Of the 50,000 species of spiders in the world, the vast majority are anything but social – even frequently eating prospective or actual mates. However, a handful live together comfortably in colonies, even sharing food, while others are classified as “subsocial”. An exploration of the differences between these spiders and their counterparts has shown parallels with the genes that make other social creatures.
“One of the conditions for social animals is that they have a bigger brain size,” said Professor Alexander Mikheyev of the Australian National University in an emailed statement. “Not only do they need to store information about the physical environment, but also the social one.”
However, as Mikheyev noted, spiders don’t have brains, instead distributing their neurons throughout their bodies. In Nature Communications, Mikheyev and co-authors note social spiders’ nervous systems are more developed than those of their closest solitary counterparts.
The authors chose to study genes associated with sociability in spiders because, while rare, it has evolved several times independently. By comparison, they note that; “Bees…are often described as ideal study systems because they include the full range of social complexity….However, there are only an estimated two-three independent origins of sociality within bees.”
The most famous example of spider sociability comes from the Kalahari where large colonies form huge webs. A recent study looked at the practice of Delena cancerides mothers protecting their young, which the paper refers to as subsocial. Investigating the genetics of 22 social and subsocial species, Mikheyev and co-authors found no amino acid substitutions common to all cases.
Nevertheless, they found certain genes where changes were often associated with sociability, for example, substitutions in the Bromodomain-containing protein 4 gene, found in five social and two subsocial spider species, many of them not closely related. This contrasts with insect genetic research, which has found low overlap in the genes associated with social evolution for social bees from different lineages.
Moreover, social spider species generally experienced faster molecular evolution across the genome compared to nonsocial counterparts. The authors attribute this to inbreeding and biased sex ratios.
Despite the vast evolutionary space between arachnids and mammals, the research could tell us quite a bit about ourselves. One of the genes that has experienced intensified selection in social spiders is known as Autism susceptibility candidate 2 because of the effects of variants observed in mice and humans.
Some animals become social so they can hunt together, particularly tackling larger prey. Terrifyingly, there are cases of this. Nevertheless, Mikheyev said D. cancerides, the Australian huntsman spiders he’s most familiar with, which live in colonies under acacia bark, don’t do this, but have been seen sharing meals with their neighbors.
Nevertheless, Mikheyev told IFLScience: “To a large extent we are seeing a tolerance rather than co-operation. It’s not like ants.” Still, as he noted, “if we think about the basis of spider behavior they are very aggressive, so it does not take a lot to attack each other.” For spiders to even share space is a challenge, let alone letting others partake in their hunting work.
Sociability can evolve for many reasons, such as information shared within a colony about predators or food resources. However, Mikheyev said spiders are more likely taking advantage of limited resources such as nesting sites.
The paper is open access in Nature Communications.