Ecological drivers of group living in two populations of the communally rearing rodent, Octodon degus

Intraspecific variation in sociality is thought to reflect a trade-off between current fitness benefits and costs that emerge from individuals' decision to join or leave groups. Since those benefits and costs may be influenced by ecological conditions, ecological variation remains a major, ultimate cause of intraspecific variation in sociality. Intraspecific comparisons of mammalian sociality across populations facing different environmental conditions have not provided a consistent relationship between ecological variation and group-living. Thus, we studied two populations of the communally rearing rodent Octodon degus to determine how co-variation between sociality and ecology supports alternative ecological causes of group living. In particular, we examined how variables linked to predation risk, thermal conditions, burrowing costs, and food availability predicted temporal and population variation in sociality. Our study revealed population and temporal variation in total group size and group composition that covaried with population and yearly differences in ecology. In particular, predation risk and burrowing costs are supported as drivers of this social variation in degus. Thermal differences, food quantity and quality were not significant predictors of social group size. In contrast to between populations, social variation within populations was largely uncoupled from ecological differences.Funding was provided by FONDECYT grant #1060499 to LAE by National Science Foundation grants #0553910 and #0853719 and Louisiana Board of Regents Research and Development grant (LEQSF 2007-09- RD-A-39) to LDH and by the Institute of Ecology and Biodiversity (grants ICM-P05-002, and PBF-23-CONICYT-Chile) to RAV. Other funding sources were the Program 1 of Centro de Estudios Avanzados en Ecología and Biodiversidad (FONDAP 1501–001), the American Society of Mammalogists, and Sigma Xi. RS was funded by a scholarship from CONICYT (Chilean Government). This study was approved by the ULM Institutional Animal Use and Care Committee and adhered to U.S. and Chilean laws (permit 1-58/2005 [2711] by the Servicio Agrícola y Ganadero, Chile