Birds do it. Bees do it. Even spiders in their webs do this: cooperate for more peaceful colonies.
That’s one of the surprising results of a new study of orb-weaving spiders in Peru by UCLA students.
The study also found that when there are more females than males in orb-weaving spider colonies, the males fight less with each other – and that females fight less in female-dominated colonies than in male-dominated ones , which leads to colonies that are somehow more peaceful. The spiders also showed slight animosity toward individuals from different colonies, a finding not previously documented for colonial spiders.
The study was published in Journal of Arachnology.
“We used to think that animals like honey bees and elephants lived together,” said the paper’s senior author Gregory Graeter, a professor of ecology and evolutionary biology at the University of California, Los Angeles. “But normally spiders live alone, so we were excited to study these colonial spiders and find out how they interact with colony mates as well as with individuals from other colonies.”
Orb-weaving spiders weave webs connected to each other in extensive webs attached to the surrounding vegetation. Within colonies, individuals protect their own networks from intruders and often compete with each other for food and mates. They retreat to common areas for protection when threatened, and some species defend common areas.
The four student authors — Catherine Wu, Chaiti Bhagawat, Modan Goldman, and Nihal Punjabi — participated in field courses taught by Graeter at the Cocha-Cashu Biological Station in southeastern Peru. Their four-day journey to reach the field research station included a long, winding bus ride through the Andes and a boat ride down the Madre de Dios River and up one of its tributaries.
Over 18 days, the students studied 34 colonies of a species called Philoponella republicana to see if the location of the webs, the ratio of male to female spiders, the size of the webs or the spiders themselves affected the animals. level of aggressiveness. Their work was supervised by Graeter, along with Debra Shier, an adjunct professor at UCLA, and Roxana Arauca-Aliaga, a social insect and spider researcher who was Cocha Cashu’s research coordinator and associate director at the time.
They watched the spiders build their webs together and wrap their prey in silk, although the spiders did not share food – only one spider ate a given prey item.
Students staged invasions by placing spiders in different locations and relocating spiders from other colonies. Some spiders defended their own web of balls against all intruders, but the spiders did not cooperate to repel the intruders.
When colonies had more females than males, males fought with other males and females fought with males less than in male-dominated groups, leading to more peaceful colonies.
However, when the colonies had many large and medium-sized females, these females grabbed the most prey and fought the most for the insects they caught, creating somewhat more aggressive colonies.
After returning from the field, the students learned from previous research that group living in spiders is extremely rare – occurring in less than 0.1% of species. Despite this, arachnologists recognize several types of sociality. Non-territorial spiders that interact in prey capture, web building, and brood care are most similar to truly social animals such as ants, honey bees, and naked diggers.
Territorial colonial spiders, which cooperate in web building but also compete aggressively with other members of their colony for food and mates, appear to have evolved many times from solitary species, presumably when environmental conditions favored group living. They occupy a position on a continuum of sociality similar to the group life of primates, including humans.
All research students graduated from UCLA. Wu currently works in outdoor education at UCLA Recreation, Bhagawat is pursuing a master’s degree at Ghent University, and Goldman and Punjabi are medical students at the Illinois Carroll College of Medicine and Case Western Reserve University, respectively.
Graeter has been leading groups of UCLA students into rainforests for field research since 2001. He plans to return to Kocha Kasha to uncover more secrets with a new group of students in January 2023.