Body-size evolution on islands: are adult size variations in tiger snakes a nonadaptive consequence of selection on birth size?

Mean adult size has been used as the traditional measure of body size to explain trends of insular gigantism and dwarfism in a wide array of taxa. However, patterns of variation in body size at birth have received surprisingly little attention, leaving open the possibility that adult body-size differences are nonadaptive consequences of selection acting on neonate body size. Here I used an empirical and correlative approach to test this hypothesis in a mosaic of 12 island and mainland snake populations in Australia. Data collected on 597 adult and 1,084 neonate tiger snakes showed that (1) both adult and neonate mean body sizes varied strongly across populations; (2) prey diversity and size convincingly explained birth-size variations: birth size-notably, gape size-correlated with prey size; (3) neonate snout-vent length was significantly correlated with neonate gape size; and (4) neonate snout-vent length was significantly correlated with adult snout-vent length. Postnatal growth rates recorded under common-garden conditions differed across populations and were correlated with mean prey size. These data collectively suggest that (1) prey size is the main driver for the evolution of body size at birth in gape-limited predators, (2) adult size variations may reflect selective forces acting on earlier life stages, and (3) adult size variations may also reflect resource availability during ontogeny (notably, prey diversity).