Summary We examined the effect of diet on pre‐hibernation fattening and the gut microbiota of captive arctic ground squirrels ( Urocitellus parryii ). We measured body composition across time and gut microbiota density, diversity and function prior to and after five‐weeks on control, high‐fat, low‐fat (18%, 40% and 10% energy from fat, respectively), or restricted calorie (50% of control) diets. Squirrels fattened at the same...