Some female spiders sacrifice themselves for the children of others

You probably don’t think of spiders when it comes to cooperation or self-sacrifice for the greater good. But according to a new study published in Animal Behaviour, virgin females of the social spider Stegodyphus dumicola care for spiderlings much like their actual mother would, from tending the egg sac to the point of becoming lunch for the little ones.

This spider is just one of about 20 known species of social spiders in the world. They live in dry and warm areas of southern Africa, building large and dense nests on trees and shrubs. Unlike your usual orb spider web, this species builds webs lacking glue droplets, using instead a very fine capture wool that is just as efficient at capturing prey yet does not dry out, says Anja Junghanns, an evolutionary biologist at Ernst Moritz Arndt University in Greifswald, Germany, who led the new study.

This means that their web does not need to get renewed, and it can grow quite large in relation to the size of a single spider, Junghanns explains. “Spiders will not only cooperate in building the web, but also in capturing prey in it, that can be as large as 10 times the size of a single spider,” she adds.

When spiderlings hatch, their mother provides them with some nutritious fluids. When that runs out, mum is the next item on the menu. After eating their mother, the spiderlings remain in their web and live socially. Over time and after multiple generations the colony grows, sometimes forming groups composed of more than a thousand individuals, all closely related, like a huge family.

In such large colonies of social and highly related animals, cooperation is taken to new levels. This includes the development of adult helpers that care for the offspring of other individuals. While such behaviour has been reported in social spiders before, it was not clear if only females that already had their own offspring could become helpers.

To test this, Junghanns and colleagues set up nearly 200 individual experiments where three virgin females and two mated females were colour-marked and observed over a 10-week period.

Their observations revealed that in more than 97% of their experiments, both virgin and mated females tended the eggs and spiderlings.

“The most interesting finding here, perhaps, is not that there is a degree of task differentiation, but rather such a high degree of task overlap between virgins and mothers,” says Nick Keiser, a junior fellow at Rice University who specialises in behavioural ecology and has studied task participation in social spiders.

The extreme maternal care of these spiders provides spiderlings with the best possible conditions to increase their chances of survival. “To enable virgins to provide this form of brood care, an evolutionary adaptation had to take place and our next goal is to investigate the nature of this adaptation,” Junghanns says.

Now Junghanns plans to address the question of how the virgins are actually able to provide this extreme care, which is comparable to a human woman starting to lactate without giving birth, she says. The answer to this question could provide a deeper insight into how sociality evolved in spiders, Junghanns adds.