Humans do not only form complex and long-lasting social relationships with friends, family, and romantic partners, but also keep track of threats to these relationships and protect them jealously against outsiders. We also observe changes in the relationships around us and try to prevent alliances that could harm us in the long run. Researcher of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, show that wild sooty mangabeys and chimpanzees, both living in complex social groups, monitor the interactions of others and take active steps to prevent friends from defecting any alliances from being formed.