Observations of non-feeding gulps in dwarf minke whales Balaenoptera acutorostrata sensu lato confirmed the axial rotation and lateral divergence (omega rotation) of the lower jaw suggested for rorquals. Gulps were either restricted to the inter-mandibular area or involved expansion of the whole ventral pouch; the extent of filling appears to be under voluntary control. Gulps may have different functions, e.g. feeding or display. Maximum gape (about 70º) occurred during inter-mandibular gulps, involving both depression of the lower jaw and elevation of the head and upper jaw. The lower jaw was depressed only to
about 40º, much less than the 90º generally illustrated in the literature for rorquals. The mouth was closed as the ventral pouch was still filling; closure was rapid, associated with the moderate depression of the lower jaw. The whole ventral pouch contracted uniformly to expel water. The fibrocartilage skeleton of the ventral pouch was involved in outpocketing of the mental (“chin”) region both at the beginning and end of gulps. During expulsion of water, partial axial rotation of the lower jaw maintained a groove just lateral to the baleen plates, opening as a vertical slit posteriorly. This would allow water expelled between the baleen plates to flow backwards, especially from the angle of the mouth. Incorporating these new observations, we discuss evolution of filter feeding and suggest that suction feeding was the primitive condition for baleen whales
