Black widow spiders in the U.S. are being killed off rapidly by invasive brown widows—but scientists are not sure why. Brown widows were first spotted in the U.S. in 1935, and as their population has risen and spread, black widow populations have plummeted. That’s especially true in Florida, where a southern variant of black widows has gone “locally extinct” in some areas. The two spider species don’t compete for the same resources, leaving scientists confused about brown widows’ hostility toward their relatives. While studies showed brown widows peacefully cohabitated with other spider species most of the time, they actively attacked and killed black widows when placed together. When young spiders of both species were put together, brown widows killed black widows 80 percent of the time, revealing stark differences in the two’s behavior. “Brown widows are boldly aggressive and will immediately investigate a neighbor and attack if there is no resistance from the neighbor,” Deby Cassill, an ecologist at the University of South Florida who co-authored the study, said in a statement. “But the black widows are extremely shy, counterattacking only to defend themselves against an aggressive spider.”