January
1992]
Short
Communications
andCommentaries
dak4" x 5"type4162),andthermoprints(Sony110mm
x 20 mm) to documentplumulaceousstructures.We
use an alphanumericgeneratorto label the face of
photomicrographsusingspeciescodesfrom Edwards
(1982, 1986). Eachphotomicrographis labelled with
species,tract, vane, vanule, and positionof the barbules along the rachilla or ramus.We include technical information,suchas the type of SEM, working
distance,and magnification(if not shown on the face
of the photograph).The photomicrographsare stored
in file-card boxes or notebooks(in systematicorder)
following alphanumeric codes developed by Edwards.
We gratefully acknowledgethe guidanceof Susann
Braden and Waiter Brown (Smithsonian Institution,
Washington,D.C.), Jan Endlich (GeorgeMason University, Fairfax, Virginia), and Mary-JacqueMann
(National Fish and Wildlife ForensicsLaboratory,
Ashland, Oregon) in the use of the microscopes.
Use
of the Hitachi SEM at GeorgeMasonUniversitywas
made possiblethrough NSF Grant BSR-8511148.M.
Ralph Browning and Waiter Brown reviewed an earlier version of this manuscript.We thank Stephen
Busackfor editing the final draft of the manuscript
and Mary-JacqueMann for the photomicrographs.
197
of ethnographicfeatherwork in aqueoussolutions. Stud. Conserv.
31:125-132.
BROM,T. G. 1990. Villi and the phyly of Wetmore's
order Piciformes (Aves). Zool. J. Linn. Soc. 98:
63-72.
DAVIES,
A. 1970. Micromorphologyof feathersusing
the scanningelectron microscope.J. ForensicSci.
Soc. 10:165-171.
DYCK,J. 1973. Featherstructure:Thesurfaceofbarbs
and barbules. Zool. Jahrb. Anat. 90:550-566.
EDWARDS,E. P. 1982. A coded workbook of birds of
the world, vol. 1. E. P. Edwards, Sweet Briar, Virginia.
EDWARDS,
E. P. 1986. A coded workbook of birds of
the world, vol. 2. E. P. Edwards, Sweet Briar,
Virginia.
REANEY,B. A., S. M. RICHNER,AND W. P. CUNNINGHAM.
1978. A preliminary scanning electron microscopestudyof the minutemorphologicalfeatures
of feathers and their taxonomic significance.
ScanningElectronMicros. 11:471-478.
ROBERTSON,
J., C. HARKIN, AND J. GOVAN. 1984. The
identification
of bird feathers.
Scheme for feather
examination. J. Forensic Sci. Soc. 24:85-98.
SCHMALZ, G. 1982. A SEM view of the Cedar Wax-
wing. Oriole 37:29-30.
LITERATURE CITED
Received5 February1991,accepted15 July 1991.
