| Extrinsic environmental factors are often invoked to explain the origin of social systems such as cooperative breeding. However, few general associations with specific environmental variables have been identified. This study reports the results of a comparative analysis of the impact of specific environmental variables on variation in sociality in species of Campylorhynchus wrens (Aves: Passeriformes). A phylogenetic hypothesis of relationships among genera of wrens was generated based upon variation in DNA sequences from both mitochondrial and nuclear loci. This hypothesis indicated that cooperative breeding had at least three origins within the wrens, and that the sister-group to Campylorhynchus is non-cooperative. A phylogenetic hypothesis of relationships among species of Campylorhynchus was also generated, based on DNA sequence variation in two mitochondrial loci. Incorporating this phylogenetic hypothesis, tests of potential correlates of group size in species of Campylorhynchus were performed. Group size was not significantly correlated with body mass, or with a measure of unpredictability of resources among years. However, significant correlations were found between group size and a measure of the environmental harshness in the non-breeding season. The observed correlation suggested that either: (i) habitat saturation may favor group formation in equable environments, or (ii) groups could be favored in all environments, but extrinsic environmental constraints on group formation might exist in harsh environments. |
