Rapid prey-induced shift in body size in an isolated snake population (Notechis scutatus, Elapidae)

Geographic divergence in phenotypic traits between long-isolated populations likely has a genetic basis, but can phenotypic plasticity generate such divergence rapidly in the initial stages of isolation? Australian tiger snakes (Notechis scutatus, Elapidae) provide a classic model system for the evolution of body size: mean adult sizes are relatively invariant in mainland populations, but many offshore islands have dwarf or giant populations. Previous work has shown a genetic basis to this divergence in long-isolated islands (>10 000 years), but what of the initial stages of this process? Human translocation of mainland snakes to Carnac Island 90 years ago gives us a unique opportunity to assess the proximate reasons for the giant size of Carnac Island animals compared with mainland conspecifics. Our data suggest a major role for phenotypic plasticity. Feeding trials on captive snakes from both island and mainland populations showed a strong link between food intake and growth rates, similar in the two populations. Snakes given abundant food grew much larger than we have ever recorded in the wild, demonstrating that observed mean body sizes are driven by food availability rather than genetic limits to growth. In combination with earlier work showing genetic divergence in growth rates in snakes from long-isolated islands, our data suggest that geographical divergence in mean adult body sizes in this system initially is driven by a rapid shift due to phenotypic plasticity, with the divergence later canalized by a gradual accumulation of genetic differentiation.