KINETICS OF THE SKULL IN THE CHICKEN (GALLUS-GALLUS-DOMESTICUS)

Domestic fowl have prokinetic skulls and are equipped with a flexible but inextensible postorbital ligament, connecting the braincase with the external process of the mandible. According to BOCK (1964), the latter point of attachment acts as a fulcrum for the rotating mandible, whenever this ligament is taut (loaded), establishing a 'coupled cranial kinesis' in which the movements of upper and lower jaws are not independent of one another but are coupled mechanically. In this view coupled cranial kinesis will be the result, for instance if the M. depressor mandibulae contracts alone or ahead of the M. protractor pterygoidei when opening the beak. BOCK (1964) allows for the fact that "considerable independent movement of the jaws is possible" in a coupled kinetic skull. Cinematographic and electromyographic analyses were used to determine if and when coupled kinesis is utilized by the chicken during pecking behaviour. The results show that coupled cranial kinesis does not play a dominant role during feeding. Opening of the beak prior to the grasp of the food is the result of protractor activity starting some 20 msec ahead of depressor activity, precluding the possibility of coupled kinesis. According to the analyses it seems unlikely that coupled kinesis is used during any beak opening stages of the pecking behaviour. However, coupled kinesis could have been utilized in closing of the bill. If coupled kinesis is operative the elevated upper jaw cannot be depressed without simultaneously raising the mandible. Contrary to what may be expected in a coupled kinetic skull, in 26 out of 34 grasps the lower jaw is not raised when the upper jaw is lowered. Nevertheless, simultaneous closing of both bills occurs in 40% and possibly even up to 75% of food transporting acts. Whereas the primary and necessary condition for coupled kinesis, viz. loading of the postorbital ligament, could as yet not be measured successfully, a firm conclusion on coupled kinesis during closing of the beak is still pending. © 1991 BRILL.