Male Spiders Fight Less When There Are More Females Around

Sometimes, it’s all about keeping the peace.

Male orb-weaving spiders fight less with each other when there are more female spiders in the colony than males, research has found. Females also spar less in female-dominated colonies than in male-dominated ones.

When female spiders are dominant, spiders also are less hostile toward spiders from other colonies. The result is a more peaceful environment all around when there are more females.

For the past two decades, senior author Gregory Grether, a professor of ecology and evolutionary biology at UCLA, has been taking undergraduate students to tropical rainforests to teach them how to study wild animal behavior in their own environments.

“But instead of assigning them research projects, I challenge the students to come up with the project ideas, literally sending them out on the trails (in pairs) to look for interesting and tractable research subjects,” Grether tells Treehugger.

“On this particular trip to the Peruvian Amazon, two pairs of students noticed the spider colonies and identified some interesting and experimentally approachable questions. In short, the students' curiosity was the impetus.”

Researchers know that species like honeybees and chimpanzees live in cooperative societies. But spiders typically live solitary lives.

Grether says that by some estimates, less than 0.1% of spider species live in communal webs, so the spiders they found were quite unusual.

Although he points out that there are around 45,000 described spider species, so even that tiny percentage includes several hundred species.

There are different ways that spiders can show social traits.

“The so-called ‘social’ spiders, which cooperate in multiple ways including brood care, have attracted the most attention in animal behavior because helping other individuals reproduce is considered altruistic. (How altruism evolves has long been a controversial topic.)” Grether says. 

“There have been relatively few studies on territorial colonial spiders, and some of the questions we were able to answer had not been answered previously for spiders with this type of sociality. For instance, we were able to show that, unlike some social insects like ants, these spiders do not discriminate between colony members and spiders of the same species from other colonies. Perhaps this is because, unlike ant colonies, spider colonies are not in competition with each other for food.” 

Why Size and Sex Matter

For their research, students studied 34 colonies of the spider species Philoponella republicana over a period of 18 days. They noted whether the web’s location, the ratio of male to female spiders, or the size of the web or the spiders had an impact on how aggressive they were.

They found that the two factors that had an impact on how much fighting occurred were the ratio of males to females and the size of the spiders in the colonies.

“As in many other animal species, males compete with each other for matings and females get harassed,” Grether says. “In the spiders, females are larger than males and often respond aggressively when males approach them. Colonies with proportionally fewer males are more peaceful presumably because the males don't have to compete with each other so intensely and females are harassed less frequently.”

Female spiders compete for higher positions in the community and over large prey.   

“Larger females are more likely to resort to aggressive tactics, such as snatching prey or attempting to take over an orb web by force, presumably because they are more likely than small females to get away with it,” Grether says. “Consequently, colonies with larger numbers of large females have higher overall levels of aggression.”

The results were published in the Journal of Arachnology.

No Sharing Food

Although the spiders were found to cooperate in many ways in the colony, they didn’t share food. For example, sometimes spiders would help each other wrap up an insect in silk, but in the end, only one spider would get to eat it.

“Similar kinds of interactions have been observed in other animals, and in some cases, it is a ‘you scratch my back now, I’ll scratch your back later’ type of arrangement,” says Grether, noting how baboons groom each other or vampire bats share blood.

“Whether the spiders are capable of that kind of social reciprocity, I have no idea, but it would be an interesting question for further research.”