Sociality in birds

Broadly defined, mammalian social behavior includes all activities that individuals engage in when interacting with conspecifics (Eisenberg, 1966). Such interactions may be agonistic or affiliative and reflect a wide array of functional contexts, including - but not limited to - foraging, predator defense, mate choice, reproductive competition, and parental care. Indeed, these social interactions are a central feature of the biology of all mammals (Lukas & Clutton-Brock, 2013). Sociality - defined as the degree to which individuals live with conspecifics in groups or societies (Eisenberg, 1966) - varies markedly across mammalian species. Sociality shapes multiple aspects of the mammalian phenotype, including mating and breeding success (Silk, 2007), physiology (Creel, 2001), and neurobiology (Fleming, et al., 1999; Carter, 2003; Hofmann, et al., 2014; Young & Wang 2004). These effects are evident over ecological and evolutionary time scales, indicating that social behavior is a powerful selective pressure. Sociality is widespread, occurring in all major lineages of non-primate mammals. Among metatherians (marsupials), sociality is found primarily in the Diprotodontia (kangaroos, wallabies, and wombats) (Jarman, 1991). Among eutherians (placentals), sociality is well-documented in the Cetacea (whales and dolphins, Connor, 2000), Hyracoidea (hyraxes, Hoeck, et al., 1982), Proboscidea (elephants, Moss, et al., 2011), Perissodactyla (odd-toed ungulates, Cameron, et al., 2009; Sundaresan, et al., 2007), Cetartiodactyla (even-toed ungulates, Clutton-Brock & Guinness, 1982; Coté & Festa-Bianchet, 2001; Jarman, 1974), and Lagomorpha (rabbits, hares, and pikas; Chapman & Flux, 1990). Although members of these taxa have greatly contributed to our understanding of mammalian cooperation, communication, grouping and dominance, here we focus primarily upon the social members of three prominent orders of non-primate mammals: the Carnivora, the Rodentia, and the Chiroptera. First, collectively these three orders represent more than 70 percent of extant mammal species (Wilson & Reeder, 2005). Second, because these lineages include many of the best-studied species of social mammals, they offer rich opportunities to explore variation in social structure. Third, because each of these orders contains multiple social species, they allow for informative comparisons of behavior across multiple taxonomic scales. To characterize sociality in these lineages, we begin by reviewing the leading hypotheses developed to explain why groups form. We then explore variation in the life history traits characterizing social non-primate mammals to explore whether group living is associated with particular suites of such variables. Finally, we outline the major empirical and conceptual advances emerging from studies of these social systems.