The Function and Evolution of the Supraorbital Process in Ducks
THE FUNCTION AND EVOLUTION OF THE SUPRAORBITAL PROCESS IN DUCKS ROBERTJ. RAIKOW THE anteriormargin of the orbit in ducksis formedby the lacrimalbone, which articulateswith the anterolateralmargin of the frontal boneand the posterodorsalcorner of the maxillary processof the nasal bone. The evolu- tion of this bone in birds generally has been dealt with recently by Cracraft (Amer. Midl. Naturalist, 80: 316, 1968), whoseterminology for the parts of the lacrimal bone is followedhere. In many ducksthe postero- dorsalcorner of the lacrimal is marked by a small tubercle,which serves as the site of attachment of the anterior end of the orbital membrane,a sheetof connectivetissue that coversand protectsthe dorsalaspect of the eyeball. In some forms this tubercle has become elongated to form a stout, finger-like projection, the supraorbitalprocess. This appears to provide mechanicalprotection to the eyeball and salt gland (Figure 1). Table 1 lists the occurrenceand degree of developmentof this process in the skulls of all living generaof ducks. CORRELATION WITH FEEDING AND LOCOMOTOR HABITS In the following discussion,data on feeding habits are from Delacour (The waterfowlof the world,vols. 1-3, London,Country Life Ltd., 1954, 1956, 1959). Table 1 showsthat the supraorbital processis developed significantlyonly in certain groupsof ducks that feed underwater. It is absentor rudimentaryin the Tadornini, Cairinini, and Anatini, which are primarily surfacefeeders, but also in Merganetta arma.ta,the Torrent Duck, which feedsunderwater. In the Aythyini it is fairly well-developed in severalspecies of Aythya, which are excellentdivers, but is rudimentary in Netta peposaca,which is moreof a surfacefeeder. It is alsorudimentary in the Canvasback,Aythya valisineria,which dives for vegetation.Among the Mergini the supraorbitalprocess is highly developedin eiders (Poly- sticta, Somateria) , scoters( M elanitta) , and Long-tailed Duck ( Clangula byemalls), all of which feed mainly on invertebratestaken from the bottom.
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