Experimental manipulation of body size alters senescence in hydra

Body size has a fundamental impact on the ecology and physiology of animals. Large size, for instance, is often associated with increased fecundity and reproductive success. A persistent correlation between body size and individual longevity is also observed across the animal world, although this relationship proved difficult to understand due to the inseparability of body size from growth rate and the widespread collinear relationship between body size with other life history traits. Here, we used Hydra oligactis, a freshwater cnidarian with high tissue plasticity and inducible ageing as an experimental system to understand the causal roles of body size on reproduction and senescence. We first show that large size predicts accelerated sexual development, increased fecundity and reduced survival in a population sample of this species kept under common garden conditions in the laboratory. Next, using phenotypic engineering, we experimentally increased or decreased body size by reciprocally grafting pieces of the body column differing in size between hydra polyps. Experimentally reduced body size was associated with delayed sexual development and reduced fecundity. In parallel, post-reproductive survival was significantly higher in polyps with reduced size. These results suggest that small hydra can physiologically detect their reduced body size and adjust reproductive decisions to achieve a higher post-reproductive survival. Our observations offer a new perspective on why smaller individuals within a species live longer by suggesting a growth-independent link between body size, reproduction and senescence.