BARTON,
G., ANDS. WEIK. 1986. Ultrasoniccleaning
The Auk 109(1):197-199, 1992
SexualSelectionand the Evolution of ExtravagantTraits in Birds:
Problemswith TestingGood-genesModels of SexualSelection
IA• L. JONES
Department
of Zoology,
University
of Cambridge,
Cambridge
CB23EJ,UnitedKingdom
The evolutionof extravaganttraitsthat maybe favoredby sexualselectionhasreceivedmuchattention
in recent literature. Empirical studieshave focused
sexualselectionin general,and "good-genes,""runaway" and "direct-benefits" models (referencesin
Kirkpatrick and Ryan 1991)in particular,that merit
on attempts to test alternative sexual-selection mech-
further
discussion
anisms,using ornamentsof somebirds as examples
Buchholz (1991) suggested that direct-benefits
of suchelaboratetraits.However,the interpretation modelsto explainthe evolution of matingpreferences
of empirical evidence has been controversial,and redo not apply to Yellow-knobbedCurassows,because
cent papers have pointed out numerous difficulties males"do not appearto defend territoriesor carefor
in testingthesemodels(Read 1990,Kirkpatrick and
Ryan 1991). Here, I reevaluatesome findings of a
recent paper on ornaments of curassows(Buchholz
chicks." Even if this were true, it should not eliminate
this model from consideration, because direct benefits
(e.g. involving parasite, predator, or harassment
avoidance;Reynoldsand Gross1990)could favor evoandintraspecific
testsof sexual-selection
hypotheses. lution of femalepreferencesfor extravagantmaletraits
Buchholz (1991) pointed to a correlation between in lekking species,or others where malesprovide no
knob-ornamentsizeand age of Yellow-knobbedCu- care.Ornamentscouldbe favoredby sexualselection
rassows
(CraxdaubentonO
asevidencefor "good-genes" if they reflect nongenetic phenotypic differences
modelsof sexualselection.The interpretationspre- among malesthat involve these mating advantages
sentedin his studyillustrateseveralperceptionsof to females.Testsof the direct-benefits
hypothesis
seem
1991) to point out somepitfalls to considerin inter-
198
ShortCommunications
and Commentaries
to have been lostin a rush to verify good-genesmodels, while the former remainsa plausiblealternative
(Kirkpatrick and Ryan 1991).Further empirical work
will need to addresswhether mating preferencesrelate to extravaganttraits that indicate direct benefits,
and/or to the much more difficult to evaluate goodgenesmodel.
A secondproblem of interpretation concernssupposedexclusivepredictionsof good-genesand runaway modelsof sexualselection.It hasbeensuggested
that a positive relationship between an extravagant
characterand viability is an exclusivepredictionof a
good-genes
model,and thata runawaymodelpredicts
no relationship between ornaments and viability
(Heisler et al. 1987,Buchholz 1991).This may be an
oversimplificationof predictionsof thesemodels.Becauseboth models explain evolution of costly traits
that seem to be maladaptivewith respectto natural
selection,a correlationbetween expressionof an extravagant trait and individual viability is expected
regardlessof whether the trait has evolved by runawayor asa signalof goodgenes(Read1990).Imagine
an ornament that has evolved through the hypothetical runaway process.Female preferencesmay
drive evolution of this trait until its expression is
limited by naturalselection,potentiallyleadingto an
extravagantand costlyornament.Any differencesin
health or viability amongindividualsare then likely
to be reflectedin ornamentexpression,
becausehealthy
individuals will be able to afford greater costs.Thus,
a correlation between viability and ornament size
could result from a runaway processwithout any selection for a signal of good genes.Furthermore,such
correlationsare likely to be consistentwith a directbenefitsmodel.Thus,simplecorrelationsbetweenextravaganttraitsand viabilityshouldnot be takenalone
assupportexclusiveto a good-genesmodel.The search
for testablepredictionsof thesesexual-selection
models clearly hasjust begun. Buchholz(1991) suggested
that the runaway model is not falsifiable until all
possiblecorrelateswith viability have been tested.If
this is the case,then it must also be true for goodgenes models,becauselack of correlationbetween
ornamentand one aspectof viability may inevitably
lead to a searchfor other measuresof viability, rather
than rejectionof this hypothesis.
Another questionarisesover the presenceor absenceof intrasexualselectionfavoring ornamentson
[Auk, Vol. 109
have a signal function outsidethe contextof active
female choice. However, no evidence presented by
Buchholz (1991) establisheswhether curassowornamentshave been favored by either inter- or intrasexual selection.Without such evidence, the appropriate-
ness of good-genes,runaway, and other models
involving intersexualselectionto curassowknobsis
doubtful. In general, careful observationand experiments must show that ornamental
traits are favored
by activematingpreferences,while controllingfor
intrasexualcompetition(e.g. H6glund et al. 1990),
beforeproceedingto testintersexualselectionmodels.
