In certain regions of the southern United States, the venomous bite of the black widow spider has instilled a sense of fear in many. However, it seems that these notorious arachnids may have their own reasons to be afraid. Spider relatives in these areas have been observed displaying a strong aversion to black widow spiders, suggesting that they may be more formidable than their reputation implies.
Scientists have observed a trend over the last few decades wherein brown widow spiders, a closely related species of the black widow, seem to be taking over their habitats. However, recent research indicates that this isn’t a straightforward competition for resources between the two species. Instead, a study has found that brown widows have a remarkable ability to actively track and eliminate black widows that are nearby.
The study, published in the Annals of the Entomological Society of America today, reveals that brown widows have a strong tendency to kill southern black widows over other related species.
The researchers found that in container habitats, brown widows were 6.6 times more likely to attack and kill southern black widows than other related species. These findings suggest that brown widows may have developed a specialized strategy for eliminating their black widow counterparts.
“We have established brown widow behavior as being highly aggressive towards the southern black widows, yet much more tolerant of other spiders within the same family,” remarks lead author Louis Coticchio.
Although brown widow spiders (Latrodectus geometricus) are thought to have originated in Africa, they have been introduced to every continent except Antarctica. Meanwhile, black widow spiders, which consist of two closely related species – the western black widow (Latrodectus hesperus) and the southern black widow (Latrodectus mactans) – are native to North America.
Scientists are now questioning what factors may be driving the displacement of black widow spiders by their brown widow counterparts.
Louis Coticchio, the researcher who led the study on brown widow spiders’ aggression towards black widows, has an interesting background. He began his career as a zookeeper in California, specializing in venomous animals. However, he later returned to Florida to pursue a degree in biology and turned his passion for spiders into research projects.
While collecting wild spiders in Florida for his studies, Coticchio observed that brown widows were outcompeting black widows, but not other related species. This piqued his curiosity and led him to delve deeper into the relationship between these arachnids.
“I had a sneaking suspicion that Florida in particular provided plenty of food and habitat for both the brown and black widow, and that there was possibly some other area such as behavioral differences that were playing a role,” he adds. “My observations in the field showed that brown widows appeared to be much more tolerant of other species outside of their genus, and so if resources were the main factor, then we should have seen the same behavior with other spiders competing for the same resources, but that did not seem to be that case.”
Louis Coticchio collaborated with his advisor, Deby Cassill, an associate professor in the Department of Integrative Biology at USF, and spider expert Richard Vetter from the University of California, Riverside, to conduct a comprehensive study on the potential reasons behind brown widows’ displacement of black widows.
Their three-part study involved mathematical modeling to analyze the risk factors that both species faced, and the results revealed that predation was a much greater threat to the survival of both species than competition for limited resources. As the researchers put it, “competition for scarce resources is not a significant cause of mortality among spiderlings for either species.”
In addition to analyzing the risk factors faced by brown and black widow spiders, the researchers also looked into the rates of growth and fertility for both species. Their findings indicated that sub-adult female brown widows were 9.5 percent larger than black widows, and adult female brown widows reached reproductive maturity 16 percent sooner than their black widow counterparts.
On the other hand, adult male brown widows were 25 percent smaller than adult male black widows, but they reached reproductive maturity 21 percent sooner. Additionally, brown widow females were about twice as fertile as black widows, with brown widows often producing multiple egg sacs at a time, while black widows typically only produced one.
To further confirm their findings, the researchers conducted experiments placing brown widows in close proximity to black widows and other spider species, which yielded the most conclusive results.
Sub-adult brown widow females were placed in container habitats with red house spiders (Nesticodes rufipes) females in 50 percent of pairings. Out of these, 40 percent were killed and consumed by the red house spiders.
In another set of experiments, brown widows were placed in container habitats with triangulate cobweb spiders (Steatoda triangulosa) in 80 percent of pairings. Surprisingly, only 10 percent of the brown widows were killed by triangulate cobweb spiders.
However, when sub-adult brown and black widow females were paired, the brown widows killed and consumed the black widows in 80 percent of pairings. When adult brown and black widow females were paired, black widows were killed in 40 percent of trials. In 30 percent of trials, black widows killed the brown widows in self-defense, while the remaining 30 percent of pairings resulted in cohabitation.
During the experiments, the researchers observed that brown widow spiders frequently invaded the webs of black widows. Red house spiders and triangulate cobweb spiders also exhibited similar bold behavior, but black widows were never observed as the instigators of such behavior.
Surprises and New Questions
“We didn’t expect to find such a dramatic and consistent difference in the personalities of the brown widow and the black widow,” Cassill adds. “Brown widows are boldly aggressive and will immediately investigate a neighbor and attack if there is no resistance from the neighbor. For two bold spiders, the initial attack is often resolved by both individuals going to separate corners and eventually being OK with having a nearby neighbor. The black widows are extremely shy, counterattacking only to defend themselves against an aggressive spider.”
According to the researchers, the term “aggressive” used to describe brown widow spiders is relative and refers only to their behavior toward black widow spiders, not humans.
Although widow spiders are often found near human-made structures like barns, garages, and sheds, they are typically very timid and avoid confrontations with humans or larger animals that are not considered prey, according to Coticchio.
“They will run or roll up into a ball and play dead when being attacked or harassed by most other animals outside of their prey range.”
The brown widow spider’s tendency to display aggression towards the black widow spider raises several inquiries, one of which being the reason for this behavior towards a closely related species. It is worth noting that brown widow venom causes less severe reactions to humans compared to black widow venom, and human bites are uncommon.
According to the researchers, invasive species generally dominate native species by excelling in areas such as fertility, growth, dispersal, or defenses against predators. Predation of a native species by an invasive one is uncommon across the animal kingdom.
“One question I would love to answer is how brown widows interact with other species of spiders, more specifically black widows in Africa, where brown widows are believed to have originated,” Coticchio adds. “I would love to see if their behavior and displacement of black widows is something that they have adapted here in North America, or if this behavior is something they exhibit naturally even in areas where they have coevolved with black widows for much longer periods of time.”
Source: 10.1093/aesa/saad003
Image Credit: Getty