Research on the evolution of social behaviour has been dominated by genetic relatedness for a long time; however, both recent empirical studies and theoretical concepts give growing evidence for ecological factors acting as very prominent additional or alternative driving forces in social evolution. Now the time is ripe to investigate similarities and differences in the course of social evolution in different animals. This book brings together renowned researchers working on sociality in different animals. For the first time, they compile the evidence for the importance of ecological factors in the evolution of social life, ranging from invertebrate to vertebrate social systems, and evaluate its importance versus that of relatedness.