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A species of spider avoids cannibalising its siblings while they are alive, but does not seem to have qualms about eating its brethren as soon as they die. This suggests some spiders come to tolerate one another by learning to recognise a chemical or mechanical 鈥渓ife signal鈥�, which could be a key piece to understanding the evolution of rare social spiders.

at the University of Toulouse in France and his colleague collected nine egg sacs of Agelena labyrinthica, a web-building spider common in Europe. Over the next 20 days, they observed hundreds of hatched spiderlings which they either kept in isolation or in groups, without food.

When the spiders were placed with others in an 鈥渁rena鈥�, the starving spiders raised in groups were tolerant with each other. Meanwhile, the spiders raised in isolation were more aggressive, including attempting bites and full-on cannibalism of their live siblings.

However, when the tolerant, group-raised spiders were placed together with a dead sibling, they wasted no time in consuming the corpse. They ate fresh bodies even more quickly than day-old ones. 鈥淲hen [the spiders] are maintained in groups, they are perceiving this life signal,鈥� says Jeanson. 鈥淲hen they are isolated, they lose sensitivity to this signal.鈥�

Tests conducted in the dark rule out the spiders using a visual cue. 鈥淥ur main hypothesis so far is that this is a chemical cue,鈥� says Jeanson, which could stop being produced upon death. Another possibility is the spiders sense vibrations that indicate their sibling is alive.

A better understanding of how this life signal mediates social tolerance could shed light on the evolution of social spiders, says Jeanson. These rare species 鈥� only 20 of the more than 53,000 known types of spider are gregarious 鈥� tolerate one another not only as juveniles but throughout their adult lives in societies of up to thousands of individuals. 鈥淲e want to understand how, during evolution, these spiders that are normally aggressive become social,鈥� he says.