Species Richness Covaries with Mating System in Birds

The Auk 113(3):544-551, 1996

SPECIES RICHNESS COVARIES WITH MATING SYSTEM IN BIRDS

SHAIBAL MITRA, •'3 HANS LANDEL,TM AND STEPHENPRUETT-JONES 2 •Committeeon EvolutionaryBiology and 2Department of Ecologyand Evolution, Universityof Chicago,1101 East 57th Street,Chicago, Illinois 60637, USA

ABSTRACT.--Manystudies have soughtto identify traitsthat influencethe relative number of speciesin related taxa.We examinedwhether speciesrichness was associatedwith social mating systemin birds. Taxa with promiscuousmating systemstended to be more species- rich than their nonpromiscuoussister taxa. This associationwas statistically significant when examinedwith teststhat take into accountthe magnitudesof paired contrasts.The results do not arisefrom covariationbetween mating system and bodysize. We discussthese findings in the contextof the hypothesisthat sexualselection promotes speciation. Received 16 February 1995, accepted25 April 1995.

AMONGTHE MOST important questionsin evo- ation in coloration and ornamentation, whereas lutionary biology are thoseconcerned with fac- the females are relatively uniform in appear- tors affectingspeciation. Many traits have been ance. Such variation among males often pro- proposedas key innovationsor adaptivebreak- vides criteria for species'boundaries according throughs responsiblefor the diversificationof to the phylogenetic speciesconcept (Cracraft particular taxa, but empirical testsof such hy- 1983, McKitrick and Zink 1988). Moreover, these potheseshave proven methodotogicatlydiffi- traits are inferred to function in premating re- cult (see Raikow 1986, Stowinski and Guyer productiveisolation under the biologicalspe- 1993). Testsof associationin comparative stud- ciesconcept (Mayr 1942,1963). Clearly, any force ies must be based on samplesreflecting inde- tending to accelerateevolutionary changesin pendent derivationsof the trait of interestacross secondarysexual characterswould hasten the a series of taxa whose historical relationships rate at which allopatric populations achieved can be reconstructed(Felsenstein 1985, Harvey speciesrank under both the phylogenetic and and Paget 1991, Guyer and Slowinski 1993, biologicalspecies concepts. Slowinski and Guyer 1993). Traits for which Sexual selection is recognized as a process effectson relative speciesnumbers have been that can lead to the rapid modificationof a va- analyzed in this manner include phytophagy riety of characters,particularly thoseinvolved in insects (Mitter et at. 1988), breadth of ovi- in mating preferences (Fisher 1930; Kaneshiro position sites in insects(Zeh et at. 1989), and 1980, 1989;Lande 1981, 1982;Kirkpatrick 1982; viviparity in fishes(Lydeard 1993).All of these Kaneshiro and Giddings 1987; Lande and Kirk- are traits at the level of individual organisms. patrick 1988).In fact, the mutually reinforcing In our study, we examined speciesrichness in elaborationof mating preferencesand second- relation to a population-level trait, social mat- ary sexualcharacters during "run-away" sexual ing system. selectionimplies that this processcould be very It has long been recognizedthat closelyre- effective in producing the differentiation nec- latedspecies often differ moststrikingly in their essaryfor the developmentof major morpho- secondarysexual characteristics (Darwin 1871). logical differences,and even premating repro- For example,within groupsof birds as diverse ductive barriers, between populations (Kane- asgrouse, hummingbirds, and birdsof paradise, shiro 1980; Lande 1981, 1982; West-Eberhard the males of many speciesshow striking vari- 1983;Wu 1985; Lande and Kirkpatrick 1988). Although there is this theoreticaljustification • Presentaddress: Department of Natural Resources for predicting a relationshipbetween the action Science,University of RhodeIsland, Kingston, Rhode of sexualselection and the processof speciation, Island 02881, USA. the methodologicaldifficulties involved in test- 4 Presentaddress: Department of Biology,Carleton ing sucha prediction are not trivial. Precisedata College, Northfield, Minnesota 55057, USA. regardingsexual-selection gradients (Lande and 544 July 1996] SpeciesRichness and Mating Systems 545

