Differences may occur in the carnivore–omnivore–herbivore spectrum over the lifespan of a reptilian species, but it seldom occurs between adult males and females. Information regarding the dietary habits of Australian eastern bearded dragon (Pogona barbata) is also limited. We dissected museum specimens and road kills of the Australian eastern bearded dragon to compare ontogenetic shift in diet. Juveniles were insectivorous. They typically consumed larger, more active, arthropod prey than mature individuals – they are active predators. Adults were omnivorous and typically consumed small arthropod prey, and tended to be sit-and-wait predators. Mature males, particularly larger males, were primarily herbivorous. Such divergence in adult reptilian diet has rarely been reported. We suggest that the dietary switches observed are consistent with the Optimum Foraging Model. Juveniles require a high protein diet to maximise growth from juvenile to maturity. Beyond maturity females continue to require higher levels of protein for reproduction than males. At least in part, this is because males rely on sham aggression to defend territory during the reproductive season rather than resorting to aggressive behaviour. This results in a lesser requirement for protein for adult males than is required for juveniles and adult females. Males have the advantage of not being as dependent on protein, and thus are able to rely more heavily on vegetation.