The Female Of The (Social) Species . . .

by Michael Bell

The social spider, Anelosimus eximius, was first described in the 19th century, but has fascinated generations of scientists, because it seems to break many of the laws that normally apply to predatory species.

A new study led by Leticia Avil‚s, lead author and associate professor in the UBC Dept. of Zoology, shows that colonies of Anelosimus eximius can reach very large sizes (up to 20,000 members) and that this communal activity is successful in terms of 'catch'.

"The average size of the prey captured by the colony increased 20-fold as colony size increased from less than 100 to 10,000 spiders," says Avil‚s, who studied the spiders in the wild in Amazonian Ecuador. "So even though the number of prey falls sharply as the colony grows, the biomass that individual spiders acquire actually increases."

Unlike social insect species such as ants, all social spiders in a colony are able to reproduce, and all cooperate in web maintenance and prey capture. The colonies construct communal, basket-like living spaces which can be more than one metre across, and build highly complex inter-woven three-dimensional webs. The surface area of the webs does not grow as fast as the number of spiders contained in the nests, so fewer prey are caught - but the size of individual prey can be up to ten times the body size of a single spider. The study found that large prey, while making up only 8% of the colony's diet, contributed to more than 75% of its nutritional needs.

It seems likely that the social adaptation of these spiders is a consequence of the larger size of insects in the tropics: social Anelosimus species are much scarcer in higher elevations and latitudes. "There simply aren't enough large insects in those areas to sustain this type of foraging behaviour," says Avil‚s.

Aviles is diplomatic enough not to tell us that the female-to-male ratio in these social groups can be as high as nine to one, since females are comparatively more useful than males when prey is plentiful. Once most manual labour has become robotic in human societies and everyone is rich, what will be the point of having equal numbers of males and females?



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