The gray Wolf (Canis lupus) is viewed as one of the most diverse mammal species. In north America, the diversity of its forms is debated, with views conflicting on subspecies designation. The present study aimed to reinvestigate the skull morphometric variation among north American populations while attempting to unveil underlying causal factors. A large sample of vouchered museum skulls, collected from 12 ecogeographical populations spanning the north American range of the species, was examined and 21 craniodental characters were measured. Skull shape showed within-population variations but provided evidence for a high morphological affinity among populations. Allometric analyses also pointed to similar evolutionary paths among populations. However, significant size-related differentiation was revealed within and among populations. Skull size could be related to three insulin-like growth factor-1 gene (IGF-1) alleles. Ecological conditions that should determine prey type and availability accounted for most of the skull size variation. In contrast, no evidence of geographical isolation of populations was detected. The results support the existence of a single morphological pool of north American gray wolf populations that could be equated with one taxonomic unit. This study raises again the question of the diversity of forms in this species in north America and calls into question the validity of previously recognized species and subspecies based on genetics and morphology.