and Galea musteloides) and domestic (Cavia aperea f. porcellus) guinea pigs, which summarizes general rules for many group-living mammals. It is shown that social interactions have consequences not only for the individuals' reproductive success but also for their degrees of stress and welfare. The way in which individuals interact is controlled not only by the present environment but also by the previous social experiences which they have gathered during their behavioral development. Furthermore, the study of ontogeny does not begin at birth, because prenatal social factors acting on pregnant females can also affect the way in which the offspring will interact when adult. In addition, to understand the genesis of interactions between domesticated animals implies knowledge of the behavioral and physiological changes which occurred during the process of domestication. Finally, understanding the social interactions among individuals of the wild ancestor of the domesticated form requires knowledge of how their behavior patterns were brought about by natural selection during the process of social evolution.