Arnold 1983) are known for very few natural ical patterns and taxonomic artifacts. Barra- populations,and pastrates of speciationare dif- clough et al. (1995) reported a significant rela- ficult or impossible to reconstruct. Neverthe- tionshipbetween sexualdichromatism and spe- less,one can approachthe hypothesisindirectly cies richness in passefine birds. To the extent by choosingconservative indicators of the vari- that dichromatism reflects the intensity of sex- ables of interest. For example, although it is ual selection, this result also is consistent with now clear that sexualselection can occurin spe- the hypothesislinking sexualselection and species exhibiting a wide variety of social mating ciation. In a study of the relationship between systems(Birkhead and Moller 1992), sexualse- lek mating systemsand sexual dimorphism in lection generally will be strongerin speciesex- birds, H6glund (1989) presented results sug- hibiting promiscuousmating systemsthan in gesting that lek-breeding, sexually dimorphic specieswith nonpromiscuousmating systems taxa were relatively species-rich.Nevertheless, (Arnold and Duvall 1994).Promiscuity thus may he did not statethis result explicitly nor discuss be an indicator of relatively intense sexualse- it in the contextof a hypothesislinking species lection. richnessto mating systems. Similarly, for the purposes of the test de- Ryan (1986) reported that three successive scribedhere, the number of extant species(spe- transitions in the neuroanatomy of anuran am- cies richness)that we observe in a given taxon phibians coincided with increasesin species is a conservativeestimator of past rates of spe- richness.The changesin neuroanatomywere ciation in that taxon. Becausespecies richness associatedwith increasesin the range of fre- is actuallythe net differencebetween speciation quenciesover which females were sensitive to and extinction events over a given period of the mating calls of males,and Ryan attributed time, one must be careful not to mistake the the increasedspecies richness to acceleratedrates effect of reduced extinction rates with that of of speciationvia divergencein mating calls. His increased speciation rates, and vice versa. We results are consistentwith the hypothesis that must examine the likely effectsof sexualselec- differentiation of traits associated with mate tion on both processes.There is some reasonto choicepromotes speciation, but the data do not believe that sexual selection increases the like- permit a general statisticaltest. lihood of extinction through its ability to reduce the mean fitness of a population (Fisher METHODS 1930, McLain 1993). Conversely, there is no in- dication that sexual selection reduces extinction To identify phylogeneticcontrasts in the intensity rates. Therefore, a positive relationship be- of sexualselection, we comparedtaxa with promis- tween the intensity of sexualselection and spe- cuous mating systemswith taxa exhibiting other cies richnessis most reasonablyinterpreted as (mostly monogamous)mating systems.For our pur- a reflection of increased rates of speciation. poses,we defined promiscuityas a mating systemin The prediction that avian taxa with mating which there is no lastingsocial pair bond and males systemsindicative of intense sexual selection do not provide parental care or resourcescritical to femalereproduction. We alsoincluded species having have greater numbers of speciesthan do their non-resource-basedpolygyny with no male parental sister taxa with other mating systemshas not care(e.g. someAnatidae). First, we identified all cases been tested rigorously. Nevertheless, several of avian mating systemsknown to meet these criteria empiricalstudies have yielded resultsrelevant (see Appendix). Next, we sought phylogeniesthat to hypotheseslinking sexualselection and spe- identifiedthe sistertaxa of monophyletictaxa having ciation. For example, Pierotti and Annett (1993) promiscuousmating systems.We rejected casesin reported that avian families exhibiting non- which phylogenetic relationships could not be re- monogamousmating systemsand sexual di- constructedwithin groupsthat were polymorphic with morphism were more species-richthan families respectto mating system.For instance,many mem- exhibiting monogamousmating systemsand bers of the Phasianidaeare promiscuous,but their phylogeneticrelationships are ambiguous(Johnsgard sexualmonomorphism. This result is consistent 1986, 1988).Similarly, one speciesin the genusAn- with the prediction described above, but the dropadus(family Pycnonotidae)is promiscuous,but method employedto compareaverage numbers its relationshipswith its congenersare unknown. of speciesper family acrossgroups of families, In each casewhere the sistertaxon was uniformly treating each family as independent, was not nonpromiscuous,we comparedthe numbers of spe- intended to distinguishbetween purely biolog- cies in the two sister taxa. We examined whether the 546 MITRA,LANDEL, AND