10Body size has a fundamental impact on the ecology and physiology of animals. Large size, for 11 instance, is often associated with increased fecundity and reproductive success. A persistent 12 correlation between body size and individual longevity is also observed across the animal world, 13 although this relationship proved difficult to understand due to the inseparability of body size 14 from growth rate and the widespread collinear relationship between body size with other life 15 history traits. Here, we used Hydra oligactis, a freshwater cnidarian with high tissue plasticity 16 and inducible ageing as an experimental system to understand the causal roles of body size on 17 reproduction and senescence. We first show that large size predicts accelerated sexual 18 development, increased fecundity and reduced survival in a population sample of this species 19 kept under common garden conditions in the laboratory. Next, using phenotypic engineering, we 20 experimentally increased or decreased body size by reciprocally grafting pieces of the body 21 column differing in size between hydra polyps. Experimentally reduced body size was associated 22