“They’re sort of like humans, when they’re around a lot of food they become lazy and wasteful,” said J. Chadwick Johnson, a biologist at Arizona State University. “They’ll kill food they don’t need, and leave some of it uneaten.”
Johnson’s team describes the spiders’ habits in a paper called “Wasteful Killing in Urban Black Widows: Gluttony in Response to Food Abundance,” published online Jan. 17 in the journal Ethology. Wastefulness is strange to see in animals, because hunting involves spending energy, risking injury and killing prey that might be needed later.
Other research from Johnson’s lab has shown that black widows can inherit some feeding tendencies, such as a taste for cannibalism, from their parents. For this study, they wanted to see if wasteful killing was a product of a spider’s individual traits, or if most black widows — which, when they’re not being wasteful, can go two months without eating — behave the same way.
In the lab they fed crickets to spiders, then measured how much the spiders ate. Some of the spiders hadn’t eaten in two days; others had gone without food for 7 or 14 days. Johnson’s team found that the spiders were quicker to attack and ate more food when they’d gone longest without eating. Habits in individual spiders, which would suggest a genetic component of gluttony, didn’t show a trend.
“The spiders are more aggressive when they’re hungrier,” Johnson said. “The simplest explanation is that.”
It isn’t clear if this kind of feeding is adaptive for the spider, but it might be a way of attracting mates. Other studies show that male black widow spiders are attracted to the smell of well-fed females.
“Even if she’s not around her web, he’ll get turned on and begin courting like crazy,” Johnson said.
If a male black widow chooses to mate with a starving female, he risks becoming a post-nuptial snack. Johnson speculates that females might be advertising for males by leaving wasted food in their web.
It sends a signal: “Come hither, because I won’t kill you.”
Image: Female black widow spider, Latrodectus mactans. (James Gathany)