Island Tiger Snakes ( Notechis scutatus ) Gain a ‘Head Start’ in Life: How Both Phenotypic Plasticity and Evolution Underlie Skull Shape Differences

Repeated island colonisation by Australian tiger snakes ( Notechis scutatus ) has become a model system demonstrating how prey size on islands influences a snake’s body and jaw size. Tiger snakes on islands with large prey have relatively longer jaws compared to their mainland counterparts, due to diet-induced phenotypic plasticity followed by assimilation of favourable traits. We present the first examination of the effects of diet on all skull elements that are involved in feeding, by analysing shape and size differences using CT imaging and a combination of linear measurements and three dimensional geometric morphometrics. We compared two populations of tiger snakes, one from Carnac Island, where the snakes were first introduced approximately 100 years ago, and another from Herdsman Lake on the mainland (a putative source population). Each population was divided into two groups, one was fed small prey and the other large prey. While snakes from the island exhibited relatively longer trophic bones at birth, they also had slightly slower growth rates for these elements regardless of diet. The island forms showed diet-induced plasticity within specific trophic elements, the mandible and palatopterygoid, which grew longer when the snakes were fed larger prey. Importantly, skull plasticity was expressed only after prolonged dietary stress, and was not clearly observable until the snakes approached adulthood. We hypothesize that this plastic response resulting in increased gape may be adaptive, allowing ingestion of large prey items available to adult tiger snakes on Carnac Island. In contrast, no plastic response was observed in the mainland population.