By Cameron Jones
When you think of spiders, the idea of them living or working together is probably the last thing that crosses your mind. After all, aside from when an egg sac has just hatched, we rarely see spiders together let alone interacting with each other. There is good reason for this: in spiders, group behavior, or what scientists call sociality is very rare. In fact, of the more than 45,000 species of spiders known, only about
80-90 species exhibit some sort of sociality. Despite its rarity, its existence makes spiders even more amazing.
Sociality is a phenomenon that occurs in many different animals, from lions to bees to humans. There are obvious benefits that are associated with being social like protection from predators, help catching food, and assistance in raising young. But there’s a reason why not all animals are social, because with sociality comes a lot of costs such as an increase in disease risk and competition. With carnivorous animals like spiders, sociality is exceptionally rare and for good reason: you could be living with a potential predator! Yet despite this risk, social spiders still exist and scientists have sought to determine just how they evolved this way.
Not all social spiders are web-builders, but most social spider species are. These spiders can have hundreds to thousands of individuals that live together in a web such as Anelosimus eximius. When prey gets snagged in a web, they pounce on the victim and subdue it, cooperating to bring down large prey. Species of these type of spiders rarely leave the web and hardly ever encounter spiders from other colonies. Another explanation for their sociality is the fact that the costs of allowing spiders to join the colony is usually low and therefore doesn’t lower the fitness of other spiders in the colony.
There are also a few non-web building spider species that are termed “subsocial” to describe the fact that offspring leave the nest retreat after reaching sexual maturity. Unlike the web-building spiders, these species can discriminate against non-kin and are very intolerant towards non-colony mates entering their retreat. A well-studied species is Delena cancerides, a large huntsman spider from Australia. Colonies of this species consist of a single matriarch and several cohorts of her own offspring. Because they don’t rely on a web, individuals in a colony will go out and hunt for food or grab whatever prey wanders by their nest which is behind bark of eucalyptus trees. However, their sociality extends beyond the fact that they live together. Spiders in a colony will also share prey with their younger colony mates! Currently scientists are trying to determine f how these spiders distinguish colony mates from unrelated intruders as well as the factors that lead to evolution of sociality in other non-web-building spiders. As for life in a D. cancerides colony, things aren’t all loving and happy. As the matriarch gets older and weaker, her mature daughters that haven’t dispersed will kill her and fight for control over the retreat. That just goes to show you, no family is perfect.
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Cameron Jones is a PhD student in the Animal Behavior Graduate Group at the University of California Davis.