The functional morphology of the jaw apparatus in the black guillemot (Cepphus grylle) and the thick-billed (Uria lomvia) and common (Uria aalge) murres

Abstract

The jaw muscles of the thick-billed murre have been described in detail and depicted with comparative notes on the jaw muscles in the common murre and the black guillemot. Owing to adaptation to underwater hunting of mobile prey (first of all, small fish), the composition of the dorsal adductor muscles in the studied species has become simplified in part, compared with their ancestral form, and has become similar to that of another underwater fisher, the great crested grebe (Podiceps cristatus). The attachment of the superficial portion of the external adductor muscle to the postorbital projection was strengthened, especially in both murres. In the external layer of the medial portion of the adductor, the muscle fibers gained secondary insertion by the horizontal aponeurosis, similarly to those of the superficial portion, thus increasing the horizontal (retracting) force component, which lowers the long upper jaw. The caudal part of the deep portion of the external adductor became incorporated in the medial portion, but in the murres, especially in the thickbilled murre, part of it is similar to the posterior adductor, which enhances the retracting force as well. All three species show traces of a transfer of the muscle fascicle from the superficial pseudotemporal muscle to the deep pseudotemporal muscle, giving the same effect. Confirmation of the relatively separated position of the guillemot and of its generalized morphological state were found. The specific features of the murres’ adaptation to fishing, including morphofunctional prerequisites for forage segregation of the two murre species, are understandable as the basis of their sympatric existence.