Until Death Do Us Part - Aunts Sacrifice Themselves for the Offspring of Their Sisters

Photo of a group of the social spider Stegodyphus dumicola
Photo of a group of the social spider Stegodyphus dumicola Photo: Anja Junghanns
A female looking after the egg sac
A female looking after the egg sac Photo: Anja Junghanns
A mother (orange back) and an unmated female (green back) looking after the offspring
A mother (orange back) and an unmated female (green back) looking after the offspring Photo: Anja Junghanns

Social spiders live together in nests, with several hundred of their own kind. In such a nest, the animals work together to catch prey and build webs. Mothers with offspring even help each other out with the arduous care of each other’s young. They guard the egg sacs for several weeks and provide the hatched young with regurgitated food. Nonetheless, they are eaten by the growing offspring in the end.

What’s the background? More than half of the females of a colony remain unmated, because males are rare and have short lifespans. Therefore these unmated females do not reproduce. Up until now it was presumed that these females only help to catch prey and build webs, as only females who are at the stage of maternal care, are able to regurgitate. This is true for spider species that do not live in groups.

Two doctoral candidates at the partner universities of Greifswald and Aarhus investigated this theory on the African social spider Stegodyphus dumicola. The researchers put groups of mated and unmated females together and examined which females carry out the tasks of maternal care and capture of prey. It turned out that not only the mothers cared for their young, but also the unmated animals. These ‘aunts’ not only provide the offspring of other females with regurgitated food, but also allow themselves to be eaten by the offspring.

But why do unmated females look after the offspring of other females and also sacrifice their lives? This question can be answered by looking at how the females are related to one another. Social groups are usually created by only one single mother and her offspring, which then interbreed. Thus, over the course of several generations, a community with an exceptionally high degree of kinship is created. The unmated females therefore actually help their sisters with the raising of their offspring. By successfully rearing her nieces and nephews, the aunt passes on her genes to the next generation. The so-called kin selection increases the evolutionary overall fitness of the unmated females and explains how this seemingly selfless behaviour was able to evolve from an evolutionary aspect and has survived.

The researchers also discovered that the tasks performed by the mothers and the aunts varied slightly: Mothers invest more time in maternal care and aunts do more of the prey catching. This insight suggests that although unpaired females can carry out all of the tasks in the nest, including the suicidal raising of the offspring, they tend to do more of the risky prey catching, whilst mothers do more of the maternal care inside the protected spun webs of the nest.


Further Information

Journal Animal Behaviour

Original Publication Extreme allomaternal care and unequal task participation by unmated females in a cooperatively breeding spider

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Contacts at the University of Greifswald

Zoological Institute and Museum
Department of General and Systematic Zoology
Loitzerstaße 26, 17489 Greifswald 

Anja Junghanns
Tel.: +49 3834 420 4281

Prof. Dr Gabriele Uhl
Tel.: +49 3834 420 4242


Contact at the University of Aarhus

Department of Bioscience - Genetics, Ecology and Evolution
Prof. Dr. Trine Bilde
Ny Munkegade 116/building 1540, 224, 8000 Aarhus C, Denmark