Male orb spiders fight less in female-dominated colonies, finds a study of spider cooperation

Orb-weaving spiders spin webs that are linked together in vast webs; Within their colonies, individual spiders protect their own webs from invaders and often fight over food and mates. Credit: Gregory Grether/UCLA

birds do. bees do it. Even spiders in their webs do it: cooperate for more peaceful colonies.

That’s one of the surprising findings of a new study by UCLA students on wheel spiders in Peru.

The study also found that when there are more females than males in colonies of orb-weaver spiders, males fight less with each other — and that females fight less in colonies dominated by females than in colonies dominated by males, resulting in colonies leads that are somewhat more peaceful. The spiders also showed little hostility towards individuals from different colonies, a discovery previously undocumented for colonial spiders.

The research was published in Journal of Arachnology.

“We’re used to thinking of animals like honey bees and elephants that live cooperatively,” said the paper’s senior author Gregory Grether, professor of ecology and evolutionary biology at UCLA. “But spiders are usually solitary, so we were excited to study these colonial spiders and how they interact with colonymates as well as individuals from other colonies.”

Orb-weaving spiders spin webs that are interconnected in vast webs anchored in the surrounding vegetation. Within the colonies, individuals protect their own webs from invaders and often fight each other for food and mates. They retreat to common areas for protection when threatened, and some species defend common areas.

Male wheel spiders fight less in female-dominated colonies

Nihal Punjabi, who recently graduated from UCLA, was one of four co-authors on the study. Credit: Gregory Grether/UCLA

The four student authors – Catherine Wu, Chaiti Bhagawat, Modan Goldman and Nihal Punjabi – participated in a field course led by Grether at the Cocha Cashu Biological Station in southeastern Peru. Their four-day journey to the field research station included a long and winding bus ride through the Andes and a boat ride down the Madre de Dios River and up one of its tributaries.

Over the course of 18 days, the students examined 34 colonies of a species called Philoponella republicana to find out whether the position of the web, the ratio of male to female spiders, or the size of the web or the spiders themselves affected the animals’ levels of aggression. Her work was overseen by Grether, as well as Debra Shier, an associate professor at UCLA, and Roxana Arauco-Aliaga, a social insect and spider researcher who was the research coordinator and associate director of Cocha Cashu at the time.

They observed spiders working together to build their webs and wrap prey in silk, although the spiders did not share their diet – only one spider ate a specific prey item.

Students staged burglaries by placing spiders in different locations and relocating spiders from other colonies. Some spiders defended their own web of orbs against all invaders, but spiders did not cooperate to repel invaders.

When colonies had more females than males, males fought with other males and females less with males than in male-dominated groups, resulting in somewhat more peaceful colonies.

Male wheel spiders fight less in female-dominated colonies

Cole Heramb (left), Catherine Wu and Chaiti Bhagawat observing wheel spiders in Peru. Credit: Gregory Grether/UCLA

However, when colonies had many large and medium-sized females, these females snatched the most prey and fought the most over captured insects, resulting in slightly more aggressive colonies.

After returning from the field, the students learned from previous research that group living among spiders is extremely rare – they are observed in less than 0.1% of species. Nevertheless, several types of sociality are recognized by arachnologists. Territorial spiders, which cooperate in prey capture, web building, and brood care, most closely resemble truly social animals like ants, honey bees, and naked mole rats.

Territorial colonial spiders, which cooperate in web building but also compete aggressively with other residents of their own colony for food and mates, appear to have evolved multiple times from solitary individuals, presumably when ecological conditions favored group life. They occupy a position on the social continuum similar to that of group-dwelling primates, including humans.

The student researchers all graduated from UCLA. Wu now 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 Carle Illinois College of Medicine and Case Western Reserve University, respectively.

Grether has been leading groups of UCLA students to tropical rainforests for field research training since 2001. He plans to return to Cocha Cashu to unveil more of his secrets with a new group of students in January 2023.

More information:
Catherine Wu et al., Aggression in a colonial spider in the western Amazon, Philoponella republicana (Araneae: Uloboridae), The Journal of Arachnology (2022). DOI: 10.1636/JoA-S-20-093

Provided by the University of California, Los Angeles

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