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Response to J.P. Lightbody 550 Commentaries [Auk,Vol. 104 canbe compensatedfor matingpolygynously by being ritory size would be expectedif femalessettled ran- in sucha superiorbreeding situationthat they expect domly. I showed recently that a neutral mate-choice the same reproductive successas would be achieved hypothesis, in which females settle randomly and in a poorer situation. The sexy-sonhypothesis sug- malescompete for territories to gain accessto females, geststhat femaleschoosing polygyny with attractive applies to at least one population of Yellow-headed males will have attractive sons with large haremsß Blackbirds(Xanthocephalus xanthocephalus; Lightbody and, thus, these females will leave more grandoff- and Weatherhead in press).This is a viable alternative spring, rather than more of their own offspring. explanationthat Nagata failed to consider.Unless it Clearly, these two hypothesescannot be used to ex- can be shown clearly that choosinga large territory amine female choice in monogamousspecies. is adaptive, it is premature to conclude that female Nagata did not properly determine whether fe- Middendorff'sGrasshopper-Warblers choose mates on males choosemates on the basisof male quality. He the basisof territory quality. found no correlationsbetween pairing date and male morphologicalcharacteristics, but he did not examine LITERATURE CITED any male behavior. Femalesmay have chosenmates on the basisof male courtship abilities (Weatherhead BORGIA,G., S. G. PRUETT-JONES,& M. A. PRUETT-JONES. and Robertson 1977, Borgia et al. 1985). 1985. The evolution of bower-building and the Nagata did not show convincinglythat females assessmentof male quality. Z. Tierpsychol. 67: choosemates based on territory quality. He showed 225-236. a significantcorrelation of pairing date with territory LIGHTBODY,J. P., & P. J. WEATHERHEAD.In press. size and, to a lesser degree, with a food scoreßand Polygyny in Yellow-headedBlackbirds: female concluded that this demonstrated female choice on choice versus male competition. Anim. Behav. the basisof territory quality. This conclusionmay not NAGATA, H. 1986. Female choice in Middendorff's be valid for several reasons.Food score was propor- Grasshopper-Warbler(Locustella ochotensis). Auk tional directly to territory size, and when he held 103: 694-700. territory size constant the correlation between pair- ORIANS,G. H. 1969. On the evolution of mating ing date and food scorewas reduced.This suggests systemsin birds and mammals.Amer. Natur. 103: that food was not the main feature of territories upon 589-603. which femalesmade their choice.Territory size may VERNER,J. 1964. Evolution of polygamyin the Long- be synonymouswith territory quality if larger terri- billed Marsh Wren. Evolution 18: 252-261. tories contain more food and the extra food increases ß & M. F. WILLSON. 1966. The influence of reproductive successor nestling growth rates, or if habitats on mating systemsof North American larger territories contain more nest sites and males passefinebirds. Ecology47: 143-147. with large territories can attract more females. The WEATHERHEAD,P. J., & R. J. ROBERTSON.1977. Male former was not examined by Nagata, and the latter behavior and female recruitment in the Red- does not apply to a monogamousspecies. Therefore, winged Blackbird.Wilson Bull. 89: 583-592. there is no valid reason to conclude that female choice ß & --.. 1979. Offspring quality and the of large territories is adaptive in this species. polygyny threshold: "the sexy son hypothesis." In addition, a correlation between arrival date and Amer. Natur. 