An example of phenotypic adherence to the island rule? - Anticosti gray jays are heavier but not structurally larger than mainland conspecifics

Abstract

The island rule refers to the tendency of small vertebrates to become larger when isolated on islands and the frequent dwarfing of large forms. It implies genetic control, and a necessary linkage, of size and body-mass differences between insular and mainland populations. To examine the island rule, we compared body size and mass of gray jays (Perisoreus canadensis) on Anticosti Island, Québec, located in the Gulf of St. Lawrence, with three mainland populations (2 in Québec and 1 in Ontario). Although gray jays on Anticosti Island were ca 10% heavier, they were not structurally larger, than the three mainland populations. This suggests that Anticosti jays are not necessarily genetically distinct from mainland gray jays and that they may have achieved their greater body masses solely through packing more mass onto mainland-sized body frames. As such, they may be the first-known example of a proposed, purely phenotypic initial step in the adherence to the island rule by an insular population. Greater jay body mass is probably advantageous in Anticosti's high-density, intensely competitive social environment that may have resulted from the island's lack of mammalian nest predators. The island-rule proposes that small animals become larger when isolated on islands whereas large animals tend towards dwarfism. We found that gray jays (Perisoreus canadensis) on Anticosti Island, Québec were ca 10% heavier but not structurally larger, than three mainland populations, suggesting suggests that Anticosti jays are not necessarily genetically distinct from mainland gray jays and that they may achieve their greater body masses solely through packing more mass onto mainland-sized body frames. This is the first known example of a proposed, purely phenotypic initial step in the adherence to the island-rule by an insular population.