Jaw Musculature of the Mourning and White-winged Doves

by Robert L. Merz

CRANIAL OSTEOLOGY

The ability of most birds to protract the upper mandible, and the structure of the skull which enables them to do so are responsible for common reference to the skull as "kinetic" (Beecher, 1951a:412; Fisher, 1955:175). The movement is effected by muscular action on a series of movable bones that exert their forward force on the upper mandible, which in turn swings on a horizontal hinge, the "naso-frontal hinge," at the base of the beak. The bone initiating the movement is the quadrate, which is hinged posteriorly by its otic process and which ordinarily swings up or down depending on the muscle or muscles being contracted at any given moment. The upward swing of the quadrate pushes the jugal bar, which is attached to its lateral tip, along its longitudinal axis, in an anterodorsal direction, and the force is transferred to the upper mandible, which is thereby elevated. A synergetic mechanism is simultaneously initiated by the same bone—the quadrate. Since the quadrate body articulates with the pterygoid, the upward movement forces the pterygoid to slide along a ridge in the ventral midline of the cranium, the sphenoidal rostrum, thus pushing the palatine forward and exerting an upward push on the upper mandible.

In the columbids the quadrate has a bifurcated otic process that functions as the hinge. The posterior tips of the forks are situated almost vertically (one above the other) and the movement of the quadrate is not so much up and down, or vertical, as it is horizontal (fig. 12). When the quadrate moves medially the upper mandible is protracted; a lateral movement results in retraction. There is a slight, almost negligible, upward movement of the quadrate. The movements of the various bony elements were observed on a skull that had been made flexible by boiling in water for a minute as suggested by Beecher (1951a:412).

Also in the columbids the naso-frontal hinge is not constructed in the same manner as it is in many other birds as there is not a simple hinge across the entire base of the beak. In fact, there is no true hinge at all in the area of the nasals, but those bones are extremely thin and they bend or flex under pressure. Actually, the hinge is double or divided. One part is on either side of the nasals. The hinges are situated at the posterodorsal tips of two thin processes of the maxillary bones and the appearance is not unlike that of half a span of a suspension bridge having the hinges at the tops of the towers. Several other species of birds share this type of hinge construction with columbids.

The movement of the lower jaw is, of course, the primary operation involved in opening the mouth. The lower jaw possesses a deep fossa at its posterior end, or on its posterodorsal surface, which articulates with the body of the quadrate bone. The length of that part of the mandible extending behind the articulation is directly correlated with the resistance offered the mandible in opening, since it is on the posterior extension that the depressor of the lower mandible inserts. The larger the muscle the more surface is needed for attachment. Also the added length of the mandible posterior to the articulation serves as a lever in opening the mandible, and the fulcrum is moved relatively farther forward.

In birds lacking resistance to abduction of the lower mandible, as in doves, it is nevertheless necessary for a slight postarticular process to remain for the insertion of a small depressor muscle which, as mentioned previously, is necessary to counteract the relaxed adductor muscles of the lower jaw.

There are many exceptions to the rule that birds have kinetic skulls, and usually a secondary fusion and reinforcement of bones around the hinge has limited or eliminated all movement. Sims (1955) describes the Hawfinch's immobile upper jaw, which is used as a powerful press in cracking the stones of fresh fruit. Skulls of woodpeckers have been modified somewhat in the same manner as a result of their foraging and nesting habits (Burt, 1930).

The two most distantly related members of the genera under investigation are the White-winged Dove, Zenaida asiatica, and the Mourning Dove, Zenaidura macroura. They were chosen to demonstrate differences and likenesses in proportions of members of the genera.

Ten measurements were taken on each skull, but simple observation reveals that, in relation to total length of the skull, the beak of the White-winged Dove is longer than that of the Mourning Dove. Tip of upper mandible to base of beak averaged 48.6 and 42.9 per cent of the total length of the skull in the White-winged Dove and Mourning Dove, respectively. The position of the jugal bar has remained about the same with respect to the cranial part of the skull, and the entire cranial part of the skull is almost the same shape in the species studied.

Likewise, in the White-winged Dove the distance from the anterior tip of the lower mandible to the anterior part of M. adductor mandibulae externus is relatively longer in relation to the length of the lower mandible than in the Mourning Dove. Finally, the position of the jugal with respect to the naso-frontal hinge is about the same in the two species.

Measurements and calculations indicate that the longer beak of the White-winged Dove as compared with the Mourning Dove is a function of the beak itself, not of differences in other parts of the skull. Measurements of skulls of Eared and Zenaida doves support this view.