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. 2017 Nov 6;7(24):10675-10682.
doi: 10.1002/ece3.3575. eCollection 2017 Dec.

Male-specific Mortality Biases Secondary Sex Ratio in Eurasian Tree Sparrows Passer montanus

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Free PMC article

Male-specific Mortality Biases Secondary Sex Ratio in Eurasian Tree Sparrows Passer montanus

Takahiro Kato et al. Ecol Evol. .
Free PMC article

Abstract

Sex allocation theory predicts that parents bias the offspring sex ratio strategically. In avian species, the offspring sex ratio can be biased at multiple growth stages, although the mechanisms are not well known. It is crucial to reveal a cause and timing of biased offspring sex ratio. We investigated (i) offspring sex ratio at multiple growth stages, from laying to fledging; and (ii) the stage at which offspring sex ratio became biased; and (iii) the cause of biased offspring sex ratio in Eurasian tree sparrows Passer montanus. Sex determination of 218 offspring, including hatchlings and unhatched eggs from 41 clutches, suggested that the offspring sex ratio was not biased at the egg-laying stage but was significantly female-biased after the laying stage due to higher mortality of male embryos. Half of the unhatched eggs showed no sign of embryo development (37/74, 50.00%), and most undeveloped eggs were male (36/37, 97.30%). Additional experiments using an incubator suggested that the cause of embryo developmental failure was a lack of developmental ability within the egg, rather than a failure of incubation. This study highlights the importance of clarifying offspring sex ratio at multiple stages and suggests that offspring sex ratio is adjusted after fertilization.

Keywords: Eurasian tree sparrow; Passer montanus; fertility; primary sex ratio; secondary sex ratio; sex‐specific mortality.

Figures

Figure 1
Figure 1
(a) The inner tissue of an undeveloped egg. A white patch in the center of the yolk (indicated by an arrow) is the germinal disk. (b, c) Nuclei were stained with Hoechst 33342, colored blue. A large number of nuclei were observed in fertilized germinal disks after normal development (b), whereas a small number of nuclei were observed in the germinal disks that were fertilized but stopped development at around the 8‐cell stage (c)
Figure 2
Figure 2
Survival rate of male and female offspring at three stages (developmental success, hatching success, and fledging success)
Figure 3
Figure 3
Relation between the embryo development/failure of eggs in an incubator and the percentage of embryo development in the original clutch. Circle size indicates sample size (from 1 to 9)

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