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Brown widow spiders are remarkably adept at seeking out and destroying their black widow spider relatives.

Female brown widow spider (left) and black widow spider (right)

Black widow spiders have earned a fearsome reputation for their venomous bite. But in parts of the southern United States these spiders have much to fear themselves—from their brown widow spider relatives who really don’t like their company. Researchers find that brown widows have a striking propensity to seek out and kill nearby black widows. In container habitats, brown widows were 6.6 times more likely to kill southern black widows than other related species.

The study looked at the potential drivers of brown widows displacing black widows. The researchers applied mathematical modeling to the risk factors to survival that brown and black widow spiders face, which showed that both species are far more likely to die by predation than by starvation. They also compared rates of growth and fertility between brown and black widows, finding that sub-adult brown widow females were 9.5 percent larger than black widows, and adult female brown widows reached reproductive maturity 16 percent sooner. While adult male brown widows were 25 percent smaller than adult male black widows, they reached reproductive maturity 21 percent sooner. Meanwhile, brown widow females were about twice as fertile as black widows, with brown widows often producing multiple egg sacs at a time versus black widows producing just one.

However, the most striking difference between the two species was their behavior towards each other. The study found that brown widows kill and consume black widows at a high rate, with black widows being killed in 80 percent of pairings with sub-adult brown widows. The researchers noted that throughout the experiments, brown widow spiders regularly ventured into black widow webs.

The researchers also looked at the behavior of the brown widows towards other spider species. They found that brown widows were much more tolerant of other spiders within the same family. For example, sub-adult brown widow females simply cohabitated with red house spider (Nesticodes rufipes) females in 50 percent of pairings and were killed and consumed by the red house spiders in 40 percent.

The researchers noted that brown widow venom causes less severe reactions to humans than black widows, and bites to people are very rare. They also pointed out that brown widow spiders’ evident aggression toward black widows raises many questions, perhaps first and foremost: Why? What drives such behavior toward a closely related species? The researchers note that invasive species typically outcompete natives through advantages in factors such as fertility, growth, dispersal, or defenses against predators. However, direct predation by an invasive species on its native relative, across the animal kingdom, is rare.

Overall, the study provides new insights into the fascinating behavior of brown widow spiders, which are believed to be native to Africa but have been introduced on all continents but Antarctica. It also raises new questions about the impact of invasive species and the factors that drive their behavior towards native species.

Louis Coticchio, lead researcher of the study, said,

“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. 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.”

Image Credits

In-Article Image Credits

Female brown widow spider (left) and black widow spider (right) via University of South Florida by Louis Coticchio with usage type - News Release Media

Featured Image Credit

Female brown widow spider (left) and black widow spider (right) via University of South Florida by Louis Coticchio with usage type - News Release Media


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