Spider Solidarity: Scientists Discover New Species With Unprecedented Social Behavior
By Carolyn Bernhardt
Scientists have discovered a new species of spider in Madagascar that exhibits social behaviors unlike those of any other spiders currently known to science. In an article published in February in Insect Systematics and Diversity (ISD), a team of researchers reports stumbling upon a species of kite spider forming colonies of up to 41 interconnected, single-cohort adult female webs with up to 38 adult males aggregating on a central, single, nonsticky line.
Ingi Agnarsson, Ph.D., professor of zoology at the University of Iceland, says the team found the spiders by happenstance. “We were on an expedition looking for bark spiders,” he says. “We were traveling in a vehicle to a different part of the national park we were working in and noted, from the car, some webs that looked like colonies of multiple spiders.”
The researchers got out of their car to take a closer look and found that, within colonies of interconnected webs formed by adult female spiders, there were multiple male spiders sitting peacefully, next to one another, on a line in the center. The researchers continued walking and driving along that same road where they made their initial discovery and found several more colonies in a similar arrangement. In reporting their observations and describing the new species in ISD, Agnarsson and colleagues name the species Isoxya manangona, noting that “manangona” derives from the Malagasy verb for “gather” or “aggregate.”
Spiders are notoriously solitary and cannibalistic. Of the 50,000 described spider species, only about 0.1 percent exhibit colonial or social lifestyles. Previous population analyses of such species have shown that most colonies consist of multiple cohorts formed by close relatives. Territorial social spiders sometimes form colonies by interlinking their webs but rarely interact beyond that. And, if they do, interaction typically only happens among juveniles or, more rarely, adult females. Male spiders congregating for any reason other than to directly compete has never before been noted in published academic science.
The researchers say the behavior they noticed in these kite spiders suggests a lekking mating system, in which males gather in a group (called a “lek”) and perform displays for females to choose from. Lekking has been observed in other insects and animals but never before in spiders.
While the Isoxya manangona males were nestled tightly together on their line through the center of the colonies, the scientists did not find any evidence of aggression among them. The team also conducted genetic analyses and found that genetic variability of the males was somewhat less than that of the females, but most colonies consisted of unrelated individuals.
Finding these spiders in a rather disturbed part of a nature reserve has conservation implications, though. Researchers need to know more about the distribution and abundance of the Isoxya manangona kite spiders, as well as understand what conservation efforts the spiders need.
While the team did not see any males actively making a mating display to signal the females, Agnarsson says spiders have poor eyesight, so visual signals are unlikely. “They may be vibratory or through chemicals like pheromones,” he says. The team also did not see any females “choose” a male. So, many key questions remain.
“We are simply hypothesizing that these spiders show lekking based on the available evidence,” Agnarsson says. “We did not have a chance to actually observe mating in the field.” Still, the implications are that this peculiar arrangement of spiders suggests a unique mating system that scientists have not reported in published research before.
“Male spiders are practically always antagonistic towards one another,” Agnarsson says. “So, what may be the mechanisms that facilitate them tolerating one another, especially if they are in competition for mating with the females? Further research is necessary but is bound to find something fascinating.”
Insect Systematics and Diversity