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Ecologiapermit numbers0492, 00987,and 11782.We white boobyand the brown boobySula spp. at thank the Secretaria de la Marina, Sexta Zona Naval, Asension. Ibis 103b:174-234. Guaymas,for transportationand food deliveries.For DRUMMOND,H. 1987. A review of parent-offspring logisticsupport, we are indebtedto the InstitutoTec- conflict and brood reduction in Pelecaniformes. no16gicoy de EstudiosSuperiores de Monterrey- Colonial Waterbirds 10:1-15. CampusGuaymas, Rescue One, A. Robles,E. Velar- DRUMMOND, H., AND C. GARCIA CHAVELAS. 1989. de, J. Guzman, L. Gonzales, Prescott College, Baja Food shortageinfluences sibling aggressionin Expeditions,Special Expeditions, H. and V. Ballance, the Blue-footedBooby. Animal Behaviour37: and T. Pfiester. We are grateful to Eureka Tents, 806-820. OceanKayaks, S. Emlen,J. Guzman,and E. Velarde, FORBES,L. 1991. Insurance offspring and brood re- for use of researchequipment. We especiallythank duction in a variable environment: The costs and D. Able, A. Angeles,S. Ashe,X. Basurto,G. Brabata, benefitsof pessimism.Oikos 62:325-332. C. Carmona, M. Cervantes,D. Craig, G. Fernandez, KEPLER,C. B. 1969. Breedingbiology of the Blue- K. Mingo, C. Navaro, N. Nufiez, H. P•rez, and E. To- facedBooby Sula dactylatra personata on GreenIs- b6n for excellenthelp and companyin the field. We land, Kure Atoll. Publications of the Nuttall Or- are grateful to D. Anderson, H. Drummond, and nithologicalClub No. 8. BRT's graduate committee members E. Adkins-Re- MOCK, D. W., H. DRUMMOND, AND C. H. STINSON. gan, S. Emlen, T. Gavin, P. Sherman,and D. Winkler 1990. Avian siblicide. American Scientist 78:438- for their excellentreviews of the manuscript. 449. MOCK, D. W., AND L. S. FORBES. 1992. Parent-off-

LITERATURE CITED spring conflict:A caseof arresteddevelopment. Trends in Ecologyand Evolution7:409-413. ALVAREZ-BORREGO,S. 1983. Gulf of California.Pages NELSON,J. B. 1978. The Sulidae: Gannets and boo- 427-449 in Estuaries and enclosed seas (B. H. bies. Oxford University Press,Oxford. Ketchum, Ed.). Elsevier, Amsterdam, The Neth- SIMMONS,R. G. 1988.Offspring quality and the evo- erlands. lution of cainism. Ibis 130:339-357. AMERSON,A. B., Jr., AND P. C. SHELTON.1976. The TERSHY, B. R. 1998. Sexual selection in the Brown naturalhistory of JohnstonAtoll, centralPacific Booby.Ph.D. thesis,Cornell University,Ithaca, Ocean. Atoll Research Bulletin No. 192. New York. ANDERSON,D. J. 1990a.Evolution of obligatesiblicide TRIVERS,R. L. 1974. Parent-offspringconflict. Amer- in boobies.1. A test of the insurance-egghy- ican Zoologist 14:249-264. pothesis.American Naturalist 135:334-350. WOODWARD,P. W. 1972.The natural history of Kure ANDERSON,D. J. 1990b.Evolution of obligatesiblici- Atoll, Northwestern Hawaiian Islands. Atoll Re- search Bulletin No.164. de in boobies.2. Food limitation and parent-off- spring conflict.Evolution 44:2069-2082. Received23 March 1998, accepted10 February2000. DORWARD,D. E 1962. Comparativebiology of the Associate Editor: T. W Arnold

TheAuk 117(3):820-825, 2000

The 20-cm Spiny Penis of the Argentine Lake Duck (Oxyura vittata)

KEVIN G. MCCRACKEN • Schoolof Forestry,Wildlife, and Fisheries, Louisiana State University, Baton Rouge, Louisiana 70803, USA

