Research Begins to Unravel Why Some Spiders are Social
By Fern Alling
A landmark study published in Molecular Phylogenetics and Evolution in May 2022 combined extensive genetic and life history data to explore how sociality evolved in huntsman spiders. The findings set the stage for further research on the evolution of social behavior.
Of the over 50,500 known species of spiders, approximately 120 species exhibit some level of sociality. That amounts to 0.24 percent of all spider species. Scientists like Linda S. Rayor, Ph.D., a senior lecturer and senior research associate in the Department of Entomology at Cornell University, want to understand why sociality has evolved in these unique species. “Whatever concessions spiders are making in order to live in groups I think are just interesting on the face of it,” Rayor says.
Genetic Origins of Huntsman Sociality
Spiders in the family Sparassidae are often known as huntsman spiders (or “sparassids,” deriving from their taxonomic family). To understand how some of these spiders became social, Rayor and fellow researchers first had to understand the genetic relationship between social and solitary sparassids. They used genome sequences to create a phylogenetic tree, a diagram that shows the evolutionary relationships between different taxonomic groups and species. With the help of Cornell colleagues Corrie Moreau, Ph.D., and Manuele de O. Ramalho, Ph.D., undergraduate assistant Jacob Gorneau collected 70 genome sequences from field samples of huntsman spiders. Another 201 sequences came from a resource called GenBank. Gorneau, now a graduate student at the California Academy of Sciences, then sequenced two mitochondrial genes and two nuclear genes for each genome, and the team used the resulting data to assemble the phylogenetic tree.
This analysis found four independent origins of sociality in huntsman spiders: two separate origins of subsociality in the genus Isopedella and an origin of prolonged subsociality in both Delena and Damastes. Prolonged subsociality, a concept first defined by Rayor, is a form of spider sociality in which a single adult female shares a retreat with multiple generations of her offspring. Depending on the species, the spiderlings will stay with their mother for five to 12 months. During this time, the siblings tolerate each other and even share captured prey.
Life History Traits and Social Behavior
The detailed phylogenetic tree allowed the researchers to track where life history traits conducive to sociality first appeared. Life history traits are defining features of an organism’s life cycle, from gestation periods to life expectancy to the number of offspring. Prolonged subsocial species, for instance, were more likely than solitary species to establish retreats under bark and tightly secure their egg sacs to the retreat itself. “The associated life history traits of the prolonged subsocial species are suitable for facilitating large interactive groups of growing spiderlings that remain in their natal retreat for many months,” Gorneau and colleagues write in their report, “while those of the solitary species are not.”
Another life history trait associated with prolonged subsociality was the age hatchlings eat. Prolonged subsocial species start feeding in their second instar rather than right away. Delena gloriosa, the only solitary Delena species included in the study, shares many socially oriented life history traits with other Delena species but begins eating in the first instar. The study’s authors theorize that Delena gloriosa has therefore secondarily lost sociality. Interestingly, while prolonged subsocial sparassids tended to be larger when they started to feed, the study did not find prolonged subsociality associated with a larger body size or longer lifespan.
Why Be Social?
If prolonged subsociality doesn’t produce larger or longer-lived spiders, why did it evolve in two separate genera? Rayor, who is the senior author on the study, is working on a forthcoming study that may shed some light on the question. She has regularly observed the prolonged subsocial huntsman spiders sharing prey in the lab. Though it’s difficult to catch prolonged subsocial huntsman spiders eating in the wild, discarded prey parts inside their retreats are a strong indication that prey sharing is still taking place. Almost all of the huntsman spiders she’s studied share prey at some age, Rayor says. But “if [spider hatchlings are] social and … tolerant, [they] start sharing house flies, [they] share crickets. And if older siblings aren’t eating [their younger siblings], older siblings start to share larger prey with [them] … so they’re getting access to much larger prey … not just at the smallest size, but all the way through at least their adolescence,” she says.
Despite the study’s size, more work remains. Thirty seven of the 89 described huntsman genera were included in the study, and questions about the exact relationships between different groups and species persist. It’s also worth investigating how prolonged subsociality is even possible in sparassids given that they do not use webs to capture prey. Other species of social spiders form interconnected prey-capture webs or share a web, communicating with each other through the vibrations in the web. Huntsman spiders, on the other hand, are wandering hunters or live in retreats rather than webs.
Benefits for Entomology and Beyond
The study’s benefits aren’t limited to entomologists, however. “With varied life history, behavioral, physiological, and developmental traits, sparassids represent a model clade,” the authors write. “This diversity makes phylogenetic study especially informative for understanding the evolution of social behavior.”
Gorneau hopes learning about social behavior in spiders will inspire curiosity in people who otherwise wouldn’t engage with arachnids. It’s “just incredible to be able to reach people who might not be interested in [spiders], or at least promote some sort of appreciation for them, even if you’re scared of them,” he says.
Molecular Phylogenetics and Evolution
Update, December 1, 2022: The original version of this article listed incorrect numbers of total known spider species and the subset that exhibit social behaviors. The post has been updated with the most current numbers.