Birds do it, bees do it, we do it: contributions of theoretical modelling to understanding the shape of ageing across the tree of life.

Organisms of different species age differently. Current theory explains why life should get worse, i.e. why patterns of increasing risk of death should evolve. However, for some species the risk of death remains constant or even falls with advancing age. Evolutionary theory to explain the observed diversity of shapes of ageing is lacking. Theoretical models can provide insights into this diversity. Comparing assumptions of models that find increasing mortality patterns with models that find a variety of patterns, including constant and falling mortality over age, I identify conditions that licence constant or negative shapes of ageing. The results suggest that patterns of improvement and maintenance over age emerge when models potentially allow organisms to (1) escape the 'damage ratchet', (2) achieve maintenance and repair in parallel, (3) face increasing future reproductive potential and (4) incorporate flexible trade-offs. With these insights, theoretical models contribute to hypotheses about which species may follow life history strategies of negligible or negative ageing.