The vertebrate photoperiodic neuroendocrine system uses photoperiod as a proxy to time annual rhythms in reproduction. To investigate seasonal adaptation in mammals, the hinge region and the first part of the transmembrane domain of the Tshr gene were sequenced for 278 common vole (Microtus arvalis) specimens from 15 localities in Western Europe, and 28 localities in Eastern Europe. Forty-nine single nucleotide polymorphisms (SNPs; 22 intronic, 27 exonic) were found, with weak or zero correlation with pairwise geographical distance, latitude, longitude and altitude. By applying a temperature threshold to the local photoperiod-temperature ellipsoid, we obtained a predicted critical photoperiod (pCPP) as a proxy for spring onset of local primary food production (grass). The pCPP obtained explains the distribution of genetic variation in Tshr in Western Europe through highly significant correlations. Thus, Tshr, which plays a pivotal role in the sensitivity of the mammalian photoperiodic neuroendocrine system, was targeted by natural selection, resulting in optimized timing of seasonal reproduction.