sign in sign up
<<
Home , Anatinae, Anseranatidae, Malacorhynchus, Mergini, Oxyurini

THE FUNCTION AND OF THE SUPRAORBITAL PROCESS IN

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 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 , which feedsunderwater. In the Aythyini it is fairly well-developed in severalspecies of Aythya, which are excellentdivers, but is rudimentary in peposaca,which is moreof a surfacefeeder. It is alsorudimentary in the Canvasback,Aythya valisineria,which dives for vegetation.Among the the supraorbitalprocess is highly developedin (Poly- sticta, Somateria) , ( M elanitta) , and Long-tailed Duck ( Clangula byemalls), all of which feed mainly on invertebratestaken from the bottom. It is also fairly large in the , Histrionicushis- trio.nicus,which feeds on invertebrates,often in turbulent waters. In contrast, the processis absent or rudimentary in the and goldeneyes(Bucephala) and the closely related mergansers(). These speciesare largely carnivorous,the merganserseating mostly fish. Among the stifftail ducks () the supraorbitalprocess is unde- velopedin both the Black-headedDuck (Heteronettaatricapilla) and the Ruddy Duck (Oxyura jamaicensis)which feed mainly on vegetation,and

568 The Auk, 87: 568-572. July 197o July 1970] Supraorbital Processin Ducks 569

SALT GLAND

Figure 1. Lacrimal bone and adjacent structuresin a male King , Somateria spectabilis. in the carnivorousMusk Duck ( lobata). In the steamer ducks (Tachyerini) the processis quite large, resemblingthat in the eiders. Both groupsfeed widely on marine invertebrates. In general,the supraorbitalprocess is most highly developedin those forms that feed on the bottom and that might be likely to damage the eye by abrasionagainst underwaterobjects such as the hard shells in molluskbeds. Freshwaterdivers that feedon plantsor invertebratesin the soft mud of pondsor lakes are probably lesssubject to suchinjury. The mergansers,which pursue fish in open water, are less exposedto the possibilityof sucheye injuries.

EVOLUTION The most recentstudies of waterfowlevolution (e.g. Johnsgard,Water- /their biology and natural history, Lincoln, Univ. Nebraska Press, 196.8,p. 3) suggestthat the Oxyurini, Mergini, Aythyini, and Tachyerini arose independentlyfrom some Anatini-like, surface feeding ancestral group. Hence specializationfor diving and underwaterfood-gathering aroseat least four times in the history of the , and the develop- mentof an elongatedsupraorbital process presumably occurred three times. It is not clear why this structuredid not arise in such bottom-feeding forms as the Oxyurini or Merganetta armata. Perhaps the necessary geneticchanges upon which selectioncould operate simply never took place. The questionarises as to whether the elongatedprocess should be con- sideredhomologous in the differentgroups of ducks. It is generallyheld 57O ROBERTJ. R^mow [Auk, Vol. 87

TABLE 1 OCCURRENCEAND SIZE1 OF TIlE SUPRAORBITALPROCESS IN DUCKS

Actual length Relative lengths No. Range Mean Range Mean

TADORNINI C yanochen cyanopterus 1 Rudimentary3 Chloephaga melanoptera 2 Absent Neochen jubatus 1 Rudimentary Alopochen aegyptiacus 1 Absent Tadorna tadornoides 1 Rudimentary

TACHYERI•NI Tachyeres patachonichus 2 3.3-17.0 10.2 14-75 44 T. pteneres 1 6.3-7.0 6.7 24-25 24.5 T. brachypterus 1 8.4 8.4 - 49

CAIRININI Plectropterus gambensis 1 Absent Cairina moschata 2 Rudimentary Pteronetta hartlaubl 1 Rudimentary melanotos 1 Absent Nettapus coromandelianus 1 Rudimentary Callonetta leucophrys 1 Absent sponsa 4 Absent jubata 2 Absent Amazonefta braziliensis 1 Absent

ANAT1NI Merganetta armata 1 Absent Hymenolaimus 1 Absent crecca 5 Absent or rudimentary A. gibberifrons 3 Absent or rudimentary A. platyrhynchos 5 Absent or rudimentary A. rubripes 2 Rudimentary Malacorhynchus membranaceus 3 Rudimentary Marmaronetta anguirostris 1 Absent

AYTHYINI Rhodonessa caryophyllacea 1 Rudimentary Netta peposaca 2 Rudimentary A ythya valisineria 4 Rudimentary A. americana 5 0.7-1.8 1.3 3-9 6 A. marila 3 1.1-2.3 1.6 6-12 8 A. a]finis 5 1.3-2.7 2.1 7-14 11

