DEVELOPMENT AND HOMOLOGIES OF HEAD STRIPES IN THE DELPHINOID CETACEANS

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Abstract

Ontogeny and variation of head stripes were investigated in Stenella attenuata, S. longirostris, and Delphinus delphis. The eye stripe and the blowhole stripe together comprise the bridle. The Anlage of the bridle is a line extending across the back of the head in small fetuses. As the blowhole moves posteriorly and the rostrum lengthens, an inflection point develops in the line above the eye. This point moves forward, creating a progressively greater triangular area between the incipient eye and blowhole stripes. Well before birth, the inflection point is close to the apex of the melon, and the eye and blowhole stripes are narrow and well defined. This process proceeds to varying degrees in the other delphinoids. For example, in Peponocephala electra and in Cephalorhynchus spp., the inflection point migrates only part of the way to the apex of the melon, and the resulting “blowhole stripe” remains a broad caplike feature, resembling the state of development in small fetuses of Stenella and Delphinus. A simple eye spot without a stripe may be primitive, as it occurs in non-delphinoid cetaceans. Presence of the bridle only in the delphinids and phocoenids may unite them in a clade exclusive of the monodontids.

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