Phenotypic and developmental dissection of an instance of the island rule.

Organismal body weight correlates with morphology, life history, physiology, and behavior, making it perhaps the most telling single indicator of an organism's evolutionary and ecological profile. Island populations provide an exceptional opportunity to study body weight evolution. In accord with the "island rule," insular small-bodied vertebrates often evolve larger sizes, whereas insular large-bodied vertebrates evolve smaller sizes. To understand how island populations evolve to extreme sizes, we adopted a developmental perspective and compared a suite of traits with established connections to body size in the world's largest wild house mice from Gough Island and mice from a smaller-bodied mainland strain. We pinpoint 24-h periods during the third and fifth week of age in which Gough mice gain exceptionally more weight than mainland mice. We show that Gough mice accumulate more visceral fat beginning early in postnatal development. During a burst of weight gain, Gough mice shift toward carbohydrates and away from fat as fuel, despite being more active than and consuming equivalent amounts of food as mainland mice. Our findings showcase the value of developmental phenotypic characterization for discovering how body weight evolves in the context of broader patterns of trait evolution.