Sometimes Spiders Share Themselves to Death

Sometimes spiders live in huge social communities, and sometimes they share food so equitably that they all starve to death. Spiders don't usually hang out with each other because they're highly territorial – and sometimes cannibals. But of the 40-thousand-plus identified species of spiders, we know 25 to be social. One such species, the South American Anelosimus eximius, has been observed in colonies of 50 thousand individual spiders living in communal, 3D webs that span 25 feet by 5 feet, arching through several trees. These critters cooperate on web upkeep, prey capture, and childcare. Broodcare?

(It involves female spiders – which comprise between 78 and 95 percent of any given colony – regurgitating food for the youngsters. Including youngsters that aren't their own.) They self-select tasks based on what each unique spider is personally suited for. Beyond that, the entire neighborhood shares their food supply when the hunting's good, ensuring that no spider goes hungry. Or that's how it usually works. In every generation, about 21 percent of healthy, established A. eximius colonies suddenly collapse and die off. As in, no survivors. It's like the end of The Thing, except infinitely worse because there's no Kurt Russell and the entire cast is spiders. So a team of entomologists out of the University of British Columbia tried to figure out why this happens. They experimented with spider populations in the lab – but they also used MATH. They were looking at prey size and individuals' behavior in growing colonies. Their mathematical model and experiments showed that the spiders tend to hog smaller prey and share larger ones around. Meaning there's probably a limit to what a spider considers worth defending. Smaller colonies have smaller webs that catch smaller prey. In those cases, the spiders are mostly competing for food. Some individuals may starve, but the overall colony persists. In studies of animal behavior, that's called contest competition. However, when a web gets big enough to catch all large prey all the time, it seems that the spiders start sharing. They even give preferential food access to the hungrier spiders.

That's called scramble competition. But they're bringing in less overall food per spider capita. They wind up sharing it so fairly that no one has enough. And boom: Colony extinction, not with a bang but a whimper. But don't worry -- A. eximius populations remain pretty healthy overall. Which is great, because studying them has led to lots of hypotheses about the evolution of arachnid social behavior. Further research could help scientists get a better handle on how competitive animal behavior is influenced by environmental factors – and how populations are influenced by competitive animal behavior.