The hardness of food plays an important role in food selection behavior in rats.

To examine the importance of the hardness of foods, we conducted behavioral and electromyographical experiments in Wistar male rats using three kinds of pellets, a hard commercially made pellet (MF), a hard privately produced pellet (H) and a soft privately produced pellet (S). MF and H had the same hardness but contained different ingredients, and S and H had the same ingredients but different degrees of hardness (S<H). In the behavioral experiment, when the rats fed with MF during the 3 weeks following weaning were presented with two types of pellets, S and H, they preferred S to H. On the other hand, the rats fed with powdered MF during the 3 weeks following weaning could not discriminate between S and H on the 1st day of the 6th week. When we compared the EMG activities of the masseter muscle during feeding of S or H pellets, we found that the rats raised on powdered food could not control their muscular power according to the hardness of foods on the first test day; however, rats recovered from this disability after one-day feeding of H and S pellets. After ingestion of either S or H was paired with an intraperitoneal (ip) injection of 0.15 M LiCl (2% of body weight), the animals of both groups avoided the pellets paired with LiCl. However, when the presented pellets were crushed, rats did not discriminate between crushed S and crushed H. These results suggest that the hardness of food plays an important role in the selection and ingestion of food and that the hardness of food could be a conditioned stimulus for conditioned food aversion.