Is the island rule general? Turtles disagree

Aim: The island rule describes a trend toward dwarfism of large animals and gigantism of small animals on islands. Studies of vertebrates, including reptiles, report conflicting results regarding the generality of this evolutionary pattern. We studied the size evolution of insular turtles at the intra- and interspecific levels, as well as the clade level. We examined the evolutionary patterns, the relationship of habitat preferences and insular turtle size and whether the island rule extends to turtles. Location: Islands world-wide and adjacent continents. Methods: We collected data from the literature and expanded our database by measuring turtle specimens in museum collections. We used reduced major axis regression to test the island rule at the inter- and intraspecific levels, after accounting for phylogenetic effects. In addition, we compared body size frequency distributions of insular versus mainland turtles at the clade level. Results: Our analyses revealed no support for the island rule in turtles. Intraspecifically, we found no systematic pattern of body size evolution. Interspecifically, large turtle species tend to become larger on islands, in contrast to the island rule predictions. Islands generally harbour larger turtles than mainland regions. This pattern is especially apparent when recently extinct species are considered. We found no impact of habitat preferences on the evolution of body size on islands. Main conclusions: Turtles on islands do not follow the island rule. We suggest that both physiological and ecological factors drive turtle body size evolution on islands, and their tendency for insular gigantism. Large turtles can survive longer periods of water or food shortage on small oceanic islands. Increased intraspecific competition and reduced interspecific competition and predation on islands may favour large body size. A non-mutually exclusive explanation is that the presence of giant tortoises on islands may reflect founder effects rather than evolution - large turtles are better island colonizers.