We evaluated the effect of dietary fat on body fat of meadow voles (Microtus pennsylvanicus) in the field after a 6-wk supplemental feeding period with a 25% dietary-fat food. The initial 3-wk period showed no difference in body composition, but, after 6-wk, 33% of captures showed body fat exceeding 10%, levels atypical of wild populations. Fattening levels found with supplemental food were similar to body fat levels seen in lab-reared or wild animals removed and kept under lab conditions. Our results suggest that the lean body composition found in wild populations is influenced strongly by high-fiber and low dietary-fat forage, along with other environmental constraints. When provided easily available and abundant high dietary-fat food in the wild, a higher percentage of individuals increased body fat to levels rarely seen under natural conditions.