Sexual size dimorphism (SSD) is ubiquitous across animals with female bias most prominent in snakes and other ectothermic organisms. To understand how SSD evolves across species, Rensch’s Rule predicts that in taxa where males are larger, SSD increases with body size. In contrast, where females are larger, SSD decreases with body size. While this rule holds for many taxa, it may be ambiguous for others, particularly ectothermic vertebrates. Importantly, this rule suggests that the outcomes of SSD over phylogenetic time scales depends on the direction of dimorphism predicated on the difference in reproductive efforts between males and females. Here we examine SSD in the context of Rensch’s Rule in Thamnophiini, the garter and waternsakes, a prominent group composing the North American snake biota. Using a dated phylogeny, measurements of gape, body and tail size, we show that these snakes do not follow Rensch’s Rule, but rather female-biased SSD increases with body size. We in turn find that this allometry is most pronounced with gape and is correlated with both neonate and litter size, suggesting that acquiring prey of increased size may be directly related to fecundity selection. These changes in SSD are not constrained to any particular clade; we find no evidence of phylogenetic shifts in those traits showing SSD. We suggest several ways forward to better understand the anatomical units of selection for SSD and modularity