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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