In a novel study published in PNAS, researchers describe how the personality of female social spiders is strongly linked to the tendency to perform certain tasks within colonies and also task aptitude. The findings are reminiscent of the relationship between task specialization and castes (queens, workers and soldiers) in social insects such as ants and bees where morphological differences dictate the tasks performed by members.

The social spider investigated in this study, Anelosimus studiosus, does not have a caste system and the females are morphologically similar. However, they do exhibit different phenotypes (personalities). Some females are aggressive, whereas others are docile. Although previous studies demonstrated that the aggressive spiders are more likely to attack intruders, the purpose of the docile spiders remained a mystery and many scientists had their suspicions that these scroungers weren't needed to maintain successful spider societies.

“It was thought they acted as freeloaders and that colonies would perhaps be better off without these lazy spiders eating their food and contributing nothing positive to colony success,” lead author of the study, Colin Wright, told the BBC.

“But other studies performed in the field showed quite clearly that colonies of mixed personality composition outperformed colonies composed of all docile or all aggressive individuals. This indicated that the docile individuals must be contributing something of importance to colony formation.”

In order to investigate the propensity of female spiders with these distinct personalities to perform certain tasks, the researchers created mixed phenotype colonies with both aggressive and docile spiders. They then investigated preferences for four different tasks; prey capture, web building, colony defense and parental care.

The team found that aggressive females were more heavily involved in the first three tasks listed, whereas docile females generally acted as "stay-at-home-moms" spending more time looking after the young'uns.

To take this further, the scientists then separated the colonies and investigated the abilities of the single spiders to perform these tasks. They found that aggressive spiders were more proficient at the first three tasks than docile spiders; however, docile spiders were more successful at rearing large numbers of young.

According to Wright, the aggressive spiders were surprisingly bad parents because they were so aggressive that they would sometimes direct this behavior towards their young, killing some in the process.

The addition of docile spiders is therefore critical to the maintenance of a successful colony, and these spiders can now shed their freeloading reputation. Furthermore, since these phenotypes are heritable, this study suggests that intra-colony variation is maintained because of the advantages conveyed by task division.

“These spiders achieve the best of both worlds by evolving two behavioral types with specialized tasks. Aggressive spiders perform all the tasks where aggression is important, and docile spiders are allowed to focus on being good parents. It’s a win-win situation,” added Wright.

Since differences in personality have been observed in virtually every animal system investigated to date, the researchers believe that the definition of a caste system should be updated to include personalities rather than just morphological differences.