Black widow spiders are known to be poisonous and scary, but turns out there is something even worse roaming around in the southern US. A new study found that brown widow spiders are actually actively hunting down their black widow relatives and killing them!
Recently, scientists have noticed that black widow spiders are being replaced by brown widows in the same family. However, a new study shows that this isn’t just because one species outcompetes the other for food or where it lives- instead, brown widow spiders actively look for and attack nearby black widow spiders.
researchers at the University of South Florida (USF) put brown widow spiders and related cobweb spider species in a container together. In the experiment, the brown widows were much more likely to attack and kill southern black widows than any other types of spiders. This research was reported in an article on March 13th in 2023 which will be published in ‘Annals of the Entomological Society of America’.
Louis Coticchio, a researcher from the University of South Florida, did a study and discovered that brown widows are very aggressive towards southern black widows, yet much more calm around other spiders within their same family.
Brown widow spiders come from Africa, but they can be found all over the world except Antarctica. Black widow spiders are native to North America and they have two types – a western variant and a southern variant.
Investigating the Surprise Takeover of Black Widows by Brown Widows in Florida
Coticchio used to be a zookeeper that worked with dangerous animals in California. Later, he decided to go back to Florida and get an education in biology. His favorite animal was spiders and so he wanted to do research on them. While collecting wild spiders in Florida, he noticed brown widows replacing black widows but not any of the other kinds of spiders (but nothing else). This made him wonder why this was happening.
I thought Florida had plenty of food and places to live for both brown and black widow spiders. But I found out that there was something else going on too, like differences in the way they acted. My research showed that brown widows were more able to live with other types of spiders that also wanted their food and homes. This made it look like resources weren’t the only factor in how these different spiders get along.
Joseph Coticchio worked with an assistant professor, Deby Cassill and a spider expert, Richard Vetter both from the United States of America, to formulate a research study. The purpose of this study was to work out why brown widows are taking over the places where black widows used to live.
Scientists did some research on black and brown widow spiders to see what things were most likely to cause their deaths. The study showed that these types of spiders were more likely to be killed by a predator (like another animal) instead of running out of food and starving. This means that it is not hard for them to find food, so they don’t usually die from having nothing to eat.
Researchers compared the size and fertility of brown and black widows. They found that brown widow females were larger and grew faster than black widows, reaching maturity quicker too. Brown widow males were smaller, but they reached maturity more quickly too compared to the male black widows. On top of that, while a female black widow only hatches one egg sac at a time, female brown widows can lay multiple egg sacks all at once!
To better understand how spiders interact with each other, scientists put brown widows close to black widows and some other kinds of spiders. With sub-adult female brown widows and red house spider (Nesticodes rufipes) females, the former shared the same space with the latter half the time, while being eaten by them in 40% of trials. When they studied Brown widow and triangulate cobweb spiders (Steatoda triangulosa), 80% of pairings ended peacefully – 10% were fatal for the Brown widow. But when a sub-adult black widow was paired with a Brown widow, it got eaten more than 80% of the time. Now for adult pairings: 40% of Black widow were killed whereas 30% of Brown Widows were killed and in 30%, both coexisted peacefully.
The researchers said that during their experiments, brown widow spiders often went into black widow webs. Red house spiders and triangulate cobweb spiders also had this “brave” behavior, however they never saw black widows being agressive.
Does Native Behavior of Brown Widows Explain Why Black Widows Are Disappearing?
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Researchers have found a big difference in the personalities of brown widows and black widows. Brown widows tend to be brave and attack strangers if they don’t back away fast enough. But when two bold spiders end up close together, they’ll eventually get used to it. Black widows are way shyer, normally only attacking when they need to protect themselves against an aggressive spider.
Brown widow spiders are known to be aggressive, but not towards humans. They live near our homes in places like barns, garages, and sheds, but if a human or animal bigger than them tries to attack them, they can get scared and try to run away or even play dead. Even though their venom is less strong than that of black widows and bites to people are rare, it’s important to still be careful around them.
Brown widow spiders’ unexpectedly aggressive behavior towards black widows has scientists really puzzled. They are trying to figure out why this is happening. Usually when a new species enters an area, it out competes existing species by doing things like producing more babies, growing faster, having better ways of spreading around and protecting itself from predators. But in the animal kingdom, we rarely see one species directly killing another related species like what is going on with the two widow spiders here.
Coticchio wants to know if brown widows (a type of spider) act differently around other species, like black widows, in Africa where they originally come from. He is wondering if the things they do there are the same as what they do here in North America, or if it’s something different that they only do when living with black widows for longer periods of time.
Scientists recently published a new article in the Annals of the Entomological Society of America about how introduced brown widow spiders can cause native black widows to disappear in urban habitats. This research explains why some areas no longer have any black widow spiders living there.