MERGINI Somateria mollisima 4 4.7-8.9 6.2 18-33 25 S. spectabilis 3 4.6-5.2 4.9 19-23 21 S. fischeri 4 6.0-8.2 6.7 25-34 28 Polysticta stelleri 5 3.0-3.6 3.3 16-19 18 Histrionicus histrionicus 3 1.8-2.7 2.2 10-14 12 Clangula hyemalis 5 2.6-4.1 3.4 13-19 16 Melanitta nigra 3 3.7-5.0 4.6 18-24 22 M. perspicillata 5 2.3-4.0 3.2 13-18 15 M. ]usca 5 2.6-4.0 3.1 12-19 15

x Measurements in mm. -øThis is the actual length divided by the width of the orbit and expressed as a per- centage. The mean relative length is most useful for the purpose of comparing the development of the process in different species. a The process is considered rudimentary if less than 1.5 mm. in length. July 19701 Supraorbital Processin Ducks 571

TABLE 1 (Continued)

Actual length Relative lengths Species No. Range Mean Rt•nge Mean

Bucephala albeola 4 Absent or rudimentary B. islandica 1 Absent B. clangula 4 Absent Mergus albellus 1 Rudimentary M. serrator 5 Rudimentary M. merganser 3 Absent or rudimentary

OXYURIXI Heteronetta atricapilla 2 Absent or rudimentary Oxyura jamaicensis 5 Absent or rudimentary Biziura lobata 3 Absent or rudimentary that homologyoccurs when the condition or feature is also found in a form ancestralto the groupsunder discussion.As it is believed that the different tribes of diving ducks evolved separatelyfrom a surface-feeding ancestry,the conditionwould not be consideredhomologous by this defini- tion alone. It seemsprobable that in each case this advanced condition resulted by parallel evolution from a common ancestral structure, the tubercleof attachment of the orbital membrane. Bock (Amer. Naturalist, 47: 265, 1963) suggeststhat, in closelyrelated forms, featuresthat appear to be homologousmay have evolved independently in groups whose commonancestor possessed a precursorof the advancedcondition. This

TABLE 2 FAMILIES AND SPECIES OF BIRDS EXAMINED IN ADDITION TO THOSE IN TABLE 1

Spheniscidae: Pygoscelisadeliae; Spheniscushumboldti; S. magelIanicus; S. mendiculus; Megadyptes antipodes; Eudyptula minor; Aptenodytes ]orsteri. Gaviidae: Gayla adamsii; G. arctics; G. iraruer; G. stellata. Podicipedidae: Aechmophorusmajor; A. occidentalis; Centropelma micropterum; Podicepsauritus; P. caspicus;P. cristatus; Podilymbus . Diomediidae: Diomedes exulans; D. immutabilis; D. nigripes. : Daption capensis; Macronectesgiganteus ; Pterodroma leucopterus; Pachyptila ]orste•. Pelecanoididae: Pelecanoides garotii. Pelecanidae: PelecanusconspiciIlatus. : Sula dactylatra. Phalacrocoracidae: Nannopterum harrisi. Anhingidae: Anhinga novae-hollandiae. Fregatidae: Fregata magnificens. : A nseranassemipalmata. : Anser albi]rons; canadensis;B. bernicla; Cygnus strata; Coscoroba coscoroba; Dendrocygna autumnalis; D. bicolor. Rallidae: Fulica americana. Heliornithidae: Heliornis ]ulica. Alcidae: Pinguinus impennis; Uria aalge; Cerorhinca monocerata. Cinclidae: Cinclus mexicanus. 572 ROBgRTJ. Ra•ow [Auk, Vol. 87 wouldindicate that the descendentgroups share an evolutionarypotential inherited from the ancestor,and the advancedcondition should then be consideredto be homologous.The supraorbitalprocess in ducksappears to be such a case. In to ascertainwhether a similar developmenthas occurredin othergroups of aquaticbirds, I surveyedthe skullsof representativespecies of a wide variety of orders(Table 2). In mostcases no structureexists comparableto the elongatedsupraorbital process of ducks,although some penguinsand grebeshave a rudimentaryprocess that presumablyfunctions as a placeof attachmentof the orbitalmembrane. (Diomedea) and someProcellariidae (Dap.tion, Pterodro.ma) have a short,blunt process that may provideprotection similar to that suggestedfor ducks. A short processalso is presentin the Finfoot, HeliornisJulica. AmongAnatidae otherthan true ducks(Anatinae), a processof morethan rudimentarysize was found only in the swanlikeCo•scoroba coscoroba. Its significancein this nondiving,surface-feeding species is not known.

ACKN OWLEDG1V•ENTS

I would like to thank Nicolaas Verbeek and Wilbur Quay for reading and criticizing the manuscript,and Peter Ames for discussingthe problem with me. F. B. Gill kindly measuredspecimens of Tachyeresin the collectionof the University of Michi- gan Museum of Zoology. Gene M. Christman offered helpful advice in the prepara- tion of the illustration. I am also grateful to the following personsfor loaning or providing accessto specimensin their care: Dean Amadon, American Museum of Natural History; D. W. Snow, British Museum (Natural History); Robert W. Storer, University of Michigan Museum of Zoology, and Richard L. Zusi, U.S. National Museum.

Museum of Vertebrate Zoology, University of California, Berkeley, California 94720.