Black widow spiders face extinction at the hands of their smaller but just-as-deadly cousins the brown widow, according to new research.
In the past couple decades, the notorious black widow has been disappearing from parts of the US as their close relative spreads – being outnumbered 20 to one in some areas.
It was believed they were being outcompeted for food or habitat. Now a more sinister reason has been identified by scientists investigating the trend.
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According to the study, in Annals of the Entomological Society of America, pairing brown widows in containers found they were 6.6 times more likely to kill southern black widows than other related species.
Lead author Louis Coticchio, a PhD student at the University of South Florida, said: "We have established brown widow behaviour as being highly aggressive towards the southern black widows, yet much more tolerant of other spiders within the same family.
"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 behavioural differences that were playing a role [in declining numbers]."
When young females of both species were placed together, the brown widows killed and consumed the black widows in 80% of cases.
In pairings of adults, black widows were killed in 40% of trials – and defensively killed brown widows in 30%.
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Throughout the experiments, brown widow spiders regularly ventured into black widow webs, the researchers say.
Co author Professor Deby Cassill said: "We didn't expect to find such a dramatic and consistent difference in the personalities of the brown widow and the black widow.
"Brown widows are boldly aggressive and will immediately investigate a neighbour and attack if there is no resistance from the neighbour.
"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 neighbour.
"The black widows are extremely shy, counterattacking only to defend themselves against an aggressive spider."
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