Rats defend different body weights depending on palatability and accessibility of their food.

Normal adult rats lived on powdered diets adulterated to contain as much as 1.6% quinine sulfate, on a palatable high-fat diet, or in Skinner boxes with 45-mg Noyes pellets available on fixed-ratio (FR) schedules as high as FR 156. They maintained lower body weights over periods of months in proportion to the percentage of quinine adulteration or the fixed ratio. Rats exposed to the high-fat diet overate as much and gained weight as rapidly as rats recovering from food deprivation, and became moderately obese. Rats having become lean or obese contingent on the palatability or accessibility of their diet defended body weight by eating more in the cold, less when force-fed by gavage, and more to restore weight after food deprivation. Yet on chow they restored and defended body weights typical of rats whose diet had been confined to commercially prepared chow. These results are interpreted to be inconsistent with motivational models that rigidly distinguish drive from incentive, that treat body weight changes as evidence for failure to regulate energy balance or body weight, or that rely exclusively on deprivation of food or reduction of body weight for definitions of need for calories. Instead, caloric homeostasis in rats may incorporate ecological constraints.