PRIJETT-JOIqES [Auk, Vol. 113 speciesrichness of the promiscuoustaxon exceeded Our data set comprised 14 comparisons,as that of its sistertaxon more frequently than expected detailed in Table 1. One of these, comparison by chance,using three statisticaltests: a sign test,a 14, involved a group (Quiscalusgrackles) for Wilcoxonsigned-ranks test, and a bootstraprandom- which two availablephylogenies differed in the ization test. All tests were one-tailed because of the reconstruction of sister-group relationships a prioriprediction that the associationbetween mating (Lanyonunpubl. data, BjiSrklund 1991). Includ- systemand speciesnumber would be positive.In one case,where two sourcesimplied different phyloge- ing comparison14a, the number of speciesin neticrelationships for a given promiscuoustaxon, the the promiscuoustaxon was higher than that of resultsof the alternativescenarios were analyzedsep- its nonpromiscuoussister group in eight cases, arately.In determiningthe numberof speciesin each equal in four cases,and lower in two cases. taxon,we followedSibley and Monroe(1990). A major Including comparison14b, the number of spe- difficultyarises in a phylogeneticcomparison of species in the promiscuoustaxon was higher than ciesrichness because most publishedphylogenies do that of its nonpromiscuoussister group in seven not includeevery speciesof the ingroup.Unless stat- cases,equal in five cases,and lower in two cases. ed otherwise,we assumedthat higher taxatreated in Simple sign testsperformed on thesedata were this manner were monophyletic. not statisticallysignificant (P = 0.055including Body size covaries with mating system in some groupsof birds (Webster1992). To controlfor the 14a, and P = 0.090 using 14b). When variation possibleconfounding effects of body size, we tested in the magnitudes of differences among the for an associationbetween body size and mating sys- paired contrastswere taken into account,the tem, and for an associationbetween body size and resultswere significantfor the analysesinvolv- speciesrichness for the taxa in our sample.The sign ing comparison14a, but not for thoseinvolving testsemployed in this part of the analysiswere two- comparison14b (Table 2). tailed becauseany association,positive or negative, No associationbetween mating system and involvingbody size would be relevant to ouranalysis body size was evident, as speciesin the pro- of matingsystems and species richness. We calculated miscuoustaxon had a larger mean body mass a meanbody massfor eachtaxon by averagingacross in 5 of 12 cases(two-tailed sign test,P = 0.774). all speciesfor which data representingmales and females (or birds not identified to sex) were available Speciesrichness and body size also were not in Dunning (1993).For the exceptionallyspecies-rich associated;species in the more species-richtax- familiesApodidae and Trochilidae,we selecteda sin- on were larger in 3 of 10 cases(two-tailed sign glespecies from each genus and calculated an average test, P = 0.344). value from these for each family. In this case,the representativespecies from eachgenus was either the DISCUSSION first one listed for which the samplesize was at least 10 individuals,or the one with the largestsample if A hypothesisas general and historically long- all specieswere representedby fewer than 10 indi- lived as the one linking sexual selectionwith viduals. speciationdeserves careful empirical consid- eration.Because the temporalscale over which RESULTS sucha relationshipmust be observedprecludes experimental testing, we must resort to com- Promiscuityor polygynywith no paternalcare parativetechniques, despite their vulnerability was identified in 14 families: Megapodiidae, to the difficulties inherent in reconstructing Phasianidae, Anatidae, Psittacidae, Trochilidae, historicalpatterns of phylogeneticdifferentia- Otididae, Scolopacidae,Tyrannidae (Tyranni- tion and mating system evolution. Further- nae, Cotinginae,Piprinae), Menuridae, Ptilon- more, limitations in our knowledge of variation orhynchidae,Corvidae (Paradisaeini), Pycnon- in the intensity of sexualselection, and in our otidae, Passeridae(Ploceinae, Estrildinae), and ability to infer past rates of speciation,require Fringillidae (Icterini). Adequatephylogenetic that we approachthe issuein terms of the re- data (allowing direct comparisonsof sistertaxa lationshipbetween mating systemsand species with different mating systems)were available richness. for promiscuoustaxa in nine of thesegroups: Clearly,the comparisonswe usein our study Megapodiidae,Tetraoninae, Anatidae, Trochil- are vulnerable to errors in phylogenetic recon- idae, Tyrannidae(Cotinginae, Piprinae), Men- struction.Any sucherror would tend to distort uridae,Ptilonorhynchidae, Paradisaeini, and Ic- the inferred pattern of evolutionary transitions terini. between mating systemsand would make a July1996] SpeciesRichness andMating Systems 547 548 MITRA,LANDEL, AND PRUETT-JONES [Auk,Vol. 113