The genitalsof male birds generallyare not noted mutual juxtapositionof the protruded cloacae,and for their sizeor extravagantornamentation. For most there is relatively little contactbetween males and fe- speciesof birds, well-developedpenes or intromit- males comparedwith the processof coital penetra- tent organs are absent(e.g. King 1981, Briskie and tion in mammals,reptiles, some species of fish,and Montgomerie1997). Sperm transfer occursvia the most insects (Eckstein and Zuckerman 1956, Dowl- ing and Savage1960, Eberhard 1985). Several groups of birds, including ratites,screamers, waterfowl, and • Presentaddress: University of Alaska Museum, cracids,possess well-developed male copulatoryor- 907 Yukon Drive, Fairbanks, Alaska 99775, USA. E- gans (Forbes1882, King 1981, Lake 1981). In these mail: [email protected] groups,the penis(or male intromittentorgan) arises July2000] ShortCommunications 821 from the ventral wall of the cloacaand generallyis J. Fullagar pers. comm.;see also Marchant and Hig- believed to facilitate sperm transfer via a series of gins 1990), and unpublishedobservations of similar longitudinal grooves(internal spermducts homolo- intromittent organs in Ruddy Ducks (Oxyurajamai- gousto thosein mammalsare not presentin birds). censis;R. B. Brua pers. comm.),no recordsdetailing The penisof an Ostrich(Struthio camelus), for exam- the size, texture, or conformation of this structure ple, can measureas long as 20 cm when flaccidand within the abdomenare known to exist. Unusually is bright red in color (King 1981). The intromittent large testessize, elaborate sexual display repertoires, organs of various ducks, on the other hand, have and a putatively promiscuousmating system offer been reported to vary in length between50 and 90 opportunitiesfor speculationabout the relationship mm, sometimesexceeding that range (Liebe 1914, betweenform and functionof the unusualpenis in Hochbaum1942). this species. The presenceof a well-developedpenis in many Methods.--Sevenadult male ArgentineLake Ducks dabbling and diving ducks may derive in part from were collectedearly in thebreeding season on 22 No- their tendencyto copulatein the water (Lake 1981). vember 1998 from a small wetland (n = 2) and irri- Geese,which tend to copulateon land, typicallypos- gation canal (n = 5) adjacentto the Rfo Negro near sessmuch smaller penes,even thoughtheir overall General Conesa, Rfo Negro, Argentina (40ø04'S, body size is larger (Coker1998). Another probable, 64ø16'W).Mass (+ 50 g) of eachbird was measured but not necessarilyexclusive, explanation is that at the time of collection.Carcasses subsequently large ornamentedpenes have evolved in responseto were placedon ice,frozen, and exportedto the Lou- sperm competitionwithin the female reproductive isiana State University Museum of Natural Science tract, such that males with larger penes achieve (LSUMNS). Carcasseswere thawed three monthslat- greater reproductive successthan others (Waage er and prepared as museumskins (n = 6) and a skel- 1979, Thornhill and Alcock 1983, Eberhard 1985). etal specimen(n = 1). Speciesthat experiencegreater sperm competition After discovering unusually large cloacal swell- are expectedto possesslarger penes that may in- ingsand correspondinglylarge penes in the first two crease the probability of successfulfertilization specimens,I dissectedand measuredthe intromit- (Birkheadand Moller 1992,Briskie and Montgomerie tent organsand testesof the five birds collectedon 1997). Thus, sperm competitionprobably also ex- the canal(LSUMNS B34019,B34025, B34034, B34036, plainsthe contrastbetween the larger penesof dab- B34037). Genitalia subsequentlywere removed and bling ducksand the smallerpenes of geese.Dabbling further dissectedto verify the extent of internal ducksshow some of the highestlevels of forcedand spine-likestructures. I madea seriesof photographs unforced extrapair copulations among waterfowl, from the secondspecimen to documentboth the po- whereasmost geeseshow low frequenciesof extra- sition and conformation within the abdomen; dissec- pair copulations(Mineau and Cooke1979, McKinney tion of the other four specimensrevealed approxi- et al. 1983; but see Dunn et al. 1999). An element of mately identical features and no need for further female choicealso might play a role if changesin photographs.Manually everting the organ in its physiologyand femalebehavior also affect the out- erect