Complex sentence: Muscle tissue grows when stimulated, so it has been suggested that the calorie-restricted test animals exercised more than their companions on higher calories, perhaps because animals enter a foraging state during calorie restriction.
Split into simpler sentences: Muscle tissue grows when stimulated, so it is possible that the calorie-restricted test animals exercised more than their companions on higher calories. The reason behind this may be that animals enter a foraging state during calorie restriction.