Body Attributes of Both Parents Jointly Affect Offspring Sex Allocation in a Socially Monogamous, Size-Monomorphic Passerine
Total Page：16
File Type：pdf, Size：1020Kb
Current Zoology 59 (2): 271–277, 2013 Body attributes of both parents jointly affect offspring sex allocation in a socially monogamous, size-monomorphic passerine Xin LU*, Xianhai ZENG, Bo DU Department of Zoology, College of Life Sciences, Wuhan University, Wuhan, 430072, China Abstract Theory predicts that because males are more variable in reproductive success than females, a mother should produce more sons to maximize fitness return from the sex allocation if she is of high-quality (the female quality hypothesis) or mates with a high-quality male (the male quality hypothesis). While most previous studies have looked at each hypothesis, we tested both of them simultaneously in the white-rumped snowfinch Montifringilla taczanowskii, a socially monogamous, sexually monomorphic passerine where body size is a potential indicator of individual quality in intrasexual competition and territory de- fense. Brood sex ratios at the population level did not deviate from random expectation. Among individual broods, the proportion of sons did not depend on body size of either male or female parent, but on interaction of this trait of both parents. Further analy- ses revealed that brood sex ratios were independent of body size of male or female parents when their mates were smaller, but positively related with body size of male or female parents when their mates were larger. These results suggest that mechanisms underlying the two hypotheses may act jointly on offspring sex allocation. The mechanisms are expected to evolve through size-assortative mating which is often reached by sexual selection [Current Zoology 59 (2): 271−277, 2013]. Keywords Brood sex ratio, Parental quality, Size-assortative mating, White-rumped snowfinch Ever since Trivers and Willard (1973), many evolu- 2002; Parker, 2002) or male quality hypotheses (Elle- tionary ecologists have attempted to investigate whether, gren et al., 1996; Sheldon et al., 1999; Rutstein et al., how and why female birds are able to adaptively allo- 2005; Du and Lu, 2010; Zielińska et al., 2010), some cate the sex of their offspring (West, 2009). Relying on others provided no evidence for either (the female qual- a basic premise that sons may be highly variable in ity: Genovart et al., 2005; the male quality: Saino et al., reproductive success due to prevalence of polygynous 1999; Dreiss et al., 2006). or extra-pair mating among populations, whereas In birds, males and females can mate assortatively daughters usually experience lower risks of gaining no with respect to various quality-related phenotypes such mates, several hypotheses have been proposed to as body size (Olsen et al., 1998; Tryjanowski and Simek, address this issue. Of these, the female and the male 2005), ornament size (Jawor et al., 2003), age or breed- quality hypotheses are two most influential. The former ing experience (Cézilly et al., 1997; Ludwig and Becker, predicts that mothers in good condition would overpro- 2008; Taff et al., 2011). Sexual selection, which may act duce sons to obtain greater fitness returns providing that through either male-male competition (intrasexual se- good condition is transferred to the offspring, while lection), female or male choice (intersexual selection), mothers in poor condition would give rise to more or a combination of these mechanisms, has been appre- daughters to minimize the risk of reduced fitness (Triv- ciated the major contributor to formation of the mating ers and Willard, 1973). The latter suggests that females pattern of many taxa (Crespi, 1989). The non-random mated to high-quality and thus sexually-attractive males mating is evolutionarily important because different will benefit from yielding more sons, since the sons may combinations of males and females with specific traits inherit their father’s traits and reach a greater success in may have different fitness consequences. It has well social and extra-pair mating (Burley, 1981). However, been known that the mated pairs consisting of both while some empirical data support the female quality high-quality males and females enjoy an enhanced re- (Nager et al., 1999; Heg et al., 2000; Whittingham et al., productive success by many ways (Andersson, 1994). Received July 1, 2012; accepted Oct. 28, 2012. ∗ Corresponding author. E-mail: [email protected] © 2013 Current Zoology 272 Current Zoology Vol. 59 No. 2 Given that manipulation of offspring sex ratio through egg-laying period; paired females participate in territory the potential mechanisms associated with quality of defense. Egg-laying occurs between late April and early either female or male parents is a fundamental part of June. Clutch size averages 4.7 (2−6). Some pairs (9%, n = successful reproductive strategies, we expect that the 147) make two breeding attempts in a single year. There size-assortative mating can generate fitness benefits in is no evidence that a pair produce a replacement clutch terms of offspring sex allocation. In other words, the if nesting failure occurs during the middle of breeding two hypotheses of adaptive sex allocation may operate period. Nest-building, incubation (lasting 13 days on simultaneously or they would be effective only when average) and brooding are undertaken by females only, individual quality of both parents is combined. However, and both parents share young caring (21 days on aver- most previous empirical tests for theory only focused on age). Males do not feed their mates that are engaged in one of the two mechanisms, and to our knowledge, only incubating or brooding. two studies of sex ratio manipulation have simultane- We searched snowfinch nests within a 390-ha plot ously taken traits of both parents into account (Dowling covered with alpine meadow throughout the breeding and Mulder, 2006; Addison et al., 2008). season every year. The snowfinch species is present in Here we examine variation in offspring sex ratio for a the study area year round. There was 71% of 24 breed- socially monogamous, sexually size-monomorphic pas- ing males and 50% of 30 breeding females marked in serine, the white-rumped snowfinches Montifringilla 2006 summer in the study plot were rediscovered in taczanowskii. Although the birds are sexually size 2007 early spring. Of these resighted individuals, only monomorphic, body size should still be important for 4% males and 10% females had nests located within the individual quality because there is individual variation study plot and all of them remated. Young recruitment in this trait for each sex. Moreover, during the breeding rates from 2006 to 2007 were 21% for males (n = 61) season, both male and female snowfinches are fre- and 16% for females (n = 43), with 13% in males and quently engaged in direct aggressive interactions in- 14% in females being found to nest within the plot. cluding physical flights in which larger body sizes could We mist-netted adult females at the entrance of nest- be advantageous (Zeng, 2008). We correlate brood sex ing burrows during the incubation period and adult ratio not only with parental body condition ⎯ a quality males during the nestling period. It was difficult to indicator depending more on current ecological regimes check the nest contents that were inside the pika bur- (Moya-Laraño et al., 2008), but also with parental rows. Therefore we captured nestlings by hand or mist- structural size ⎯ a quality indicator that signifies more net when they received food from parents at the nesting genetic components (Kölliker et al., 1999; Blanchard et burrow entrances shortly before fledging. As a result, al., 2007). Specifically, we test the predictions that fe- this study only focused on brood sex ratio at fledging. males with better condition or larger size would produce Offspring sex ratios can be influenced by sex-specific male-biased broods when they mate to a better-condi- mortality rates of nestlings. However, such effect was tioned or larger male. minor in this study because the snowfich nestlings 1 Materials and Methods reared in burrows experienced little brood reduction (Lu et al., 2009; Zeng and Lu, 2009b). 1.1 Field data collection For each individual, we measured body mass (with White-rumped snowfinches were studied during the an electronic balance, to the nearest 0.1 g) and linear breeding seasons of 2006 and 2007 at Gahai Natural dimension (body length, beak length, tail length, wing Reserve (34°14′N, 102°20′E, 3450-3800 m elevation), length and tarsus length, with a calipers, to nearest 0.05 in the northeastern Tibetan Plateau. Our previous work mm; Sutherland et al., 2004). A blood sample (10−20 µl) at this site (Zeng and Lu, 2009a, b) has shown that the was taken from the femur vein of each individual, col- birds nest in the burrows of the black-lipped pika lected in a capillary tube and transferred to a tube con- Ochotona curzoniae, with a density of 0.13−0.17 nests taining 100 μl anticoagulant (5% EDTA in Sodium per ha within the study area. Both male and female Chloride Physiological Solution) and 1 ml ethanol (95%) snowfinches reach sexual maturity at the second year for preservation. after hatching. Mating system is socially monogamous. 1.2 Molecular sexing Males establish territories (size 0.1−0.5 ha) and guard We sexed the birds using two primer-pairs of P2 ⁄ P8 their mates strongly from pair formation through (Griffiths et al., 1996) and 2550F ⁄ 2718R (Fridolfsson LU X et al.: Offspring sex allocation in Montifringilla taczanowskii 273 and Ellegren, 1999). DNA was extracted from blood body length (0.72) and tail length (0.85) for males. No samples following the protocol of a DNA mini kit (SBS correlation was evident between body mass and struc- Genetech Co., Beijing). PCR reaction conditions (total tural size for female (r = −0.05, P = 0.74) and male volume = 10 μL) were conducted as follows: 1× PCR breeders (r = −0.18, P = 0.26), suggesting them to act buffer (50 mM KCl; 10 mM Tris-HCl pH 9.0; 0.1% independently.