conformationproved impossible given the ab- come of subsequent sperm competition. Teasing senceof a vasculareversion mechanism (King 1981). apart the two processescan be difficult, however, In waterfowl,the eversionmechanism is lymphatic, particularly when the contestoccurs inside the fe- and the unusuallength of the structurerelative to its male.Females also might chooseamong males in yet width prohibitedpulling the entire length of the or- another manner, based on the size and appearance gan inside out. However,longitudinal dissectionand of the penis itself (Briskieand Montgomerie1997). subsequentinversion of the outer surfaceto view the I presenthere a descriptionof the penisof the Ar- inner surface revealed what is believed to be a close gentineLake Duck (Oxyuravittata). The use of the approximationof the everted,but nonethelessflaccid waterfowl penis as a means of sex and age identifi- conformation (see below). Genitalia and testessub- cationof live birds is commonplace(e.g. Hochbaum sequentlywere fixed in 10% formaldehyde,pre- 1942).What is noveland fascinatingin this speciesis servedin 70% ethanol,and depositedat the Louisi- therelative size (ca. half thebody length) and thorny, ana State University Museum of Natural Science. brush-liketexture of its penis. Dissectionand prep- Results.--Eachof the five birdsthat I dissectedap- arationof the ArgentineLake Duck'spenis reveal a pearedto be in breedingcondition, as indicated by a massivestructure (approximately equal in length to bright blue bill, black head, and ruddy breeding the Ostrich penis) with a peculiar array of dense plumage. Mean body mass was 630 + SD of 24 g spinesrunning the entirelength of the organ.At the (range 600 to 650 g). Mean testes dimensionswere baseof the penis,the spinesare hardenedand sharp. 40.8 + 3.6 x 17.8 ___1.8 mm for the left testis and 41.0 With the exceptionof a video sequenceand numer- +_2.3 x 17.2 + 3.1 mm for the right testis(left length ous observations of copulating Australian Blue- testisrange 37 to 45 mm, left testiswidth range 15 billed Ducks(Oxyura australis) everting and preening to 20 mm;right testislength range 37 to 43 mm, right (postcopulatory)similar-sized penes (E. Slaterand P. testiswidth range 14 to 22 mm; Fig. 1A). Massesav- 822 ShortCommunications [Auk,Vol. 117

FIG.1. Photographsof thetestes and genitalia of a maleArgentine Lake Duck (LSUMNS B34025) col- lectednear General Conesa, Rio Negro,Argentina, including (A) rightand left testesas viewedfrom the ventralsurface of the abdomen,anterior margin at top;(B) cloacal area with skinremoved to revealcloacal musclering, everted base of penissheath, and foreskin, dorsal margin at right;(C) foldedpenis enclosed in two-layeredperitoneal membrane as viewed from the ventral surface of the abdomen, anterior margin at top; (D) foldedpenis with membranes removed to revealinternal conformation, posterior margin at lowerleft; (E) unraveledpenis loop and dissected cloacal muscle ring as viewed from the ventral surface, posterior marginat right;(F) outersurface of thepenis sheath as it mightappear in theeverted but nonetheless flaccid conformationarranged to revealspines and ornamentation patterns, proximal margin at bottom. July2000] ShortCommunications 823 eraged5.4 + 1.3 g for the left testisand 5.4 --_1.9 g Mechanicaleversion of the penis in its erect con- for the right testis(left testisrange 4.3 to 7.0 g; right formationwas not possiblegiven its unusuallength testisrange 3.9 to 8.6 g). The left testiswas the larger and narrow width. Geometricrelationships of the of the two in all but one individual. Mass of the two loop structure,nonetheless, predict what the penis testescombined averaged 10.8 q- 3.1 g (range8.2 to should look like in its erect conformation. At maxi- 15.6 g), or 1.7 q- 0.5% of total body mass (range 1.4 mum extent,the evertedpenis can be no longerthan to 2.4%). one-halfthe circumferenceof the loop,or a distance Eachindividual possesseda large,well-developed equivalentto the distancefrom the baseto the head cloacalswelling ventral to the baseof the tail (Fig. of the loop (seeFig. 