Within the colonies of brownish-orange social spiders called Anelosimus studiosus, females fall into one of two behavioral types: aggressive or docile. These pea-sized spiders make cobwebby nests that range “anywhere from the size of a golf ball to the size of a Volkswagen Beetle,” says Jonathan Pruitt from the University of Pittsburgh. They live along rivers and form organized groups capable of preying on small vertebrates. “We have found carcasses of rats and birds inside their colonies,” he adds.
So what ratio of docile-to-aggressive females make the most successful spider cities? Pruitt and Charles Goodnight from the University of Vermont created an array of 53 artificial colonies and deployed them at six field sites in Tennessee and Georgia. Half of these sites were rich in resources, while the other half were limited. These artificial colonies contained up to 27 females, and their compositions varied from zero aggressive females to 100 percent aggressive females. Most of these experimental colonies contained “native” individuals taken from the site where they were later deployed, and the rest contained “foreign” spiders that came from sites with opposing resource levels.
In places where there are plenty of resources, small colonies are dominated by docile females. But as the colonies grow, so does the frequency of aggressive females. In these places, social parasites may be the biggest threat to a growing colony’s long-term survival, making aggressive females more useful. The opposite happens in places with fewer resources. Small colonies there are dominated by aggressive females, and docile ones appear more frequently as the colonies grow. In these places, egg cannibalism by aggressive females are the primary threat to survival.
This shifting ratio -- which is a collective trait of the whole colony -- is adapted to specific locations. Certain ratios yield high survivorship at some sites, but not others. Their findings were published in Nature last week
When the team threw off the docile-to-aggressive ratio, the spiders seemed to sense the risk, and they readjusted their ratio back to normal over just a couple generations. When the artificially assembled colonies were placed in new locations, the spiders recreated the ratio that would have been ideal back home. “But they seem to have no idea that they’re at a new site and that what they’re doing is going to doom the whole colony,” Pruitt says in a news release. “All the friends die.”
According to the duo, their findings suggest that these spiders evolve together in groups. So-called “group selection” is caused by the differential extinction or proliferation of whole populations based on differences among them -- but this idea of “for the good of the group” has been dismissed for decades. (Think about those lemming myths.) “Biologists have never shown an adaptation in nature which is clearly attributable to group selection,” Goodnight says. “Our paper is that demonstration.” If that’s the case, this may be the first-ever experimental evidence that group selection can shape collective traits in the wild.