Morphological differences between wild and game-farm Mallards (Anas platyrhynchos) in North America

Abstract

Large-scale releases of domesticated, game-farm Mallards Anas platyrhynchos to supplement wild populations have resulted in widespread introgressive hybridization that changed the genetic constitution of wild populations in eastern North America. The resulting gene flow is well documented between game-farm and wild Mallards, but the mechanistic consequences from such interactions remain unknown in North America. We provide the first study to characterize and investigate potential differences in morphology between genetically known, wild and game-farm Mallards in North America. We used nine morphological measurements to discriminate between wild and game-farm Mallards with 96% accuracy. Compared with their wild counterparts, game-farm Mallards had longer bodies and tarsi, shorter heads and wings, and shorter, wider and taller bills. The nail on the end of the bill of game-farm Mallards was longer, and game-farm Mallard bills had a greater lamellae:bill length ratio than wild Mallards. Differences in body morphologies between wild and game-farm Mallards are consistent with an artificial, terrestrial life whereby game-farm Mallards are fed pelleted foods, resulting in artificial selection for a more ‘goose-like’ bill. We posit that: (1) game-farm Mallards have diverged from their wild ancestral traits of flying and filter feeding towards becoming optimized to run and peck for food; (2) game-farm morphological traits optimized over the last 400 years in domestic environments are likely to be maladaptive in the wild; and (3) the introgression of such traits into wild populations is likely to reduce fitness. Understanding the effects of game-farm Mallard introgression requires analysis of various game-farm × wild hybrid generations to determine how domestically derived traits persist or diminish with each generation.