Another sourceof difficulty in interpretationof sexual-selection
models concerns whether
a correlation
between an extravagant trait and age representsev-
idence that the trait signalsgood genes.Buchholz
(1991)suggested
thatonly good-genes
theorypredicts
that maleswith big ornamentsare older or healthier
than their small-knobbedconspecifics.
However, such
relationshipsare not exclusivepropertiesof a goodgenes mechanism;they are likely to appear under
other sexual-selection models, or even in the absence
of sexualselection.Measuresof viability are known
to correlatewith age within individuals, either for
developmentalreasonsor becauseolder individuals
are more experienced.For example,individualsmay
becomebetter foragers(e.g. Greig et al. 1983),or increasein dominancestatus(e.g. Ekman and Askenmo
1984) with age, either of which could lead to a correlation between age and viability. Expressionof a
costlyornamentcouldreflectthesenongeneticdifferencesamong individuals,or the ornamentmight
itself be constrainedby developmentalprocesses
relatedwith age.Femalepreferences
couldthen be favoredby directbenefitsassociated
with matingwith
older, more ornamented males. Thus, the only unequivocalevidenceconsistentwith ornamentseven
correlatingwith good geneswould be if ornament
size remains correlated with viability within age
groups(i.e.aftercontrollingfor agein a longitudinal
study).Ultimately, we would need to know if these
individual differencesin viability are heritable.
Buchholz (1991) contended that size of Great Curassowknobsis not an accurateindicator of age.How-
ever, this was not properly testedbecause,with a
sampleof only nine birds, correlationbetweenage
and knob would have to be overwhelmingly strong
Yellow-knobbed Curassows. We are told that no maleto be significant(seeForbes1990).Basedon data premale combat was observed in Yellow-knobbed
Cusentedin figures3 and 5 of Buchholz(1991),there is
rassows or Great Curassows (C. rubra), so curassow no significantdifferencein strengthof the relationornamentsare unlikely to be favoredby intrasexual shipof knobheightandagebetweenYellow-knobbed
selection. Lack of male-male
combat does not rule out
occurrenceof intermale competition,nor doesit rule
out intrasexual selection favoring curassow ornaments. Male-male interactions that determine, for ex-
ample, favored positionsin an exploded lek may be
extremelysubtle. Observationsthat curassowornamentsdo not changeseasonallyor during courtship
(Delacourand Amadon 1973)suggestthat knobscould
and Great curassows(ANCOVA of standardized data
extractedfrom figs.3 and 5;species-by-age
interaction
term is nonsignificant,Fi,is= 0.03,P = 0.9).There are
simply insufficientdata to indicatewhether the relationshipsare different.
Buchholz(1991)suggested
further that,becausethe
GreatCurassowis monogamous,
sexualselectioncannot favor evolution of an extravagantornament.This
January 1992]
ShortCommunications
and Commentaries
suppositionis contradictedby recenttheoreticalmod-
199
DELACOUR,
J.,ANDD. AMADON. 1973. Curassowsand
els that show intersexual selection can favor elaborate
related birds. American
ornamentsin monogamousspecies,where both sexes
tory, New York.
Museum
of Natural
His-
provide parentalcare,and even in sexuallymonomorphicspecies
(Kirkpatricketal. 1990),andby many
examplesof monogamousspecieswith brighter
EKMAN, J. B., AND C. E. H. ASKENMO. 1984. Social
plumage and more elaborateornamentsthan curas-
FOR•3ES,
L.S. 1990. A note on statisticalpower. Auk
rankandhabitatusein Willow Tit groups.Anita.
Behav. 32:508-514.
107:438-439.
sows(e.g. tropicbirds,Phaethon;
egrets,Egretta;parrots,Platycercus
and Trichoglossus;
someauks,Aethia; GREIG,S. A., J. C. COULSON,AND P. MONEGHAN. 1983.
puffins,Fratercula
spp.;bee-eaters,
Merops;sunbirds,
Age-relateddifferencesin foragingsuccess
of the
Nectarinia;kingfishers,Tanysiptera;
jays,Calocittaand
Herring Gull (Larusargentatus).
