Among spiderdom's violent loners, a few species develop family life.
|Tiny Delena huntsman spiderlings live with mom from the time they leave their big white egg sac until they start hunting on their own, which is unusual among loner spiders.© LINDA S. RAYOR|
Among the many species of velvety Delena huntsman spiders, four from Australia exhibit what is unusual for their species: a mother and her live-in offspring. "Cannibalism may occur on occasion, but with Delena cancel rides, it almost never occurs," says Cornell University behavioral ecologist Linda Rayor, who has studied them in the wild and in her lab for 20 years.
The oddity here is that they don't eat their own kind: Cannibalism varies greatly between spider species, according to Rayor, but it's mostly a move of solitary spiders if they meet outside of flirting or baby-guarding demands (SN: 4/25/22).
However, a small number of Delena species generally tolerate their own kind. Rayor's only exception in D. cancerides is a wild-caught female taking down a full-grown male still in her cage after she lays eggs. "Wild-caught females are very intolerant of males who stay around for an extended period of time," she says.
The shared family life of these spiders is truly bizarre, at least in the arachnid world. Only about 80 of the more than 50,000 known spider species are truly social, according to biologists. Hundreds or even thousands of females spin a great airy silken city in the most complex, where some stay their entire lives. Rayor, on the other hand, claims that "my huntsman don't do that."
Some biologists disagree that the Delena spiders Rayor studies are fully social — no spun-silk Sydney. However, these spiders are not truly solitary. A female does not spin a web but instead seeks refuge in a crevice, such as behind peeling bark on an acacia tree or beneath a slab of rock. The bodies of these crevice dwellers appear unusually flattened in profile: "a credit-card spider," one scientist called the species, with poetic license.
When a woman is successful in finding real estate, her children can stay for months in her magnificent mom cave — the children who do not yet have a sex life, that is. Never mind that older offspring are already out hunting alone at night, or that one or two later clutches of babies hatch before the older ones. Spider mothers are frequently seen guarding poppyseed-sized hatchlings. In the arachnid world, having capable, nearly grown offspring is unusual.
D. cancerides, the most sociable of these kind-of tolerant moms, allows a group of young to hang out at the house for about a year after hatching. That's a long time for spiders, who only live two and a half years. One of the reasons for doing so may sound familiar: finding suitable housing can be difficult.
Rayor used to believe that hanging out with mom was a uniquely Australian experience, but she has since learned about tolerant moms from Madagascar in the Damastes species. Jacob Gorneau, now at the California Academy of Sciences in San Francisco, and his Cornell colleagues used four genes from spiders in 37 huntsman genera to create the most ambitious genealogical tree of the family yet. Rayor then looked through her decades of data to see what behaviors appeared on branches that led to lingering children.
The researchers report in the September issue of Molecular Phylogenetics and Evolution that a type of egg sac called "plastered" — not drunk but spun like a firmly anchored splat on a surface of spider retreats — appears in branches with family-tolerant behavior.
Spiders spin a variety of outer sac shapes that deter egg raiders. Unlike some other moms who carry eggs, spiders in a fixed retreat can easily anchor a sac. Also, Rayor speculates that the more spread-out, the plastered form might fit better in tight crevices than the puffier throw-pillow styles of egg sacs.
When Delena spiderlings clamber out of the egg sac, they still have an abundance of egg yolk with them. They do not eat until they have completed their first molt. "They have these big, fat, green abdomens," Rayor observes. "Because they don't have long legs, they waddle."
One advantage of this family lifestyle may be table scraps for the littles once they molt. Young spiderlings are "ridiculously small," according to Rayor, and can only handle food the size of a fruit fly. The opportunity to steal extra meat shreds from an older sibling could significantly improve nutrition. Older siblings may prefer not to share, but Rayor sees "tolerated theft."
Cute babies may not matter to a spider mother, but they may to humans. Gorneau, who was once "very afraid of spiders," wants to dispel the myth that spiders are aggressive dangers. Rayor's spiders strike him as "stoic," sitting quietly most of the time. Rayor's lab, which is full of spiders in their homes, is now a "calming environment."
Reference : doi: 10.1016/j.ympev.2022.107530