Personality plays a similar role in social spider societies to physical castes in other social species Social spiders' personalities determine the tasks they perform and the division of labour in their societies, new research has shown.Females lack physical differences, instead they display either aggressive or docile behaviour.
Scientists observed how often each personality type participated in tasks like catching prey and parental care.
They showed clear links between personality, preference for specific tasks and proficiency at those tasks.
The US-based research team reported their findings in the journal PNAS.
Much has been written about the division of labour between specialised groups - or castes - in animal societies by looking at differences in their bodies.
Many social insects have queens and workers that differ considerably in size or structure and the roles they perform.
But studies on social invertebrates without these physical differences are limited.
The social spider, Anelosimus studiosus, tends to live in groups or colonies. They mainly live in tropical regions but can be found in the eastern United States and other areas with mild climates.Females show either aggressive or docile "personalities" and previous research has shown aggressive females are more likely to attack prey and intruders than docile ones.
But the role of the docile personality type within such colonies was unclear.
"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, Colin Wright, from the University of Pittsburgh, US said.
"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 function."
The authors created colonies of two aggressive and two docile spiders and examined how they participated in four tasks - colony defence, prey capture, parental care and web repair.Aggressive females took part in colony defence, prey capture, and web repair nearly three times more often than docile females, whereas docile females spent three times as long caring for the young as aggressive females.
The research team also studied how well each personality type carried out the tasks and again found strong differences.
When the spiders were made to live outside of a colony, aggressive females were more effective at capturing prey, constructed better webs and were more likely to attack and repel intruders than docile individuals.
However aggressive types were less successful at rearing large numbers of offspring than docile ones.
Mr Wright said: "We were surprised at how poor parents aggressive spiders were. It turns out that they are so aggressive that they will even act aggressively towards their own offspring, killing many in the act."
But having both personalities present is key to a colony's success.
"These spiders achieve the best of both worlds by evolving two behavioural types with specialised 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," Mr Wright added.
Having provided the clearest evidence for division of labour organised around behavioural rather than physical differences, the authors say personality may play a similar role in many other animal societies.
"We argue that perhaps researchers should rethink their notion of what constitutes a caste to include difference solely in personality," Mr Wright said.
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