Invariant synaptic density across species

The nervous system scales with animal size, maintaining function. However, it is unknown how variations in neuronal size are compensated to preserve excitability. Here, we measured neuronal cell structure and connectivity from fly and zebrafish to mouse and human and discovered an invariant feature that stabilizes neuronal responses: an average of one synapse for every micrometer of dendritic cable. We provide evidence of how this pattern emerges from precise spatial positioning, electrical activity, and resource conservation in neurons, suggesting a simplifying scaling principle across species.