1E). When everted,the penis lB). Dissectionrevealed that the cloacalswelling is thus conformsto the topology of a two-layered composedprimarily of a large ring of muscle;the sheathconnected to the cloacaat both ends,contig- baseof the penis, seminalvesicles, and intestineare uousat the distal end, with an orificeat the tip and embedded within this muscle and constitute the re- hollow in the middle (seeKing 1981).The outersur- mainder of the cloacal mass. Within the cloacal ori- faceof the sheath(which appears inverted in the ab- fice, the base of the penis is clearly visible as an in- dominalcavity) possessesthe spines.The inner sur- verted blind-end tube arisingfrom the ventral wall face is not visible in this conformation but forms a of the cloacaand penetratingwell into the body cav- hollowtube running the entireinternal length of the ity, not unlike the Mallard (Anasplatyrhynchos) penis penis. Longitudinal dissectionand examination of (King 1981).Manual eversionbeyond a couplecen- ornamentationpatterns corroborate this hypothesis timeters, however, was not possible.Eversion to a (Fig. 1F).When viewed like this, spinesare observed centimeteror so,nonetheless, revealed a densearray alongthe entirelength of theouter sheath (22.3 + 2.1 of black-tipped,white spinesat thebase of the penis. cm) but not on the inner sheath. At the base of the Spinedensity is higheston theventral side of thepe- penis(outer sheath), the spinesare numerous,large, nis, and their anglesare suchthat I believethey em- well developed,and sharp.As one proceedsdistally, bedinto and claspthe wall of the femalecloaca at the however,the spinesdiminish in number,become soft point of insertion,whereby the male may evert the and brush-like, and take on a more ochre color, until remaininglength of the penis.A smallforeskin-like at the distal terminusthey disappearaltogether; at structurewith no spinesand measuring7.2 + 1.3 this point the outersheath inverts to becomethe in- mm (range6 to 9 mm) arisesdorsally from the base ner sheaththat extendsproximafly toward the base of the penis(see Fig. lB). No othernotable traits were of the penis.Spines also are arrangedalong a series revealedby further manipulationof the cloaca. of spiral groovesextending the entire length of the Internal dissectionof the abdominal cavity re- everted penis (Fig. 1F). vealed a far more extensive coiled structure placed Discussion.--Thepenis of theArgentine Lake Duck slightlyto the left of the centeraxis of the body,an- measuresapproximately half the body length and terior to the mass of muscle that encircles the cloaca exhibitsproportions comparable to thosefound in and adjacentto the intestine.Here the invertedpenis fleasand somenematodes and flies;male genitaliain was folded and enclosedwithin a two-layeredperi- these invertebratesoften exceedthe length of the toneal membranemeasuring 25.5 _+1.7 mm x 19.3 body and can be the most structurallycomplex or- q-2.2 mm at its two widestdimensions (n = 4; length gansin the entirebody (Hyman 1951,Thornhill and range 24 to 28 mm, width range 17 to 22 mm; Figs. Alcock 1983).Without regard to size,such structures 1C,D). Subsequentdissection and unraveling from are uncommonin birds but are presentin nonavian the two membranesrevealed the invertedpenis to be groupsincluding crocodiles and turtles (King 1981). an elongated,blind-end, tube-like structure attached As such,the presenceof a penis in birds is a shared to the cloacaat both ends.Total lengthof the penis ancestraltrait, and an explanationfor multiple ori- tube as measured from the base of the cloaca to