Experiments on the food preferences of wild rats (Rattus norvegicus Berkenhout)

1. Colonies of wild Rattus norvegicus Berkenhout were kept in closed rooms containing nesting sites. The conditions permitted breeding. 2. Seventy-six experiments, each lasting 6–35 days, were done to test the rats' food preferences. In each experiment except two, a choice was given between two foods, both available in excess. In the two exceptional experiments a choice between three foods was given. 3. Although marked preferences were shown, a food was rarely quite rejected in favour of an alternative. 4. Among cereals, wholemeal was preferred to wheat grains, but wheat grains were preferred to white flour; little or no white flour was eaten if wholemeal was available. Wheat germ was eaten only in small quantities when an alternative cereal was available, but in larger quantities if the alternative was sugar or a sugar-cereal mixture. 5. The addition of sugar or saccharin to cereals increased acceptability; sugar was preferred to saccharin. 6. The addition of arachis oil to wholemeal or wheat increased acceptability. 7. Cod-liver oil at 2·4 % acted as a deterrent. 8. Total calorific intake remained steady over short periods, but over several weeks rose as the mass of the rat colony increased. The higher the mean weight of the rats in a colony, the lower the calorific intake per unit weight of rat. 9. The amount of food eaten was partly determined by its calorigenic capacity: in particular, wet foods were eaten in much larger quantities than dry foods. 10. The food preferences of wild rats differ in several respects from those of white Rattus norvegicus. 11. Apart from palatability or flavour, the main factors influencing food preference were (a) previous experience and probably (b) the nutritional value of the food. The effect of previous experience was often observed when rats, accustomed to one food, gradually changed from the accustomed food to another. The stages of a change to a new food, or the establishment of a particular preference, varied, and are discussed in detail.