113: 201-208. territory size need not indicate females prefer large territories. A correlation between arrival date and ter- ReceivedI December1986, accepted27 February1987. Responseto J.P. Lightbody HISASHI NAGATA • Lightbody (1987)argued that the polygyny-thresh- in monogamousspecies, however, if there are vari- old hypothesis(Verner 1964,Verner and Willson 1966, ations among territories or genetic qualities, and if Orians 1969) and sexy-sonhypothesis (Weatherhead those variations reflect upon the fitnessof females. A and Robertson 1979) are useful to explain only why threshold never need be exceeded.The polygyny- somemales in polygynousspecies acquire more than thresholdhypothesis assumes that territory quality is one mate. Femaleswill try to mate with fitter males the main cue of female mate choice,and for monog- amous speciesor primary females predicts that the male with the bestterritory will be chosen.The sexy- Departmentof Biology,Faculty of Science,Kyu- son hypothesisextends the conceptof the polygyny shu University 33, Fukuoka 812, Japan. threshold to the quality of a female's offspring. The July1987] Commentaries 551 sexy-sonhypothesis assumes that a sexycharacter is male fitness.To negate the possibilitythat females highly heritableand impliesthat a femalethat choos- choosemates on the basisof male courtshipability esa male with sucha charactercan gain compensation requiresexamination of the correlationbetween male for the reductionof her childrenin future generations behaviorsand pairing date. through the successof their sexysons. Lightbody (1987) proposedthat a neutral female- There are some differencesbetween the two hy- choice hypothesiswas applicable to Middendorff's pothesesin the definition of the fitnessgain of the Grasshopper-Warbler.If females choosemales ran- females.In thepolygyny-threshold hypothesis fitness domly, polygyny is expectedmore frequently. Mo- is defined as immediate reproductive success,while nogamy is the predominant mating system in this in the sexy-sonhypothesis it is definedas the number species,however. There was evidence that the loca- of descendants, i.e. the ultimate contribution to the tion where females arrived first was distant from the gene pool. Therefore, if females assessthe territory territory where they settledand that somemales led quality and choosemates to maximize the current one female into their territory. These observations reproductivesuccess, the polygyny-thresholdhy- suggestthat females choosetheir mates selectively pothesisis applicablefor female choicein monoga- rather than randomly.Moreover, arrival date did not mousspecies. If femalesassess male quality and choose correlatewith territorysize (r = 0.38,not significant; matesto maximizethe number of grandoffspring,then Nagata1986). Therefore, the neutralmate-choice hy- the sexy-sonhypothesis is applicablein monogamous pothesiscannot apply to the Middendorff's Grass- species. hopper-Warbler. As pointedout by Lightbody(1987), the correlation between pairing date and food score for Midden- LITERATURE CITED dorff'sGrasshopper-Warbler (Locustella ochotensis) was reduced when territory size was held constant(Na- LIGHTBODY,J.P. 1987. Female choice in Midden- gata 1986),which indicated only that femalesdid not dorff's Grasshopper-Warbler?Auk 104: 549-550. use the density of available food to assessterritory NAGATA, H. 1986. Female choice in Middendorff's quality. Larger territories may contain more food, Grasshopper-Warbler(Locustella ochotensis). Auk however,as preferable food resourcesare distributed 103: 694-700. randomly, and larger territories may contain more ORIANS,G. H. 1969. On the evolution of mating potential nest sites. Females that failed to breed re- systemsin birdsand mammals.Amer. Natur. 103: nestedat a distancefrom their first nests(Nagata un- 589-603. publ. data). Large territories will provide alternative VERNER,J. 1964. Evolutionof polygyny in the Long- nestsites for femalesand allow the possibilityof po- billed Marsh Wren. Evolution 18: 252-261. lygynousmating, if a polygynythreshold is exceeded. , & M. F. WILLSON. 1966. The influence of ! observedonly two casesof occasionalpolygyny over habitatson mating systemsof North American 6 yr of observation(Nagata in prep.). In those cases passerinebirds. Ecology47: 143-147. males with large territories acquired additional fe- WEATHERHEAD,P. J., & R. J. ROBERTSON.1979. Off- males.Therefore, I concludethat femalegrasshopper- springquality and the polygyny threshold:"the warblers choosemales with large territories. sexyson hypothesis." Amer. Natur. 113:201-208. The factthat I foundno correlationbetween pairing date and morphologicaltraits of males(Nagata 1986) Received28 February1987, accepted5 March 1987. indicatedonly that suchcharacteristics did not reflect .
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