Older males have bigger knobs : correlates of ornamentation in two species of Curassow

-I examined the correlation between the size of male facial ornamentation and three measures of viability in the polygynous Yellow-knobbed Curassow (Crax daubentoni) and the relationship between age and knob size in the monogamous Great Curassow (C. rubra), to determine which of three sexual selection models could account for the maintenance of these anomalous structures. Field studies of both species suggest that these ornaments are not used in intrasexual signaling. The data for the Yellow-knobbed Curassow support the "good genes" model of sexual selection, although not the Hamilton and Zuk (1982) model of extravagant characters as indicators of male resistance to parasites. Ornament size was not associated with body size or coccidian load, although the latter was negatively correlated with tarsus length. No hematozoa were found in blood smears. Knob height, wattle height, and wattle width were reliable indicators of age in this species and may be used by females in mate choice. Because older males have demonstrated their presumably heritable ability to survive, female preference for males with big ornaments is consistent with the "good genes" model. The absence of a correlation between knob size and age in male Great Curassows supports the hypothesis that age-character relationships will be strongest in long-lived polygynous species. Received 14 February 1990, accepted 30 July 1990. EXTRAVAGANT characters are morphological or behavioral traits that may be maladaptive under natural selection but provide a differential mating advantage under sexual selection (Nur and Hasson 1984). Both intrasexual and intersexual roles have been proposed for these traits. In Borgia's (1979) War Propaganda model a male assesses the competitive abilities of other males by examining the quality of their extravagant characters. By clearly and honestly signaling their status (Kodric-Brown and Brown 1984), high-quality males avoid energetically costly battles with weaker males. In addition to or instead of intrasexual signaling, female assessment of male extravagant characters may occur. There are three major schools of thought on the adaptive function of female choice (Johnson and Marzluff 1990). The first holds that female choice is based on the ability of males to provide immediate benefits to her and their offspring (e.g. courtship feeding, foraging areas). However, extravagant characters are most elaborate in mating systems where males provide little or no paternal care (Sigurj0nsd0ttir 1981, Payne 1984, Bjorklund 1990). This suggests that these traits are not used to assess parental abilities. Therefore in this paper I concentrate on models of extravagant character evolution that address the nondirect benefits most likely associated with ornamentation. The "good genes" hypothesis proposes that extravagant characters reflect the genetically based ability of a male to survive, gain access to essential resources, and reproduce (Trivers 1972, Zahavi 1975, Kodric-Brown and Brown 1984, Andersson 1986). Females could increase their reproductive success if their offspring carried such "good genes," and they should prefer those males with the biggest, brightest, or most intense characters. A study of mate choice in Drosophila provides the only direct empirical support for this model (Partridge 1980). The third hypothesis for the evolution of extravagant characters, first proposed by Fisher (1930), implies that, although females may have initially chosen males for a specific adaptation that promoted survival under natural selection, this adaptive reason for choice has been lost in a "runaway process." In the process he described, a genetic correlation evolves between the male character and the female preference for that character. Once the female-choice gene is established, female preference for the male character may continue under sexual selection alone, which would cause exaggeration of the trait until countered by natural selection (i.e. when the cost in decreased survival due to the 153 The Auk 108: 153-160. January 1991 This content downloaded from 157.55.39.69 on Sun, 06 Nov 2016 04:18:05 UTC All use subject to http://about.jstor.org/terms 154 RICHARD BUCHHOLZ [Auk, Vol. 108 character is greater than the benefit it provides in mate attraction) or until additive genetic variance for the male character is exhausted. Genetic models support the runaway process and demonstrate that it can be initiated by stochastic factors such as genetic drift (O'Donald 1980; Kirkpatrick 1982, 1985; Lande and Arnold 1985; but see Nichols and Butlin 1989). Heisler et al. (1987) have proposed a methodology for distinguishing between the end products of these two intersexual processes at evolutionary equilibrium. Variance in an extravagant character that evolved via the good genes process must be positively associated with measures of individual viability (sensu Andersson 1986), whereas a nonpositive relationship with viability should be found for characters that are the result of runaway selection. The "runaway" hypothesis is not falsified, however, until all the possible viability correlates of ornamentation have been tested; a difficult task if selection is episodic or the species' natural history is not well known. Clearly most shortterm studies will not be able to disprove this model convincingly. In any case isolating the mechanisms underlying phenotypic variation in extravagant characters remains the first step in determining how these traits are maintained (Zeh and Zeh 1988). I examined age, size, and parasite load as correlates of an extravagant character in captive Yellow-knobbed Curassows (Crax daubentoni), and I examined age in Great Curassows (C. rubra; Galliformes: Cracidae). Males of both of these strikingly sexually dimorphic species have yellow "fleshy" knobs-absent in females-on the maxilla. In addition the male Yellow-knobbed Curassow has two pendant wattles on the mandible. Although the adaptive significance of this ornamentation is not known, sexually selected functions have been proposed for "fleshy" ornaments in this species (Delacour and Amadon 1973, Buchholz 1989) and have been described in related species (Brodsky 1988, Zuk et al. 1990). My goal was to determine which sexual selection model might account for the evolution of curassow knobs and wattles. Across curassow species, male ornament size is correlated positively with sexual size dimorphism, a measure of male-male competition in other Galliformes (Spearman rank correlation, rs = 0.97, n = 7, P < 0.02; Fig. 1; Payne 1984). However an absence of any direct male-male combat in field studies of the Yellow-knobbed and Great curassows does not support the "intrasexual signaling" hypothesis (Sermeno 1986, Buchholz 1989). Sexual dimorphism in curassows may be the result of female choice or alternative processes (Hedrick and Temeles 1989). Because courtship displays appear to emphasize the size and color of the ornamented head by placing it against the black back of the animal (Delacour and Amadon 1973), it seems likely that female choice has played the predominant role in the evolution of knobs and wattles. Both the "good genes" and the "runaway" hypotheses predict that females will prefer to mate with males with the biggest knobs. Only the "good genes" theory predicts that males with big ornaments are older or healthier than their small-knobbed conspecifics (Heisler et al. 1987). If curassow ornaments evolved via the Fisherian runaway process, big-knobbed males should be no more likely to survive than smallknobbed ones.