the gins of the avian penisis not necessary.What needs bendof the loop was22.3 q-2.1 cm (range19.0 to 24.5 to be explainedis why birdshave lost the penisre- cm; Fig. 1E). The width at the base(the end of the peatedly,and moreover,why it hasbeen retained or penisvisible upon cloacalexamination) was 6.5 q- 1.3 more complexlydeveloped in somegroups like wa- mm (range4.5 to 8 mm).From this point distally, the terfowl.In both cases,the comparativemethod has penistapered gradually to the oppositeend where it the potentialto bring to light interestingsocial and insertedthe cloacaventral to the insertionpoint of ecologicalcorrelates that evolvedin tandemwith the the baseof the penis.At this juncture,the width of avian penis. the penistube was3.4 + 0.5 mm (range3 to 4 mm). For the ArgentineLake Duck, a numberof func- A largeblood vessel extended along the entirelength tional questionsremain. First is the questionof size of the organ.Black pigments in the spineson the ex- and seasonal recrudescence. The testes of birds un- ternal surface of the penis, likewise, were visible dergo substantialchanges in size, often enlarging from the abdominalsurface, thus imparting a mot- greatlyduring the breedingseason and then shrink- tled gray appearanceto the basalhalf of the penis; ing to becomealmost invisible (Witschi 1935, Wing- the distalhalf of the peniswas moreflesh-colored. field and Farner 1980). Does the Argentine Lake 824 ShortCommunications [Auk,Vol. 117

Duck penisrecrudesce after the breedingseason in likewise,suggest that conventionalpair bondsdo not a similar fashion, and if so, do the same hormones existin this group(Matthews and Evans1974, Sieg- that regulatethe testescycle control the seasonalde- fried 1976,Ladhams 1977, Gray 1980,Marchant and velopmentof the penis?Either scenariocan be con- Higgins 1990).Pair bondsthat havebeen observed strued as energeticallyexpensive (i.e. annual devel- probablyreflect nothing more than short-termrela- opment vs. year-round maintenance). Year-round tionshipsfor copulationor mate guarding.Copula- maintenancemay be more probablebecause Argen- tion in Ruddy Ducks and Australian Blue-billed tine Lake Ducks are likely to be sexually active Ducks usually is tumultuous and boisterousand throughoutmost of the year,and like otherSouthern generallyis precededby prolongedpursuits across Hemispherewaterfowl, they lack the sharply defined or under the surface of the water (Wheeler 1953, seasonsof breedingactivity typical of ducksin the Gray 1980).In most instances,the evertedpenis is far north. However,it is not knownwhether the pe- clearly visible prior to and after copulation(D. K. nes of any waterfowl speciesrecrudesce seasonally. Rushtonand P.J. Fullagar pers. obs., R. B Braupers. Is it a coincidencethat sucha large structurehas comm.;see Marchant and Higgins1990). Postcopu- evolvedin a divingduck with highwing loadingthat latorypreening of the penis,likewise, appears to be rarely flies? commonin both species(P. J. Fullagar and B. Brua Another questionthat comesto mind concerns pers. comm.). spermflow and ejaculation.Other birds lack sperm Observationsof thesethree Oxyura species suggest ducts in their penes,and insteadrely on spiral groovesto channelthe flow of sperm.Does sperm that well-developedintromittent organs are present proceedvia a longitudinalsulcus in the Argentine in other membersof the group. In this respect,a LakeDuck asit doesin otherspecies? I was not able large ornamentedpenis appears to be a synapo- to identify the openingof the sperm duct, so the morphiccharacter that setsstifftail ducks apart from question remains unanswered. One last functional otherwaterfowl. The probability that the complexity questionconcerns the topicof spermdisplacement. of this organ developedin responseto male com- Thebase of the penisin thisspecies is generouslyor- petitionfor