Black widows got their namesake from the idea that females eat their mates after copulation. This is not necessarily the case: If the males are fit, females should not get a chance to sink their teeth into them before they scurry off.
"If the male's a real loser, doing his little courtship dance for you, why give him the opportunity? Just eat him," says Jonathan Pruitt, an assistant professor who studies spiders at the University of California at Santa Barbara. "Being inseminated by some loser is absolutely a bad outcome."
Just as their mothers sometimes snag a meal from a would-be mate, black widow spiderlings sometimes decide that a full belly trumps brotherly love.
Johnson had previously noticed that a black widow's eggs were usually all the same size and that spiderlings hatched and molted at the roughly same time — something he thought might occur by design, to keep spiderlings at the same stage of development. In the new study, he and his fellow researchers conducted an experiment to see whether the size of baby spiders had any effect on how much they cannibalized each other after hatching.
They kept some western black widow spider eggs at hot temperatures and some at cold temperatures to control different rates of development. In groups where spiders of different sizes were mixed together, it only took a few days before the bigger hatchlings started feasting on their smaller siblings. Spiders kept at roughly the same size avoided sibling rivalry for longer.
"Just a few days' difference drastically increased the amount of cannibalism," Pruitt says. He was not involved in Johnson's study, but he called the research "a slam dunk."
The experimental arachnids were deprived of non-cannibalistic food sources, but things aren't much different in the wild. Food sources are usually scarce for newborn spiderlings, which are too small to take a fly and often get by on carcasses left in their mothers' webs. It usually takes a few molts before they can make their own webs and eat meals that are not leftovers (or relatives).
In the meantime, Johnson says that the spiders might prefer to avoid a fight to the death if possible — the battles can be costly, even for the winner. But if one spider is the heavyweight champ of its age cohort, it may seize the chance for an easier meal.
Pruitt says that this difference in weight and the resulting cannibalism, which is probably not unique to black widows, is disastrous for mothers. "The last thing a mother wants is out of her 300 babies, to have one giant one and 299 dead ones," he says. "It really suggests that females have been able to provision their eggs very precisely, with a precise amount of yolk and chemicals and things, so their development is in lockstep."
Regardless of what it means for raising a happy spider family, cannibalism may have implications for population growth and pest control. "If we could figure out how to feed them to each other, we might be better off," Johnson says.
The new study is part of a larger effort to understand why black widows do so well in urban environments. But counterintuitively, having spiders that eat one another might not lower populations. Having 250 black widow cannibals might actually be worse than having 500, because the bigger spiders might reproduce more quickly. And the cannibals could probably pack a bigger bite — larger spiders can break human skin, passing venom on more easily.
"It could be that cannibalism is a terrible thing to do if you want to limit their population growth," Johnson says.
Joshua Learn has contributed articles to National Geographic, Smithsonian Magazine, Hakai Magazine and other publications. Check out his Twitter feed @JoshuaLearn1.