Anita. Behav.31:
1237-1243.
Cyanocorax;
and tyrant flycatchers,Muscivora).ElabS. J. ARNOLD,C. R. BOAK,
orate traits expressed
in malesand femalesmay be HEISLER,L., M. B. ANDERSSON,
the result of mutual sexual selection related to vari-
G. BORGIA, G. HAUSFATER,M. KIRKPATRICK,R.
ation in mating success
of both sexes(Kirkpatricket
LANDE,J. MAYNARDSMITH, P. O'DONALD, A. R.
THORNHILL,AND F. J. WFaSSING.1987. The evo-
al. 1990).Thus,thelogicof good-genes,
runaway,and
direct-benefits
modelsmayapplytoornamentsof monogamous
nonlekkingspecies,
but understanding
of
which modelbestexplainsevolutionof extravagant
traitswill dependon carefullydesignedfield experimentson a varietyof species
with differentmating
systems.Eventuallywe may find that all three models
lution of mating preferencesand sexually selected traits. Pages97-118 in Sexual selection:
Testingthe alternatives(J. W. Bradburyand M.
B. Andersson,Eds.).JohnWiley and Sons,New
York.
H•GLUND, J., M. ERIKSSON,
AND L. E. LINDELL. 1990.
maywork in nature,perhapsevensimultaneously.
Femalesof the lek-breedingGreatSnipe, GallinaFinally, data indicatinga lack of correlationof cugo media,prefer males with white tails. Anim.
Behav. 40:23-32.
rassowornamentswith parasiteprevalenceisnot consistentwith good-genesmodelsof sexualselection. KIRKPATRICK,
M., T. PRICE,AND S. J. ARNOLD. 1990.
It ismoreconsistent
with Buchholz's(1991)depiction
The Darwin-Fishertheory of sexualselectionin
of the runaway model, or with the idea that knob
monogamousbirds. Evolution 44:180-193.
ornamentsarearbitrarywith respectto viability.Suc- KIRKPATRICK,M., AND M. J. RYAN. 1991. The evocessful evaluation of these sexual-selection models
lution of mating preferencesand the paradoxof
the lek. Nature 350:33-38.
awaitsderivationof testablemutuallyexclusivepre-
dictions,and on field studiesthat experimentally
READ, A. F. 1990. Parasites and the evolution of host
measureactive mating preferencesand intrasexual
sexualbehaviour.Pages117-157in Parasitismand
competition,
while controllingfor confoundingfac-
host behaviour (C. J. Barnard and J. M. Behnke,
tors suchas age.
Eds.).Taylor and Francis,London.
REYNOLDS,J. D., AND M. R. GROSS. 1990. Costs and
benefitsof female mate choice:Is there a lek par-
LITERATURE CITED
adox? Am. Nat.
BUCHHOLZ,
R. 1991. Older maleshavebiggerknobs:
Correlatesof ornamentationin two speciesof
136:230-243.
Received
25 April 1991,accepted
26 April 1991.
curassow. Auk 108:153-160.
The Auk 109(1):199-201, 1992
Confusing Models with Tests in Studies of Sexual Selection:
Reply to Jones
RICHARD BUCHHOLZ
Department
of Zoology,223 BartramHall, Universityof Florida,Gainesville,
Florida32611,USA
Jones'(1992)thought-provoking
commentary
points
to some statistical issues and intricacies of sexual-se-
wronglycontendsthat the predictionsof the "goodgenes"and "runaway" modelsfor the evolution of
ornamentsare not exclusive.He hypothesizesthat
lection theory not discussedin my original paper.
However, I believethat someof the "pitfalls" he de- runaway traits may become good indicators of the
scribesare moot in the empiricalrealm.
bearer'sfitnessas they becomemore burdensome.
In the major thrust of his commentary,Jones This scenariois not unreasonable.Nevertheless,once