__Shrinking Alpine chamois: Higher spring temperatures over the last 27 years in Switzerland are linked to a 3 kg reduction in body mass of yearlings__ __Abstract__ Although climate change is considered to be partly responsible for the size change observed in numerous species, the relevance of this hypothesis for ungulates remains debated. We used body mass measurements of 5635 yearlings (i.e. 1.5 years old) Alpine chamois (Rupicapra rupicapra) harvested in September in the Swiss Alps (Ticino canton) from 1992 to 2018. We identified that warmer temperatures during birth and the early suckling period (May 9 to July 2 in the year of birth) had the strongest impact on yearling mass. In our study area, during this period, yearlings shrank by ca. 3 kg while temperatures between May and July rose by 1.7°C. Further analysis of year-detrended mass and temperature data strongly support the hypothesis that temperature increases during this particularly sensitive time window for development and growth are responsible for the decrease in body mass of yearling chamois. Altogether, our results suggest that rising temperatures in the Alpine regions could significantly affect the ecology and evolution of this wild ungulate. Link to the publication: https://doi.org/10.1098/rsos.231295