Spiders are notoriously antisocial, with a number of species known for making a meal out of recent mates. But there are some notable exceptions to that rule, called the social spiders. These can form groups of thousands of spiders, which cooperate to capture prey and build nests that can grow up to six meters long. Their group construction projects allow them to hunt prey that is much larger than any of them could capture individually.

For one species, Anelosimus eximius, however, these big nests appear to come with big risks. Some population surveys indicate that more than 20 percent of nests end up going extinct each generation. And, since only mature females that have already mated can successfully start up new nests, this means that most of the spiders in the nest end up dying.

Now, researchers from the University of British Columbia have proposed an explanation for the population busts. It seems that the spiders are a bit too good at sharing so that even weak and immature colony members are generally able to get something to eat. And, if food ever gets a bit short in the nest, that means that none of the spiders may get enough food to survive.

In many species, dominant individuals monopolize access to food. In species that live in isolation, this can come by that individual controlling territory that provides lots of resources; for social species, this can come about by dominating the social hierarchy. Other populations can have what's called scramble competition. Here, everyone gets equal access to food. And, if food gets short, that means that everyone is equally likely to go hungry. Scramble competition has been seen both in the wild and in labs for individuals that don't form social groups.

Ruth Sharpe and Leticia Avilés of the University of British Columbia suspected that social spiders may have a form of scramble competition for the food the colony captures, which could explain why the entire colony could collapse suddenly. So, they set out to test the idea by going into the field and catching a bunch of spiders.

These spiders were then split up to make a number of artificial colonies, each with a group of spiders that had a similar range of sizes so that all of them were roughly equivalent. The spiders were marked (there is apparently specialized insect paint) so that their activities could be tracked. Each of these colonies were then given large or small prey daily, and researchers noted which spiders attacked and disabled the prey, along with which ones fed on it. The weight and size of the spiders were also tracked for the length of the experiment.

Sharpe and Avilés found that, when small prey was placed in the container, it was largely eaten by the spiders that helped incapacitate it. While that was also true for large prey, larger food was shared more evenly. This allowed individuals that weren't in top physical condition, including younger members of the colony, to have greater access to food. In part, this could be a function of visibility; large prey may just be easier to spot, drawing spiders from around the colony.

The authors suggest that this is precisely the problem, at least from the spiders' perspective. As nests get larger, they tend to capture larger prey. This prey is then shared more evenly among all the members of the colony, which means that the large, healthy spiders get relatively less to eat. When resources become scarce, that may put the largest, healthiest spiders at the greatest risk of starvation. "Insufficient resources coupled with scramble competition," Sharpe and Avilés write, "may result in delayed growth of individuals so that offspring fail to grow to maturity in time to replace a dying maternal generation."

Journal of Animal Ecology, 2016. DOI: 10.1111/1365-2656.12559 (About DOIs).