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Sociality in cobweb spiders (Anelosimus spp.) : evolutionary consequences and the role of pre-existing traits Samuk, Kieran Mikhail


Sociality – cooperative group living – is ubiquitous in the natural world, yet our understanding of its evolution is still in its infancy. In this thesis, I explore two poorly understood aspects of the evolutionary origin and consequences of sociality using social cobweb spiders (Anelosimus spp.) as a model system. First, I examine how pre-exisiting traits have contributed to the evolution of alloparental care – the care of non-descendant offspring – in social cobweb spiders. I begin by showing alloparental care is extensive in wild social cobweb spider nests. I then test the hypothesis that alloparental care occurs as a result of a lack of discrimination against foreign egg sacs. In support of this hypothesis, I show that subsocial species from clades sister to the social species freely care for foreign egg sacs. This suggests that a lack of offspring discrimination is ancestral to sociality in cobweb spiders and alloparental care likely emerged spontaneously along with group living. This may have facilitated the evolution of sociality by immediately providing the group-level benefits of alloparental care. Secondly, I examine how social life may have altered natural selection acting on social cobweb spiders. In social cobweb spider nests, the protection offered by a communal nest and the presence of alloparents may have relaxed natural selection on individual maternal care behaviour. Using a comparative approach, I test the hypothesis that sociality is associated with reduced maternal care behavioural phenotypes. I show that social species from independently derived social clades score significantly lower than their subsocial sister taxa on six different assays of maternal care, including the probability of repairing damaged egg sacs and of abandoning egg sacs in the face of simulated predation. Integrating a number of supporting facts, I interpret this result as suggestive of relaxed natural selection on maternal care behaviour as a consequence of sociality. Together, the two comparative studies I present reveal a key role for pre-existing traits in the origin of sociality and that the forces of evolution are likely altered in concert with the onset of social life.

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