Mosaic ageing occurs when senescence of different traits is not synchronous. Within-individual variation between traits in both the pace and shape of senescence appears common, but why traits evolved to have different senescence trajectories is not well understood. Senescence in an evolutionary sense (declining reproduction/survival) is an emergent property of senescence of all body functions combined. Hence an understanding of mosaic ageing is essential to understand organismal senescence. In this project we will 1) develop theory to explain the evolution of mosaic ageing, 2) study mosaic ageing in an innovative ecologically relevant ageing model, and 3) test the theoretically derived predictions with existing and new data.