Parental Care and Sociality

Abstract

Spiders are famously aggressive and cannibalistic, and nearly all are solitary. Only about 20-25 out of over 46,000 known species display highly social behavior. Nevertheless, sociality has arisen in multiple families independently in spiders, probably via the 'maternal care route', with an apparent concentration of social species in the Neotropics. We review aspects of reproduction and maternal care and how these may interplay with the evolution and maintenance of social cooperative behavior, focusing on Neotropical spiders. We also discuss the behavioral, ecological, and evolutionary contexts in which these behaviors have evolved in spiders, and highlight the unique opportunities that exist for research due to the multiple independent evolutionary experiments that replicated origins of sociality offer. We ponder why social species appear concentrated in the Neotropics, with the outstanding example found in the genus Anelosimus. Curiously, highly social Anelosimus are restricted to the Neotropics, while the genus is distributed globally and ubiquitously displays extended maternal care. We discuss traits that are shared among these independently derived social species and thus form a part of a social 'syndrome'. Such traits include absence of dispersal, inbreeding, biased sex ratios, and even shared patterns of colony composition of individuals differing in personality type. Ecologically, social Neotropical spiders are mostly restricted to tropical lowland and mid-elevation forests where prey size tends to be greater than in areas where sub-social species are found. They are especially common in areas of high rainfall, where their very dense 3-dimensional webs may not only allow capture of large prey, but also serve as a predator defense, for examples where ants are particularly common. Neotropical social spiders receive benefits from collaboration in web construction, care of young, nest defense, and prey capture, where they can handle much larger prey than other similarly sized spiders, and more effectively fend off predators. Colonies seem to benefit from a mix of personality types within colonies with both bold and shy individuals being crucial to colony success, but with larger colonies having more shy individuals and thus characterized by lower overall aggression. While sociality seems to offer short-term benefits in certain environments, a switch to an inbred breeding system that is tightly linked to sociality in spiders seems responsible for a loss of genetic variability that may restrict diversification due to vulnerability to climate change, disease, and parasitism.