Comparative studies on social evolution are ideally based on large datasets to ensure high statistical power, but their scientific validity also relies on the quality of the data. However, even though social organization, i.e., the composition of social units, is measured in many field studies testing specific hypotheses, these data are often not adequately reported. Here, 2 we summarise which data on social organization should always be reported regardless of the study focus to make them available for comparative studies. As an example, we report data from a literature survey on one of the four superorders of placental mammals, the Xenarthra from South America (armadillos, anteaters and sloths), of which all 30 species are generally assumed to be solitary living. In total we found 4510 articles. The titles and abstracts of 61 publications indicated that data on social organization were collected, but only 12 publications contained sufficient information for us to determine the social organization. We found reliable information on nine species, of which seven were strictly solitary. Two species showed a combination of solitary-living and pair-living with one including female groups. This review therefore indicates that Xenarthra may not be exclusively solitary. Our literature survey further shows that valuable data are often not reported even though it can be assumed that these data had been collected. We report examples from 23 additional studies on monotremes, marsupials, and two other placental superorders showing similar issues in reporting data. It is important to make authors aware that this information would be valuable for comparative studies. In sum, we recommend including data on the composition of social units, sex of individuals, occupancy of sleeping sites, frequency of observations and trapping events, home range overlap, and the proportion of the individuals in the study area, independent of the study question in all publications.