Black widow spiders have long inspired fear for their potent venom, but in the southern United States, they’ve met a surprising threat—one from their own family.
The brown widow spider, a close relative, is rapidly displacing black widows across parts of the region, and new research reveals that aggression, not competition for food, may be the cause.
Researchers at the University of South Florida (USF) found that brown widow spiders don’t just outcompete black widows—they actively hunt and kill them.
In controlled experiments, brown widows were more than six times likelier to attack and kill southern black widows than other related species. The findings, published in Annals of the Entomological Society of America, point to behavior-driven domination.
An Aggressive Invader Takes Hold
Brown widows (Latrodectus geometricus) are believed to have originated in Africa but now inhabit every continent except Antarctica. Their North American cousins, the black widows (Latrodectus mactans and Latrodectus hesperus), are native to the United States and are among the most recognizable spiders in the world.
Louis Coticchio, who led the study as an undergraduate researcher at USF, noticed an unusual pattern while collecting spiders in Florida. Brown widows were replacing black widows but leaving other cobweb species alone.
“It seemed like food or habitat wasn’t the issue,” he explains. “Behavioral differences appeared to be the key.”
Digging Into the Science
Coticchio collaborated with USF biologist Dr. Deby Cassill and spider expert Richard Vetter from the University of California, Riverside, to explore why brown widows were so successful.
Their research combined behavioral experiments, population modeling, and measurements of growth and fertility rates to identify what gave brown widows an edge.
Mathematical modeling revealed that both black and brown widow spiders were far more likely to die from predation than starvation. This finding suggested that the competition for food was not the driving factor behind black widows’ decline. Something else—most likely aggression—was shaping the outcome.
Fast Growers and Fertile Hunters
When comparing development, the brown widow proved to be the more efficient breeder. Sub-adult female brown widows were about 9.5 percent larger than their black widow counterparts and matured 16 percent faster.
Male brown widows reached adulthood 21 percent sooner, and females produced roughly twice as many egg sacs. Faster growth and greater fertility make for a powerful survival advantage.
However, the biggest difference came from behavior. When paired in controlled habitats, brown widows killed and consumed black widows in 80 percent of sub-adult pairings.
Adult brown widows killed black widows in 40 percent of encounters, while black widows defensively killed brown widows in only 30 percent. In many cases, the brown widow initiated the attack.
Predator or Bully?
Throughout the trials, brown widows boldly entered black widow webs—something black widows never reciprocated. When paired with other species like the red house spider or triangulate cobweb spider, brown widows were far more tolerant, often sharing space without aggression.
The black widows’ shy, defensive nature made them particularly vulnerable.
“We didn’t expect to find such a dramatic difference in personality,” says Cassill. “Brown widows are bold and investigative, while black widows are extremely shy, responding only when attacked.”
This boldness likely explains how brown widows so easily invade black widow territories.
No Threat to Humans
Despite their aggression toward other spiders, brown widows pose little danger to people. Coticchio emphasizes that both species avoid humans when disturbed. “They’ll play dead or roll up into a ball,” he says. Additionally, brown widow venom is milder than that of black widows, and bites are rare.
Still, the behavioral aggression between these species raises intriguing evolutionary questions. Why would one species target its close relative so persistently? Across the animal kingdom, direct predation between close relatives is uncommon.
The researchers hope future studies will explore whether this hostility is unique to North American populations or present in the brown widow’s African range as well.
The Mystery of the Widow Wars
For now, one thing is clear: the brown widow’s rise is not just an ecological coincidence—it’s a behavioral takeover. As these bold invaders continue spreading through the southern U.S., black widows are being pushed out of their long-held habitats.
Understanding this “widow war” may shed light on how subtle behavioral differences can transform entire ecosystems.
FAQs
Why are brown widow spiders replacing black widow spiders?
Research shows brown widows are more aggressive than black widows. In controlled studies, they actively attacked and killed black widows in most encounters, suggesting behavioral dominance—not competition for food—is driving their displacement.
Are brown widow spiders dangerous to humans?
No. Brown widow venom is milder than that of black widows, and bites to humans are extremely rare. These spiders usually avoid confrontation, preferring to hide or play dead when disturbed.
Where did brown widow spiders come from?
Brown widows (Latrodectus geometricus) are believed to have originated in Africa. They’ve since spread globally and now live on every continent except Antarctica, thriving around human-made structures.
What advantages do brown widows have over black widows?
Brown widows mature faster, reproduce more frequently, and lay multiple egg sacs at a time. Their bold behavior and higher fertility give them a clear survival advantage over black widows.
Do black widows ever fight back?
Black widows are typically shy and defensive. They rarely initiate attacks, only countering when threatened. This timid nature makes them vulnerable when brown widows invade their territory.
