A study of social spider communities has revealed that bad leadership can destroy entire colonies. Potential lessons for human societies could come from the simple, but revealing, model.
Stegodyphus dumicola, an inhabitant of the Kalahari Desert, is one of a very small number of species of social spider. They form collective webs which, according to The University of Pittsburgh’s Dr Jonathan Pruitt can range in size from a golfball to a veritable Mirkwood as large as “several SUVs.” When an animal blunders into the web the spiders may collectively attack it, and Pruitt told IFLScience that the numbers responding are proportional to the size of prey. “For a small insect a dozen might attack, for a large animal it can be thousands.”
Individual spiders have varying personalities, most importantly where they lie on a spectrum from shy to bold. Pruitt told IFLScience that boldness is measured by blowing a jet of air at a spider. “They can’t see well so this is the way they detect birds,” Pruitt said. “They all curl up their legs, but the bold ones bounce back quickly, while the shy ones feign dead for twenty minutes to an hour.”
What might seem like a curious quirk dominates spider society dynamics. When something enters the web, the spiders take their cues from the boldest individual in their vicinity.
“Colonies containing just one very bold individual attack prey more rapidly and with 400% more attackers,” Pruitt told the Behaviour 2015 conference. The decision on whether to attack is important because sometimes a new entrant is more threat than food.
Pruitt and his colleagues taught spiders to associate one sort of vibration with a tasty moth, another with an ant, the spiders' most lethal predator. The trained spiders then joined a colony of 14 spiders that had never experienced either sort of vibration before, and used the vibrations to accompany the arrival of insects, but not always the ones the trained spiders expected.
“The training of shy generic individuals always had very little influence,” Pruitt told the conference. Bold individuals were another matter. They would quickly spur the colony to attack the source of vibrations that they had learned to associate with food. However, the bold but misinformed spiders taught to associate a particular vibration with ants not only refused to attack potential prey accompanied by the same vibration, but somehow deterred their webmates as well.
Entire colonies starved to death in the presence of abundant food if a single bold leader had learned to associate its vibration with ants.
Other studies of animal personality have also found that bold individuals can wield disproportionate influence, but Pruitt told IFLScience that the extent of bold individuals' power was a surprise. “Our prediction was that good information in the hands of leaders would be important, but we didn't expect it to be so one-sided,” he said. “You would think having these leaders would be advantageous, but in our experiments it was more likely to hurt with the bad than help with the good.”
Pruitt drew analogies with the current US election contest.
At the same conference Pruitt's student Colin Wright reported on S. dumicola colonies placed in areas where the ant Anoplolepsis custodiens was common.
Colony responses varied with personality, “If all the spiders in a colony were bold they would attack the ants aggressively,” Pruitt told IFLScience, “If all were shy they would try to hide. If there was a mixture they would build a silken wall to hide behind.” Tragically for the spiders, Pruitt and Wright found that none of these strategies worked in ant-rich zones, with every colony wiped out within 24 hours.
Earlier work by Dr Jonathan Pruitt on the differences between shy and bold spider colonies. geobeats/Youtube.