The role of the postorbital ligament in the zebra finch (Taeniopygia guttata)

Cranial kinesis in birds has been the subject of many functional morphologi cal investigations. One of the mechanisms proposed for avian jaw movement i s a mechanical coupling of the upper and the lower jaw by the postorbital l igament. Upper jaw elevation is believed to enable lower jaw movement, whic h is blocked by the limited extensibility of the ligament. In a previous st udy it was shown that in the Zebra Finch the elastic force of the postorbit al ligament is very small for low and medium amplitude gapes (< 7 mm) and d oes not block lower jaw depression in this species. However, these experime nts were not suited to detect very small ligament forces and the jaw positi ons were imposed externally, which may affect the kinematics of the skull a nd thus the stretching of the postorbital ligament. In this study, we inves tigated in vivo whether the postorbital ligament plays a role during the hu sking of red millet seeds (diameter 2 mm; max gape 5 mm) in Zebra Finches ( Taeniopygia guttata). We measured seed husking time, number of mandibulatio ns, and percentage of rejected seeds before and after surgically removing p art of the left and right postorbital ligament. Although no effect was expe cted removal of the ligament did change husking performance. Individual bir ds react differently to removal of the postorbital ligament. The data sugge st that there is an inverse relationship between husking time and percentag e of rejected seeds. Birds that reject more seeds after cutting the postorb ital ligament have an average husking time that is similar or even lower th an before surgery. When birds have the same rejection rate as before the tr eatment, husking time is significantly higher in the post-operative conditi on. Although the postorbital ligament primarily serves to limit the maximal extension of the lower jaw, it is speculated that small amplitude stretchi ng of the ligament may play a role in the fine control of jaw opening durin g dorsoventral husking movements in estrildids.