The Role of the Postorbital Ligament in the Zebra Finch (Taeniopygia Guttata)

Abstract

Cranial kinesis in birds has been the subject of many functional morphological investigations. One of the mechanisms proposed for avian jaw movement is a mechanical coupling of the upper and the lower jaw by the postorbital ligament. Upper jaw elevation is believed to enable lower jaw movement, which is blocked by the limited extensibility of the ligament. In a previous study it was shown that in the Zebra Finch the elastic force of the postorbital ligament is very small for low and medium amplitude gapes (< 7 mm) and does not block lower jaw depression in this species. However, these experiments were not suited to detect very small ligament forces and the jaw positions were imposed externally, which may affect the kinematics of the skull and thus the stretching of the postorbital ligament. In this study, we investigated in vivo whether the postorbital ligament plays a role during the husking of red millet seeds (diameter 2 mm; max gape 5 mm) in Zebra Finches (Taeniopygia guttata). We measured seed husking time, number of mandibulations, and percentage of rejected seeds before and after surgically removing part of the left and right postorbital ligament. Although no effect was expected removal of the ligament did change husking performance. Individual birds react differently to removal of the postorbital ligament. The data suggest that there is an inverse relationship between husking time and percentage of rejected seeds. Birds that reject more seeds after cutting the postorbital ligament have an average husking time that is similar or even lower than before surgery. When birds have the same rejection rate as before the treatment, husking time is significantly higher in the post-operative condition. Although the postorbital ligament primarily serves to limit the maximal extension of the lower jaw, it is speculated that small amplitude stretching of the ligament may play a role in the fine control of jaw opening during dorsoventral husking movements in estrildids.