Spiders are often viewed as creatures of solitude, silently weaving their intricate webs or lurking in hidden corners. This perception largely holds true for the vast majority of spider species across the globe. However, within the diverse world of arachnids, there are fascinating, albeit rare, instances where spiders exhibit collective behaviors, challenging the common understanding of their social lives.
The Solitary Nature of Most Spiders
Most of the approximately 50,000 known spider species lead solitary lives, due to biological and behavioral traits favoring individual existence. A significant factor is territoriality, where a spider defends its web or burrow against intruders. This behavior can be intense, with confrontations escalating into aggression. Such interactions are pronounced when resources, like prey or suitable web-building sites, are limited.
Another deterrent to group living is the risk of cannibalism. Spiders are opportunistic predators, and conspecifics can become prey. This occurs between sexes, especially after mating, or between individuals of similar size, driven by hunger or territorial disputes. Individual hunting strategies, such as ambush predation, active pursuit, or the construction of individual webs like orb-webs or funnel webs, suit a solitary existence, allowing each spider to secure sustenance without competition. Even when many spiders are observed in close proximity, such as a field teeming with orb weavers, they typically maintain individual territories and do not cooperate, merely coexisting in a resource-rich environment.
True Social Spiders: A Rare Exception
While most spiders are solitary, a small fraction, estimated at 25 to 60 species out of over 50,000, display true sociality. True sociality in spiders is characterized by several behaviors:
Cooperative prey capture.
Shared maintenance of a communal web.
Collective care of offspring (brood care).
Overlapping generations living together within the same colony.
These characteristics distinguish truly social spiders from those that merely aggregate, or live communally, where individuals might share a large web structure but still hunt and raise their young independently.
Examples of social spiders include genera like Anelosimus, Stegodyphus, and Delena. Some Anelosimus species form colonies with thousands of individuals. These social species construct large, complex communal web structures that can span several meters, covering entire bushes or trees. The development of social behavior is thought to have evolved independently multiple times across different spider families, suggesting it offers advantages under specific environmental conditions.
Benefits of Group Living for Spiders
The rare evolution of group living in spiders is driven by advantages that enhance survival and reproduction. One primary benefit is the collective ability to capture larger prey. While a single spider might be limited to smaller insects, a coordinated group can subdue prey many times their individual size, leading to a consistent, substantial food supply for the colony. This cooperative hunting often involves multiple spiders attacking and injecting digestive enzymes into the prey, which is then shared among colony members.
Group living also provides defense against predators. A large communal web can act as a physical barrier, and the sheer number of individuals can deter potential threats. Colonies may collectively respond to disturbances, increasing the chances of survival for the group. The maintenance of large communal webs becomes more efficient with multiple contributors, as individuals share the task of silk production and web repair. This cooperative effort ensures the integrity of the shared habitat and foraging structure.
Communal brood care, where multiple females contribute to raising the young, can lead to higher survival rates for spiderlings. They benefit from shared protection and feeding, including regurgitated food from multiple adults.