A central question in developmental biology is how embryonic patterning remains robust when developmental tempo changes. In ectotherms, temperature strongly alters developmental speed and biochemical reaction rates, whereas diffusion is expected to change only weakly. How morphogen gradients required for patterning remain correctly scaled under these conditions is unclear. We addressed this problem in medaka embryos by quantifying developmental speed, morphogen clearance, diffusion, and endogenous Nodal dynamics across a broad temperature range. Developmental tempo, morphogen production, clearance, and tissue movement increased approximately two- to threefold per 10°C, whereas effective morphogen diffusion remained largely unchanged. Despite this mismatch, germ-layer proportions and morphogen patterning remained robust. Our mathematical simulations suggest that this robustness arises because the system operates in a reaction-dominated regime, in which changes in reaction and growth kinetics - rather than diffusion - are sufficient to rescale morphogen gradients. Together, our experiments and simulations provide new insights into how morphogen gradients adjust to maintain accurate tissue patterning across changes in developmental time and tempo. ### Competing Interest Statement The authors have declared no competing interest. European Research Council, https://ror.org/0472cxd90, 637840, 863952 Deutsche Forschungsgemeinschaft, EXC 2117 – 422037984, SFB1756 – 550938463 FWF Austrian Science Fund, J 4507-B

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Nature Physics - Zebrafish can regenerate amputated fins back to their original size. It is now shown that gradients in extracellular signal-regulated kinase activity act as rulers during fin bone...