limited accessto femalesalso seems high namentedwith coarsespines, but the distal half is giventhe obvioussocial and phylogeneticcorrelates soft and brush-like.Do maleArgentine Lake Ducks (Birkheadand Moller 1992, Briskie and Montgomerie use the coarsespines at the baseof their penisto 1997). However, the extent to which female choice claspthe femaleas morphologywould predict,and hasfactored importantly in developmentof thistrait doesthe softerbrush-like distal half of the penisre- is unknown.Excellent potential certainly exists for a move sperm depositedby other males?Sperm-re- comparativestudy of penismorphology across wa- moval tacticsof this type occurin variouselasmo- terfowlspecies and the investigationof otherpoten- branchfishes and insects(e.g. Leigh-Sharpe1922, tially correlatedtraits suchas matingsystem, testes Simmonsand Siva-Jothy1998). size,and the incidenceof forcedor extrapaircopu- Why has sucha large spinypenis developed (or lations. beenmaintained) in this particularspecies? Limited Acknowledgments.--Ithank Daniel Blanco, Ra61 informationabout the socialbehavior and mating Card6n, Claudio Cheh•bar, Ra61 Clarke, Mike Chris- habitsof ArgentineLake Ducks suggests that pecu- tie, Donna Dittman, Martin Funes,Sergio Goldfeder, liaritiesof size,ornamentation, and the potentialfor Manuel Nores, and Alejandre del Valle for their as- sperm displacementhave evolved in conjunction sistancelocating stifftail ducks or helpexporting and with a promiscuous,group-display mating system. importing specimens.Mike Grogan offered assis- Similarcausal mechanisms also might be postulated tancein the field, and RichOlsen loaned a shotgun. for the relativemass of the testesin this species, James Briskie, Bob Brua, Chris Coker, Peter Dunn, which ranks among the top 18% of birds (Moller PeterFullagar, William Johnson,Bob Montgomerie, 1991,P. Dunn unpubl.data). Like otherstifftails (No- Fred Sheldon,and two anonymousreviewers made monyxand Oxyura),the Argentine Lake Duck is nest- ed within a cladeof promiscuous,dichromatic con- helpfulcomments on the manuscript.Collection was genersthat sharemany derivedsocial and morpho- madepossible by the Centrode EcologlaAplicada logical characters(McCracken et al. 1999; see also del Neuqu•n, Delegaci6n Regional Patagonia,Ad- Livezey 1995,Johnsgard and Carbonell1996). The ministraci6n de Parques Nacionales, Direcci6n de most notable of these includes a series of shared ste- Faunay Flora Silvestredel Argentina, Direcci6n de reotypedsexual displays that collectivelyset Oxyura Fauna Silvestre del Chubut, Direcci6n de Fauna Sil- (and probablyNoraonyx) apart from all other water- vestre del Rfo Negro, Direcci6n de FaunaSilvestre fowl (Carbonell1983, Marchant and Higgins 1990, del Santa Cruz, Louisiana CooperativeFish and McCrackenet al. 1999).These displays, which can be Wildlife Research Unit, and Louisiana State Univer- quite vigorous and prolonged, typically are per- sity AgriculturalCenter, College of Agriculture,Mu- formed in the presenceof many malesand often in seum of Natural Science,and Schoolof Forestry, arena-like contexts. Other sources of information, Wildlife, and Fisheries. July2000] ShortCommunications 825

LITERATURE CITED phology of the secondarysexual charactersof elasmobranchfishes. Journal of Morphology36: BIRKHEAD,t. R., AND A. P. MOLLER.1992. Sperm 191-243. competition in birds: Evolutionary causesand LIEBE,W. 1914. Die m•innlichesBegattungsorgane consequences.Academic Press,London. der Hausente. JenaischeZeitschrift ffir Natur- BRISKIE,J. V., AND R. MONTGOMERIE.1997. Sexual se- wissenschaften 51:627-696. lection and the intromittent organs of birds. LIVEZEY,B.C. 1995. Phylogeny and comparative Journalof Avian Biology28:73-86. ecologyof stiff-tailed ducks(Anatidae: Oxyuri- CARBONELL,M. 1983. Comparative studies of stiff- ni). Wilson Bulletin 107:214-234. tailed ducks (tribe Oxyurini: Anatidae). Ph.D. MARCHANT, S., AND P. HIGGINS. 