Between and within Clutch Variation of Egg Size in Greater Rheas

—We describe egg characteristics, and analyze between and within clutch variation in egg size and mass in a natural population of Greater Rheas (Rhea americana). We assess the effect of this variation on nesting success and egg success. Yolk represented 29.5% of egg mass whereas albumen was 63.9%. Yolk mass increased with egg width but not with egg length, while mass of albumen increased mainly with egg length. The largest and smallest eggs were 10.3% larger and 25.3% smaller, respectively than mean intra-clutch values. The widest egg was 11.9% wider while the narrowest egg was 20.5% narrower than mean intra-clutch values. There was a significant decrease in egg size between clutches during the breeding season as a result of a decrease in egg length. There was no effect of laying order on intra-clutch variation in egg size, but we detected an increase in the variation of egg length within clutches with clutch size. We did not detect a relationship between egg size and nesting success, and between egg size and egg success. The relatively low intra-clutch variation in egg size and lack of effect of egg size on hatching success do not support the hypothesis that females invest in eggs according to expected chick fitness. Received 23 November 2007. Accepted 15 April 2008. Intraspecific egg size variation is relatively common in birds (Slagsvold et al. 1984, Williams et al. 1993, Christians 2002). Most variation is among rather than within clutches and it is generally accepted this variation is the result of a strong genetic component of egg size (Christians 2002, Stryrsky et al. 2002, Valkama et al. 2002). However, variation among or within clutches laid by the same female should be attributed to phenotypic or environmental-mediated variation (Nager et al. 2006). Nutrient or energetic constraints could be responsible for egg size variation in some bird species (O’Connor 1979, Pierotti and Bellrose 1986). Egg size could also vary with parental age or experience with eggs laid by older and experienced females larger than those laid by younger females (Davis 1975, Blomqvist et al. 1997, Hipfner et al. 1997). Alternatively, egg size variation could respond to an adaptive investment of females, thereby influencing offspring quality. Larger eggs may contain more water and nutrients, which may benefit hatchlings by increasing their hatching success, growth rate, and survival (Parsons 1970, Reid and Boersma 1990, Magrath 1992, Williams 1994, Smith and Bruun 1998, Pelayo and Clark 2003). Egg size in some species 1 Departamento de Ecologı́a, Genética y Evolución, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellón 2, Ciudad Universitaria, Buenos Aires, Argentina. 2 Corresponding author; e-mail: [email protected] decreases with laying order (e.g., Nisbet 1978, Shaw 1985, Custer and Frederick 1990). This decrease could be interpreted as an adaptive strategy because the reduction in size of last laid eggs could be a mechanism for shortening incubation period and favoring synchronous hatching (e.g., Birkhead and Nettleship 1982, Perrins 1996, Kennamer et al. 1997). Maternal investment in eggs also could be related to mate quality or prospects of success. Some studies have found that females mated with non-preferred males produced fewer and smaller eggs than females mated with the males they chose (Yamamoto et al. 1989, Cunningham and Russell 2000). Most studies of egg size variation were conducted on species where the clutch is laid by a unique female. In species with communal egg laying (where more than one female laid eggs into a single nest), differences in egg investment by females could have more important consequences for offspring fitness. For example, larger eggs in Guira Cuckoo (Guira guira) produce heavier and larger chicks that may out compete smaller and lighter nest companions (Macedo et al. 2004). As a consequence, larger communal clutches in this species had eggs with more nutrients than in smaller clutches. Females of species with communal laying could manipulate the nutrient investment in their eggs depending upon social condition to enhance individual fitness (Macedo et al. 2004). We analyzed interand intra-clutch varia675 Fernández and Reboreda • EGG SIZE VARIATION IN GREATER RHEAS tion of egg size in a natural population of Greater Rheas (Rhea americana). Greater Rheas are communal nesters where two to 10 or more females lay eggs into a single nest that a male incubates. We analyzed the effect of season and clutch sizes (total number of eggs in the nest) on both interand intraclutch variation of egg size to assess if egg size was associated with hatching success. The relationship between egg size and nesting success was analyzed because it is possible females reduce their investment in eggs laid for low quality males (i.e., young inexperienced or in poor physical condition) that are prone to desert the nest. We also analyzed the relationship between egg components (yolk and albumen) and egg size. We expected a positive relationship between egg size and yolk content, and between egg size and nest success, and hatching success.