If I asked you to describe spiders, I bet “social” would not make your list of adjectives. Spiders are, after all, notorious cannibals as likely to eat potential mates as, well, mate with them. Yet some spiders are as social as bees or ants or people.
It’s true, social spiders are the exception. Out of 40,000+ species worldwide perhaps 60 species show some social behavior and only 25 of those are considered truly social. Typically, a social spider colony consists of a communal residential web with up to thousands of adults who collectively share the duties of capturing prey and raising baby spiderlings.
Oh great, you are no doubt thinking, nests of thousands of cooperating spiders, just what the world needed. Don’t worry, they live in tropical locations that few of us will ever visit, although we do share our homes with a close relative. Over half of the social spider species belong to the Theridiidae, or cobweb spider family, a family which includes the tiny American house spider. This small spider lives in houses all across North America, but at a quarter inch long you may never see one. Their social cousins live in places far more exotic, like Brazil or Australia.
Researchers like Ingi Agnarsson, Assistant Professor and Curator of Invertebrate Collections at the University of Vermont, trek all the way to the tropics to study these unique creatures. Spider sociality raises some interesting questions, including: how can such notoriously aggressive animals live together, nevermind cooperate?
Spider sociality is likely an extension of maternal care. Cannibalistic tendencies aside, many spiders are dedicated mothers. Female spiders commonly guard their egg cases carefully, some carry their spiderlings around on their backs, and others commit a kind of suicide by liquefying their innards so they can be easily eaten by their children. This extreme show of motherly affection is known as matriphagy.
Spiderlings who never leave home, but instead hang around the web to help raise their younger siblings, are one more step along the extended parental care spectrum. By never leaving their birth web, such spiderlings contribute to a permanent and multi-generational social spider nest.
Researchers have found that young spiderlings living with older siblings grow faster, as they gain access to better prey they could never catch themselves. Little spiderlings eat so little that their older siblings aren’t put out by sharing meals.
Social spiders have provided insights about the benefits of communal living, including how tasks are shared among individuals. In some species, tasks within the group are assigned by personality type: larger, bolder spiders go out and capture prey while their more timid comrades stay home and care for the young spiderlings.
Yet if social living has clear benefits, why are there so few social spider species? Dr. Agnarsson suspects sociality, for spiders, is a dead end. When they shift to cooperative colonies, they stop dispersing to mate, as bees and ants do, which results in highly inbred populations. These inbred populations lack the genetic diversity to respond to environmental challenges over the course of evolutionary time and eventually they disappear.
The next time you sweep up the cobwebs left in the corners of your house by the tiny American house spiders hiding in your home, remember their unusual tropical cousins, living socially in communities as fragile as those webs you’re brushing away.
Agnarsson, I. (2014). Social Spiders. Retrieved from the Agnarsson Lab website: http://www.theridiidae.com/
Drake, N. (2013). Spiders may have personalities, and some are bolder than others. Retrieved from the Wired website: http://www.wired.com/wiredscience/2013/07/social-spiders/
Ramanujan, K. (2013). For social spiders, preying together aids younger siblings. Retrieved from the Cornell website: http://www.news.cornell.edu/stories/2013/05/social-spiders-preying-together-aids-younger-siblings
Bilde, T. (2014). Social Spiders. Retrieved from the Spiderlab website: http://www.spiderlab.dk/social-spiders.html
Trageser, S. Evolution of Sociality in Spiders (Theridiidae). Retrieved from the University of Arizona website: http://eebweb.arizona.edu/faculty/dornhaus/courses/read_ECOL597S/Scott%20Evolution%20of%20Sociality%20in%20Spiders.pdf