1990. Handbook of dissertation,University College,Cardiff, Wales. Australia, New Zealand, and Antarctic birds. COKER,C. R. 1998.The effectsof spermcompetition Oxford University Press,Melbourne. on testessize and intromittentorgan morphol- MATTHEWS, G. V. T., AND M. E. EVANS. 1974. On the ogy in waterfowl. M.S. thesis,University of Brit- behaviourof the White-headedDuck with spe- ish Columbia, Vancouver. cial referenceto breeding.Wildfowl 25:55-66. DOWLING,H. G., ANDJ. M. SAVAGE.1960. A guide to MCCRACKEN, K. G., J. HARSHMAN, D. A. MCCLEL- the snakehemipenis: A surveyof basicstructure LAN, AND A.D. AFTON.1999. Data set incongru- and systematiccharacteristics. Zoologica 45:17- ence and correlated character evolution: An ex- 28. ample of functional convergencein the hind- DUNN, P. O., A. D. AFTON, M. L. GLOUTNEY, AND R. limbs of stifftail diving ducks.Systematic Biol- T. ALISAUSKAS.1999. Forced copulation results ogy 48:683-714. in few extrapair fertilizationsin Ross'sand Less- MCKINNEY, E, S. R. DERRICKSON,AND P. MINEAU. er Snowgeese. Animal Behaviour57:1071-1081. 1983.Forced copulation in waterfowl.Behaviour EBERHARD,W. G. 1985. Sexual selection and animal 86:250-294. genitalia. Harvard University Press,Cambridge, Massachusetts. MINEAU,P., AND E COOKE.1979. Rape in the Lesser Snow Goose. Behaviour 70:280-291. ECKSTEIN,P., AND S. ZUCKERMAN.1956. Morphology of the reproductivetract. Pages43-155 in Mar- MOLLER,A. P. 1991. Sperm competition, sperm de- shall'sphysiology of reproduction,vol. I (A. S. pletion, paternal care, and relative testis size in birds. American Naturalist 137:882-906. Parkes, Ed.). Longman's Green and Co., New York. SIEGFRIED,W. R. 1976. Socialorganization in Ruddy and Maccoa ducks. Auk 93:560-570. FORBES,W. A. 1882.Note on somepoints in the anat- omy of an Australian Duck (Biziura Iobata).Pro- SIMMONS,L. W., AND M. T. SIVA-JOTHY.1998. Sperm ceedingsof the ZoologicalSociety of London31: competitionin insects:Mechanisms and the po- 455-458. tential for selection. Pages 341-434 in Sperm GRAY,B. J. 1980.Reproduction, energetics, and social competitionand sexualselection (T. R. Birkhead structureof the Ruddy Duck. Ph.D.dissertation, and A. P. Moller, Eds.). Academic Press,London. University of California, Davis. THORNHILL,R., AND J. ALCOCK.1983. The evolution HOCHBAUM,H. A. 1942. Sex and age determination of insect mating systems. Harvard University of waterfowl by cloacal examination. Transac- Press,Cambridge, Massachusetts. tions of the North American Wildlife Conference WAAGE,J. K. 1979.Dual functionof the damselfly pe- 7:299-307. nis: Sperm removal and transfer. Science203: HYMAN,L. H. 1951.The invertebrates: Platyhelminthes 916-918. and Rhynchocoela.McGraw-Hill, New York. WHEELER,J. R. 1953. Notes on the Blue-billed Ducks JOHNSGARD,P. A., AND M. CARBONELL.1996. Ruddy at Lake Wendouree, Ballarat. Emu 53:280-282. Ducksand otherstifftails: Their biology and be- WINGFIELD, J. C., AND D. S. FARNER.1980. Control of havior.University of OklahomaPress, Norman. seasonalreproduction in temperatezone birds. KING,A. S. 1981.Phallus. Pages 107-147 in Formand Pages62-101 in Progressin reproductivebiolo- functionin birds (A. S. King and J. McClelland, gy (R. J. Reiter and B. K. Follet,Eds.). S. Karger, Eds.). Academic Press,New York. Basel, Switzerland. LADHAMS,D. E. 1977.Behavior of the RuddyDuck in WITSCHI, E. 1935. Seasonal sex characters in birds Avon. British Birds 70:137-146. and their hormonal control. Wilson Bulletin 47: LAKE,P. E. 1981. Male genital organs.Pages 1-61 in 177-188. Form and function in birds (A. S. King and J. McClelland, Eds.). Academic Press, New York. Received6 April 1999, accepted22 February2000. LEIGH-SHARPE,W. H. 1922. The comparativemor- AssociateEditor: R. D. Montgomerie