Body size has fundamental impacts on animal ecology and physiology but has been strongly influenced by recent climate change and human activities, such as size‐selective harvesting. Understanding the ecological and life history consequences of body size has proved difficult due to the inseparability of direct effects of body size from processes connected to it (such as growth rate and individual condition). Here, we used the cnidarian Hydra oligactis to directly manipulate body size and understand its causal effects on reproduction and senescence. We found that experimentally reducing size delayed sexual development and lowered fecundity, while post‐reproductive survival increased, implying that smaller individuals can physiologically detect their reduced size and adjust life history decisions to achieve higher survival. Our experiment suggests that ecological or human‐induced changes in body size will have immediate effects on life history and population dynamics through a growth‐independent link between body size, reproduction and senescence.

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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