For nearly grown spiderlings, lingering in their mother’s web instead of setting off on their own turns out to be a boon for the mom, as well as themselves.
“This was fairly surprising,” says Thomas C. Jones of University of Tennessee in Knoxville. All those extra mouths could easily have been a drag on the mother.
Weeks before they leave home, youngsters of Anelosimus studiosus are perfectly capable of spinning their own webs. Yet they often hang around, feeding from the maternal web, occasionally even mating there. They do spinning but no child care if the mom lays a new clutch.
Despite the potential for stressing the mom, both she and the hangers-on fare better together than either generation does alone, Jones and Patricia G. Parker of the University of Missouri at St. Louis report in the January Behavioral Ecology. Jones says the study supports ideas that lingering at the home web might have led to the evolution of social spiders.
Out of the 32,000 known spider species, only several dozen always live in colonies (SN: 5/8/99, p. 300: http://www.sciencenews.org/sn_arc99/5_8_99/bob2.htm). Sometimes clustered by the thousands, these spiders defend and feed each other’s young and together attack prey.
To find the origins of sociality, researchers are studying habits of less clubby spider species. A. studiosus spins dense tangles up to the size of a football. These webs, containing singletons, families, or even small colonies, dot waterside shrubbery from Argentina to New England.
In the Everglades, Jones transferred dozens of A. studiosus females to fake branches that he had bought at a silk-flower shop. Females raised young in the branches, guarding egg sacs and regurgitating baby food.
Jones collected the youngsters when they were a little over halfway to adulthood. He set some up solo on their own fake branches and returned some siblings to the moms. Jones also left some of the mothers on their own.
Solo youngsters ended up with less food on average than their homebound siblings did. Jones suspects that the bigger maternal web ensnared more flying insects than single spiderling webs did and that extra legs at the mom’s web subdued prey that would have escaped a solo spiderling. Seven percent of singletons survived to adulthood versus 25 percent of those at Mom’s place.
The mothers surrounded by their offspring lived longer than solo moms did, 20 days after Jones’s rearrangement versus 12. The mothers amid the young also laid their second clutch sooner, in about 18 days instead of 28. Jones speculates that the difference came from youngsters’ improvements in the home web.
Deborah Smith of the University of Kansas at Lawrence says she wouldn’t necessarily have predicted the mother’s benefits. “It’s one of those things that after the fact, you say, ‘Duh, it makes sense,'” she notes.
Yael Lubin of Ben Gurion University in Sede Boker, Israel, says she didn’t expect such a maternal bonus, either. Now, she wants to know how such hang-around kids cope with the search for mates and the risk of disease in crowded webs.
The Stegodyphus spiders that Lubin’s team studies also have young that delay independence. However, just before they leave, they eat their mother.