In the mammal collection of the Museum für Naturkunde Berlin (MfN), Germany a serious but inconspicuous deterioration of mammal skins and hides has been detected. The tear strength has been decreasing until the skins are falling apart, risking permanent loss of valuable specimens. At the MfN, about 80% of the 30,000 skins are affected. Although this phenomenon has been known by taxidermists for some time, there are very few publications on the subject. In this study, we surveyed the literature and conducted interviews with collections and leather industry professionals to understand the extent and potential causes of skin deterioration. In addition, analyses of skins in the collections of the MfN and the ZFMK (Bonn, Germany) showed that more than 80% of the tested skins had a very low tear strength. The tear strength appears correlated with the pH value and age of the skin. Our findings suggest that surplus acid from residual fat, preservation methods, or external sources such as air pollution might be a primary source of the degradation. Future steps should include further research on the chemical processes involved in deterioration, treatment options for threatened skins, and development of best practices, protocols for documentation, and development of a publicly available online knowledge base for museum skin preparation, preservation, and storage methods.