Social spiders do better when hunting with relatives

Social spiders are an arachnophobe's nightmare. While the vast majority of spiders work alone, the odd few live communally and cooperate to hunt and feed. Their numbers, along with the massive webs that they all have a spinneret in creating, allow them to tackle prey far larger than themselves. The aftermath of a kill opens up new conflicts for the spiders that other cooperating hunters like lions or wolves don't share. They don't divide up the carcass to eat separately, for like all spiders, they digest their prey outside their bodies.

All the colony members spit their digestive enzymes into the corpse and suck up the liquefied remains. But producing these enzymes costs energy, and since all the spiders are using the same opaque spittoon, there's no way for an individual to tell how much its teammates are coughing up. It's a system that favours cheats, who take their share of the communal resources but contribute very little toward creating them. This conflict isn't just theoretical - experiments have shown that groups of social spiders feed far less efficiently than individuals do.

In the face of this temptation to cheat, how do the spiders manage to eat anything at all? This is a perennial question faced by biologists who seek to understand the evolution of cooperation: why work together, when cheating often yields greater rewards? There are many possible answers and in the social spiders, Jutta Schneider and Trine Bilde saw a chance to test one of the most pervasive - the theory of kin selection.

The theory holds that individuals can find extra value in helping their relatives because there's a good chance that their bodies house many of the same genes. By ensuring their survival, animals can increase the chances that their own genes are passed to the next generation, no matter how indirectly. The role of kin selection among the many other theories of cooperation is still unclear, and particularly whether it could help to foster the initial stages of cooperation, paving the way for other processes to take hold.

Schneider and Bilde tested the role of kin selection in Stegodyphus lineatus, a species of spider that isn't quite fully social yet. Unlike true social spiders that live permanently in groups, Stegodyphus only forms alliances as youngsters - it's a 'subsocial' spider. The species breeds only once in its life for the mother offers her own bodily fluids as a first snack for her newly hatched young, who promptly drink her dry. For the next couple of months, the young spiders stay in the next together and hunt as a group.

Sorta-social

Schneider and Bilde collected female spiders and their egg sacs from the Greek island of Karpethos. After hatching, the spiderlings were removed and swapped around. Some siblings were kept as a group and returned to their mothers as normal. Others were mixed and matched before being assigned a parent, and some of these spiders were given a chance to get to know one another before they were returned.

Over the next 8 weeks of feeding and growing, the siblings put on weight at a significantly quicker rate than the groups of unrelated spiders and ended up weighing more on average. Their siblings' boosted growth was down to a more efficient feeding style, and they managed to extract more mass from their prey than the unrelated groups did. And because of these larger meals, fewer spiders in the related groups had died by the 8-week mark.

You might argue that related spiders are simply more familiar with each other, but Schneider and Bilde's experiment allowed them to separate the effects of acquaintance from genetic relatedness. In the two unrelated groups, the spiders that were introduced to one another beforehand didn't cooperate to any greater extent than the fresh strangers.

The fact that cooperating relatives fared better than unrelated teams provides strong support for the idea that kin selection helps to mitigate the negative effects of competition over a common resource. The advantages of this nepotism could provide a foothold for developing the type of fully social lifestyle seen in the tremendously successful social insects.

Of course, the edge displayed by the related groups suggests that Stegodyphus has ways of telling family members from strangers. How they do this is a mystery, but other studies have also suggested that they can. If they lack for prey, the spiders will turn on each other but they are much more likely to cannibalise unrelated individuals than relatives.

Reference: PNAS doi:10.1073/pnas.0804126105

Images from Joaquin Portela and PNAS