Seasonal breeding in mammals is traditionally attributed to melatonin signalling from the pineal gland, which transmits photoperiod information to biological clock pathways. However, this mechanism does not fully account for seasonal reproduction across species: wolves continue to breed seasonally even after removal of their pineal gland, indicating that melatonin is not strictly required for reproductive timing. This suggests the existence of additional, melatonin-independent pathways enabling animals to detect and respond to seasonal cues. To identify genetic determinants of seasonal breeding, we performed a genome-wide association study in domestic dogs, comparing 16 strictly seasonal-breeding Basenjis and Tibetan Mastiffs with 769 non-seasonal dogs. This analysis revealed a single genomic region that strongly distinguishes seasonal from non-seasonal breeders. The strongest signal maps to a region containing a non-synonymous substitution in a histidine-catabolism gene, encoding urocanate hydratase (UROC1), that is otherwise highly conserved across animals. Mouse studies show UV exposure increases skin urocanic acid, the substrate of UROC1, which is metabolised into brain glutamate, linking light exposure on skin to neuronal signalling. To test for functionality, we introduced the UROC1 canine variant into C. elegans using CRISPR. Tissue expression analysis using a uroc-1::GFP reporter, supported by previously reported single-cell RNA-seq annotations, showed that uroc-1 is expressed most in the tail hypodermis, seam cells and in glial-like socket and sheath cells that support glutamatergic sensory neurons. Whole-worm RNA-seq of the uroc-1 mutant revealed differential expression enriched for genes expressed in these same cells and in glutamatergic neurons. This concordance provides evidence that the canine variant alters UROC1 enzyme function, modifying histidine-derived glutamate supply within a local metabolic network surrounding glutamatergic neurons. Together, these findings hint at how a single metabolic enzyme variant could alter seasonal breeding in dogs and suggest that histidine-derived glutamate links environmental cues to neuronal activity and behaviour across the animal kingdom.