Spiders which eat together, stay together and multiply

August 9th, 2008 - 4:21 pm ICT by IANS  


Washington, Aug 9 (IANS) The ability to work together and capture larger prey has allowed social spiders to flourish beyond the strength of their numbers, according to a new study. The surface area of the three-dimensional webs social spiders use to capture prey does not grow as fast as the number of spiders in the nests. So the number of incoming prey per spider declines with colony size.

But Anelosimus eximius, a species notable for its enormous colony size - some numbering more than 20,000 individuals - have gained the ability to stretch that law by cooperating and capturing increasingly large insects as their colonies grow.

“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,” said Leticia Avilés, who studied the spiders in Amazonian Ecuador with undergraduate Eric Yip and graduate student Kimberly Powers.

“So even though the number of prey falls sharply as the colony grows, the biomass that individual spiders acquire actually increases,” said Avilés, who is also the co-author of the study and associate professor at the University of British Columbia.

The study also found that large prey, while making up only eight percent of the colony’s diet, contributed to more than 75 percent of its nutritional needs.

“But that only works to a certain point,” said Avilés, who added that the biomass of prey consumed by the colony peaks when the colony reaches between 500 and 1,000 individuals.

As for the scarcity of social Anelosimus species in higher elevations and latitudes, “there simply aren’t enough large insects in those areas to sustain this type of foraging behaviour,” she said.

The results were published in the Proceedings of the National Academy of Science.

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