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

Gray jays on Anticosti Island, Québec, exhibit a unique phenotypic adherence to the island rule—being approximately 10% heavier than mainland conspecifics without structural size differences. This challenges the assumption that insular populations must evolve distinct genetic traits to achieve larger body masses. Researchers compared body size and mass of gray jays from Anticosti with three mainland populations (two in Québec, one in Ontario) and found no evidence of structural enlargement, suggesting that increased body mass may result solely from denser packing within a mainland-sized frame. This phenomenon could be advantageous in Anticosti's high-density, predator-free environment, offering a potential model for the initial phenotypic step toward island rule compliance.

Strickland, D., & Norris, D. R. (2015). An example of phenotypic adherence to the island rule? – Anticosti gray jays are heavier but not structurally larger than mainland conspecifics. Ecology and Evolution, 5(17), 3687–3694. Portico. https://doi.org/10.1002/ece3.1557