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