Abstract
It may be expected that experience of the different kind food during postnatal development, such as milk (liquid) at preweaning stage, weaning food (semi-liquid) at weaning stage and pellet (solid) at postweaning stage, should be crucial for development of ability to adapt mandibular movement and masticatory muscle activities to physically different food. Two groups of rabbits were raised on pellet and powder diet from 6-weeks old (just after weaning) to 33-weeks old during postnatal development, respectively. Around 31-32 weeks old, short term recordings of jaw movements and EMGs of the masseter muscle were made not only during mastication of the primary diet, but also during mastication of the diet which was used in the other group. The data obtained from the two groups during mastication of the same diet were compared to examine effects of the long term raising on soft food on functional developments of mastication. It was found that the rabbits of the powder-raised group were able to masticate pellet with no difficulty and no hesitance, even though this was the first time for the rabbits to masticate pellet at all. Mandibular movement pattern of the powder-raised group during pellet mastication was found to be of grind type, which was similar to those observed in the pellet-raised group during the same diet, pellet mastication. There was no significant difference in a number of chewing cycles consisting of a chewing sequence and in duration of jaw-closing phase between the two groups during mastication of 1-g pellet. These findings suggest that long term powder raising would not necessarily lead to impairment of the masticatory function to adapt masticatory apparatus to physically different diet, especially to hard diet. However, not fundamental but notable functional differences between the two groups were found; 1) duration of the jaw-opening phase was shorter in the powder-raised group compared with the pellet-raised group and 2) the masseter muscle activities appeared in the jaw-opening phase in the powder-raised group and the corresponding difference in the trajectories of the jaw movement was found between the two groups.