TABLE2. Statisticalanalysis of comparisons ofspecies numbers across sister taxa with different mating systems. Signtest Wilcoxonsigned-ranks test Bootstrap Data set P Z P P Using comparison14a 0.055 -1.704 0.044 0.046 Using comparison 14b 0.090 -1.554 0.060 0.067 comparison of species richness meaningless. broad phylogeneticscope of our study would Similarly, our ignoranceof the historicalinten- seemto makethis source of confusionunlikely. sities of sexual selection forces us to oversim- Our analysisincludes representatives of nine plify our treatmentof the putativecausal factor. familiesin five orders,spanning much of the Although sexualselection can operate in any geographicaland ecologicalvariation observed mating system,the absenceof more direct ev- among birds. idence requires that we employ gross catego- Acknowledgingthe limitationsof our ap- rizations of mating systemsto recognize groups proach,we concludethat there is an association that are likely to have differed historically in in birds betweenpromiscuous mating systems intensity of sexual selection. This oversimpli- and relative speciesrichness that supportsthe ficationimplies that, even if our reconstruction hypothesisthat sexualselection promotes spe- of changesin mating systemwere perfect, our ciation.Our study is a preliminary steptoward comparisonsalmost certainly would overlook a rigorous empirical test of an important hy- instancesof relatively intense sexual selection pothesisin evolutionarybiology. We welcome in groupswith monogamousor mildly polyg- reanalysisof this problem at such time when ynous mating systems.We emphasizethat this larger setsof comparisonsare possible. source of error would tend to reduce, rather than exaggerate,the degreeof contrastin sexual ACKNOWLEDGMENTS selectionactually tested in our comparisons. In our analysis,the hypothesislinking sexual We thank K. Barker,J. Coyne, J. Harshman,S. Lan- selectionto speciationwas supportedwhen a yon, B. Livezey,R. Pierotti, M. Pruett-Jones,J. B. Slow- Wilcoxonsigned-ranks test and a bootstrapsim- inski, and M. Webster for helpful suggestionsand ulation were applied to one version of the data comments.Our researchwas supportedby the Com- set (including comparison 14a), but not when mittee on Evolutionary Biology and the Department they were applied to the other version(includ- of Ecologyand Evolution at the University of Chi- ing comparison14b), or when a sign test was cago,and NSF Grant IBN-9207609 to S.P.-J. employed(Table 2). This ambiguityis probably related to our small samplesize. The samepro- LITERATURE CITED portion of positivecomparisons that we report (0.8) would yield a strongly significantsign test ALATALO,R. V., J. H6GLUND, AND A. LUNDBERG.1988. Patterns of variation in tail ornament size in birds. (P < 0.01) if the sample size were increasedto 20. BiologicalJournal of the Linnean Society34:363- 374. Besides sexual selection, other factors that ANDERSSON, M. 1982. Female choice selects for ex- might affect speciationinclude gene flow, spa- treme tail length in a widowbird. Nature 299: tial variability in natural selection,and unpre- 818-820. dictable historical events (such as those in- ARNOLD,S. J., ANDD. DUVALL. 1994. Animal mating volved in polyploidy). The last of thesecannot systems:A synthesisbased on selectiontheory. reasonablybe invoked to accountfor the pat- American Naturalist 143:317-348. tern that we report, and the broad variety of BARRACLOUGH.T. G., P. H. HARVEY, AND S. NEE. 1995. taxathat we consideredeffectively rules out the Sexualselection and taxonoraicdiversity in passefinebirds. Proceedings of the RoyalSociety of other two factorsas explanations for our results. London Series B 259:211-215. We must also considerthe possibility that our BtRKHEAD,T. R., AND A. P. MOLLER.1992. Sperm results are attributable to a covariance between competition in birds. Academic Press,London. an unknown factor affecting speciesrichness BJ6RKLUND,M. 1991. Evolution, phylogeny, sexual and the mating systemsused here to identify dimorphism,and mating systemin the grackles high levels of sexualselection. Although we are (Quiscalusspp.: Icterinae). Evolution 45:608-621. not able to rule out all such possibilities,the BROM,T. G. 1990. Currentstudies on the phylogeny July 1996] SpeciesRichness and Mating Systems 549

