US scientists find evidence for group selection of spider colonies in the wild

Saturday, October 4, 2014

A female spider of related but different species Anelosimus crassipes.
Image: Akio Tanikawa.

US biologists from the Universities of Pittsburgh and Vermont have found that social spiders of species Anelosimus studiosus exhibit apparent group selection, determining a trait affecting the colony's survival. According to one of the researchers, this is the first experimental evidence of group selection in wild populations driving adaptation to local conditions. The study was published in journal Nature on Wednesday.

The researchers found that, depending on the availability of resources at the location of the colony and the size of the colony, the spider colonies have a different composition, promoting colony survival. The collective colony trait the scientists analyzed is the shifting ratio of "aggressive" and "docile" female spiders. The authors suggest two possible causes of colony extinction from inappropriate ratio in a large colony, depending on the availability of resources: egg cannibalism when there are few resources and a high proportion of "aggressive" female spiders, and social parasites when there are abundant resources and a high proportion of "docile" female spiders.

To check whether the self-regulation of spider colonies can be considered an adaptation to local conditions, the scientists placed artificially assembled colonies in different locations, with resource availability similar to or different from their home location. In cases of high risk of extinction, colonies self-regulated the ratio of "aggressive" and "docile" female spiders to match not resource conditions of the site they were placed at, but resource conditions of the home site from which they were taken.

Coauthor Jonathan Pruitt, University of Pittsburgh assistant professor of behavioral ecology, said, "These findings provide compelling evidence that the mechanisms that colonies use to regulate their compositions are themselves locally adapted, presumably because of the survival advantages they confer to the colony". "They're continuing to make the phenotypes, the trait at a group level, that would have been advantageous if they had stayed home [...] But they seem to have no idea that they're at a new site and that what they're doing is going to doom the whole colony. All the friends die".

Coauthor Charles Goodnight, at the University of Vermont, said: "Biologists have never shown an adaptation in nature which is clearly attributable to group selection [...] Our paper is that demonstration."