure, the researchers marked the specimen with colored paint.

Previous studies have termed this personality distinction “social” and “asocial.” But Wright says there are many contexts, such as prey capture, where aggressive spiders cooperate. "They just don't like each other that much."

Once the spiders had been sorted by personality, the researchers set up colonies with females of both types. At set intervals, they recorded which spiders were performing which tasks. The aggressive spiders overwhelmingly chose to build webs, catch prey, and defend the colony. Meanwhile, the docile individuals looked after the colony's brood.

To test individuals' abilities without the influence of a social environment, the researchers removed single spiders from colonies and tested them alone. According to Wright, this allowed the researchers to connect the spiders’ personalities with how well they performed tasks.

For prey capture, the researchers dropped a cricket into the spider's web and watched to see how quickly the spider subdued the prey, or if it escaped. For web repair, they damaged the web to see how well each spider could fix it. For colony defense, the researchers introduced another species of spider that preys on their young and steals their food, and watched how the spider responded. And for parenting skills, the spiders were given starting broods of 10 or 25 babies. Their proficiency as parents was assessed by how many survived into adulthood.

The aggressive spiders passed the first tests with flying colors, but bumbled through parenting. The docile ones were less successful at prey capture, web repair, and colony defense, but they raised strong broods.

Much of the scientific literature concerning insects and spiders discusses how social castes are linked to the animals’ physical characteristics, according to Wright. But the researchers have yet to spot physical differences between spiders of the two personality types. They hope future studies will examine personality as a factor in labor division, says Wright.

The next step will be to determine the ideal ratio of docile and aggressive spiders in a colony. Wright suggests that local resources and predators would influence this, so the study would have to examine colonies in their natural environments in various regions.

Wright says it's important to note that efficient division of labor isn't just for arachnid communities. "It's relevant to how human society works. We all have different personalities as well, we all perform different jobs."