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APPENDIX. Avian taxa classifiedas promiscuousor highly polygynous(males provide no resourcesto their matesor young). Names of higher taxa (e.g. family or genusnames by themselves)refer to all of the species included in that taxon accordingto Sibley and Monroe (1990).

Aepypodiusand Alectura(Jones 1990); Phasianini (some;Johnsgard 1986); Centrocercus, Tetrao,Tympanuchus, and Bonasabonasia (Johnsgard 1983); Meleagris (Campbell and Lack 1985); Cairinaand Aix (Johnsgard1978); Anas (all except waigiuensis,sibilatrix, capensis, gibberifrons, castanea, aucklandica, specularis, specularioides, versicolor; Johnsgard1978, McKinney 1991); Strigops(Merton et al. 1984); Trochilidae (Skutch 1976); Otis (Cramp and Simmons 1980);Gallinago media (Cramp and Simmons1980, H6glund and Lundberg 1987); Tryngites(Pruett- Jones1988); Philomachus (Cramp and Simmons 1980, van Rhijn 1991); Calidrismelanotos (Campbell and Lack 1985);Mionectes oleagineus, M. macconnelli,and M. rufiventris(Willis et al. 1978);Phoenicircus, Tijuca atra, Lipaugus unirufus,L. vociferans,L. fuscocinereus,Pyroderus scutatus, Cephalopterus, Perissocephalus, Procnias, Rupicola (Snow 1982); Oxyruncus(Stiles and Whitney 1983);Pipra (some;Ridgely and Tudor 1994); Chiroxiphia(some; Snow 1963; Foster 1977, 1981);Corapipo gutturalis (Davis 1949);Manacus manacus (Lill 1974);M. vitellinus(Willis and Eisenmann 1979);Machaeropterus pyrocephalus (Robbins 1985);M. regulus(Meyer de Schauenseeet al. 1978); Menura (Campbell and Lack 1985);Ptilonorhynchidae (except Ailuroedus; Cooper and Forshaw 1977); Parad- isaeini (except Manucodia;Cooper and Forshaw 1977); Andropaduslatirostris (Brosset 1982); Euplectes jacksoni (Andersson1982); Viduachalybeata (Payne 1973); V. macroura(Shaw 1984, Alatalo et al. 1988); V. orientalis,V. paradisaea,and V. obtusa(Payne 1984, Alatalo et al. 1988);Quiscalus major, Q. mexicanus(Webster 1992); Molothrus ater(Yokel 1989); M. aeneusand M. bonariensis(Campbell and Lack 1985); M. rufoaxillaris(Mason 1987); Scaphidura oryzivora(Campbell and Lack 1985);Psarocolius, Gymnostinops, Ocyalus, and Cacicus(some; Campbell and Lack 1985, Webster 1992).