Background <p>Many animals change traits over the course of their life history. It has been proposed that some of these developmental changes evolve in concert with shifts in ecology, but there are relatively few clear examples, and identifying the underlying molecular changes is challenging. Australian brown snakes (<i>Pseudonaja</i> spp.), one of Australia's most venomous snakes, may provide such an example, as previous work has shown that these species undergo a dietary shift from reptiles to mammals, accompanied by differences in venom activity.</p> Results <p>We show that the venom composition of adult brown snakes is distinguished by the collective expression of four procoagulant toxin families, which are absent in juvenile life stages, where venom composition is instead dominated by neurotoxins. The developmental onset of expression across all four procoagulant toxins correlates with a functional shift in venom activity to one that disrupts the blood-clotting system of mammalian prey. We further show that this developmental timing in both toxin expression and procoagulant venom activity coincides with a broader ecological transition in which Australian brown snakes begin to prey on mammals. In contrast, we found that the smallest brown snake species, a lizard specialist throughout its lifetime, only expresses neurotoxins and shows no comparable developmental shift in either procoagulant toxin expression or procoagulant activity.</p> Conclusion <p>Our findings suggest that a coordinated shift in the expression of four procoagulant toxin gene families transitions the venom from a neurotoxic phenotype in lizard-feeding juveniles to a potent procoagulant phenotype in mammal-feeding adults. These results provide a clear example of how the concerted upregulation of multiple genes can drive a functional shift in a key ecological trait, linking changes in toxin gene expression to venom activity and, ultimately, to prey-capture strategy.</p>