������ý

Species:

Habitat: Massive spider webs in southern Africa

It takes a lot to be a good aunt if you’re a . In fact, it takes your internal organs. After tending lovingly to your sisters’ eggs and regurgitating food for newborns, it’s time to offer yourself as the main course for the spiderlings to suck you dry.

“[The] spiders literally start feeding on the female while she is alive,” says at Aarhus University in Denmark. The spiderlings inject enzymes to dissolve her innards and suck out the semi-digested fluids, leaving only the outer shell. “But there is no apparent aggression. It looks as if females are almost inviting spiderlings to feed on them.”

S. dumicola are social spiders that live in large communal nests. Hundreds cooperate to capture prey, defend the nest and take care of the young. The nest is a dense retreat of silk and plant material, with two-dimensional webs to catch prey. Each spider only lives for a year, so can only reproduce once.

In the closely related species S. lineatus, only mated females care for spiderlings. In these spiders, the act of mating seems to cause females to care for other offspring as well as their own – an act called “alloparenting”. However, there are limits: they only let their own spiderlings eat them. Letting your kids eat you is a surprisingly common behaviour known as “matriphagy”.

Eat me!

Bilde and her colleagues wanted to find out whether unmated S. dumicola females also perform alloparenting duties. They bred spiders in the lab and placed them in groups, each with two mated and three virgin females, along with some spiderlings, to observe their behaviour.

Both virgin and mated females performed all forms of alloparenting. They tended to egg sacs, regurgitated food for spiderlings and finally offered themselves up as a meal.

This extreme behaviour makes sense because the spiders in a nest are all closely related and share genes. There are many more females than males, and only certain females reproduce, so the spiders in a colony are genetically similar.

“The investment in these offspring is an investment in her lifetime reproductive success,” says Bilde. “The more gene copies she propagates to the next generation, the better, so providing your body as food is a sensible evolutionary solution.”

“I suspect that females merely aren’t capable of discriminating between their egg cases and someone else’s,” says at the University of California, Santa Barbara. “The colony is composed of close kin, so even if females produced their own egg cases, there would still be a benefit of assisting a closely related relative.”

The spiders’ environment may also be a factor. “Spiders in the genus Stegodyphus occupy arid landscapes, deserts, where prey is mostly scarce,” says at the Israel Cohen Institute for Biological Control. A female who sacrifices herself will be “providing more food than they can find by foraging for prey”.

Journal reference: Animal Behaviour, DOI: