Non-random sex allocation may occur whenever the expected reproductive value of sons and daughters differs, as is the case
when the sexes differ in susceptibility to environmental conditions or maternal effects (e.g. egg size and hatch order). Under
such circumstances, covariation between egg and clutch characteristics and egg sex may be expected, and this covariation should
vary with maternal state or ecological conditions. In this 2-year study (2007–2008), we examined sex allocation in relation
to egg and clutch traits in the yellow-legged gull Larus michahellis, a species where male chicks are larger and more susceptible to harsh rearing conditions than female ones. In 2008, eggs
were more likely male early in the season in two- but not three-egg clutches, and large eggs were more likely males late in
the season. No egg/clutch traits predicted egg sex in 2007. Within-clutch egg mass asymmetry (the difference in egg mass between
the first- and last-laid eggs) predicted sex in both years. In 2007, clutches with smaller egg mass variation were more likely
to contain males, while in 2008 this relationship held for the last-laid egg and was reversed for the preceding egg(s). Laying
order and sex of the previous egg did not predict egg sex, providing no evidence of sex-specific oocyte clustering. Thus,
the relationships between egg sex and egg/clutch traits differed among years, suggesting a phenotypically plastic response
of females to extrinsic conditions, and involved within-clutch egg mass asymmetry, a trait likely reflecting variation in
maternal quality and/or reproductive tactics, which has been largely neglected in previous studies of sex allocation.
" egg size increased . These patterns could be driven by sex effects on egg shape and / or egg composition . Our simple index of egg shape , based on length and width , was identical for male and female eggs ( Table 3 ) , but shape could nonetheless differ in more complex ways . Egg shape can vary with laying order ( e . g . , Blanco et al . 2003 ; Rubolini et al . 2009 ) ; therefore , any sex bias with laying order could produce differences in egg shape between the sexes . Although we found no sex ratio difference with respect to egg order , our power was limited . Väisänen et al . ( 1972 ) reported that fourth eggs of 28 red - necked phalarope clutches were longer than earlier eggs ; we fail to dupli"
[Show abstract][Hide abstract] ABSTRACT: Fisher's sex ratio theory predicts that on average parents should allocate resources equally to the production of males and females. However, when the cost/benefit ratio for producing males versus females differs, the theory predicts that parents may bias production, typically through underproduction of the sex with greater variation in fitness. We tested theoretical predictions in the red-necked phalarope, a polyandrous shorebird with sex-role reversal. Since females are larger and therefore potentially more expensive to produce and may have greater variation in reproductive success, we predicted from Fisher's hypothesis a male bias in population embryonic sex ratio, and from sex allocation theory, female biases in the clutches of females allocating more resources to reproduction. We measured eggs and chicks and sexed 535 offspring from 163 clutches laid over 6 years at two sites in Alaska. The embryonic sex ratio of 51.1 M:48.9 F did not vary from parity. Clutch sex ratio (% male) was positively correlated with clutch mean egg size, opposite to our prediction. Within clutches, however, egg size did not differ by sex. Male phalarope fitness may be more variable than previously thought, and/or differential investment in eggs may affect the within-sex fitness of males more than females. Eggs producing males were less dense than those producing females, possibly indicating they contained more yolk relative to albumen. Albumen contributes to chick structural size, while yolk supports survivorship after hatch. Sex-specific chick growth strategies may affect egg size and allocation patterns by female phalaropes and other birds.
"It should be noted that the latter analysis was based on a larger number of males than females from first-laid eggs, while the converse was true in last-laid eggs (see samples sizes in Fig. 2). At present, given that our analyses are based on all chicks that survived to 4 days of age (when sex of the chicks was determined), we are unable to conclude if this uneven distribution of the sexes between first-and lastlaid eggs was due to a difference between the sexes in hatching rate or in post-hatching survival or even to an association between sex and position in the laying sequence (but see Rubolini et al. 2009). Therefore, we cannot rule out the possibility that avidin had interacted with the sex of the embryo in determining hatching and survival prospect of the chicks. "
"data) and are up to 20% smaller than a-eggs [see Table 1; see also ]. Hatching is asynchronous: chicks from c-eggs hatch on average 2 days later than their siblings from a-eggs , are smaller and suffer from competitive disadvantage compared to their older siblings, resulting in higher pre-fledging mortality [49,51]. Similar to closely-related species [52,53], Yellow-legged Gulls are sexually size dimorphic soon after hatching , with males becoming 15-20% larger than females once adult size is reached . "
[Show abstract][Hide abstract] ABSTRACT: ABSTRACT:
Maternal effects mediated by egg size and quality may profoundly affect offspring development and performance, and mothers may adjust egg traits according to environmental or social influences. In avian species, context-dependency of maternal effects may result in variation in egg composition, as well as in differential patterns of covariation among selected egg components, according to, for example, position in the laying sequence or offspring sex. We investigated variation in major classes of egg yolk components (carotenoids, vitamins and steroid hormones) in relation to egg size, position in the laying sequence and embryo sex in clutches of the Yellow-legged Gull (Larus michahellis). We also investigated their covariation, to highlight mutual adjustments, maternal constraints or trade-offs in egg allocation.
Laying sequence-specific patterns of allocation emerged: concentration of carotenoids and vitamin E decreased, while concentrations of androgens increased. Vitamin A, estradiol and corticosterone did not show any change. There was no evidence of sex-specific allocation or covariation of yolk components. Concentrations of carotenoids and vitamins were positively correlated. Egg mass decreased along the laying sequence, and this decrease was negatively correlated with the mean concentrations of carotenoids in clutches, suggesting that nutritionally constrained females lay low quality clutches in terms of carotenoid content. Finally, clutches with smaller decline in antioxidants between first- and last-laid eggs had a larger increase in yolk corticosterone, suggesting that a smaller antioxidant depletion along the laying sequence may entail a cost for laying females in terms of increased stress levels.
Since some of the analyzed yolk components (e.g. testosterone and lutein) are known to exert sex-specific phenotypic effects on the progeny in this species, the lack of sex-specific egg allocation by mothers may either result from trade-offs between contrasting effects of different egg components on male and female offspring, or indicate that sex-specific traits are controlled primarily by mechanisms of sexual differentiation, including endogenous hormone production or metabolism of exogenous antioxidants, during embryonic development.
Frontiers in Zoology 10/2011; 8(1):24. DOI:10.1186/1742-9994-8-24 · 3.05 Impact Factor
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