Yolk is a source of maternal testosterone for developing birds.

  title={Yolk is a source of maternal testosterone for developing birds.},
  author={Hubert Schwabl},
  journal={Proceedings of the National Academy of Sciences of the United States of America},
  volume={90 24},
  • H. Schwabl
  • Published 15 December 1993
  • Biology
  • Proceedings of the National Academy of Sciences of the United States of America
The sex steroid hormones that affect development in birds have been thought to be produced exclusively by the embryo or neonate. I used radioimmunoassay to measure the amounts of androstenedione, 5 alpha-dihydrotestosterone, testosterone, 17 beta-estradiol, and corticosterone in the yolk of freshly laid canary (Serinus canaria) and zebra finch (Poephila guttata) eggs. Testosterone was found in both canary and zebra finch eggs, but its contents were much higher in the former than in the latter… 

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It is unlikely that maternal androgens are a key factor in the avian sex determination mechanism, as testosterone levels in chicken eggs do not change with incubation period and A4 levels decrease between 3 and 5 days of incubation.
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It is discovered that male nestlings were more susceptible than female nestlings to growth inhibition by yolk-androgen elevation but did not find a bias in sex ratio with respect to laying order, consistent with the hypothesis that sex differences in yolks enable mothers to economically tune reproductive effort to an individual offspring’s reproductive value.
Environment modifies the testosterone levels of a female bird and its eggs.
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    The Journal of experimental zoology
  • 1996
It is concluded that here the authors have a mechanism which communicates environmental conditions from the mother to the offspring, and that this mechanism serves to optimize reproduction and/or modify offspring traits.
Maternal androgens in egg yolks: relation with sex, incubation time and embryonic growth.
Maternal testosterone in tree swallow eggs varies with female aggression
In tree swallows, Tachycineta bicolor, it is found that yolk testosterone was correlated with the aggressive interactions of the female before and during egg laying and did not vary with laying order in tree swallow.
Yolk Testosterone Stimulates Growth and Immunity in House Finch Chicks
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Offspring Sex Is Not Related to Maternal Allocation of Yolk Steroids in the Lizard Bassiana duperreyi (Scincidae)
Yolk steroid hormones do not appear to play a critical role in sex determination for B. duperreyi, and there were no significant differences in yolk steroids, or in relative composition of steroids, between eggs destined to become male versus female.
Functional significance of variation in egg-yolk androgens in the American coot
Within clutches, early-laid eggs had higher androgen levels than late-laying eggs, and this pattern may exacerbate negative effects of hatching asynchrony on chicks from late-hatching eggs if androgens provide chicks with a behavioral or growth advantage over chicks from eggs with lower androgens levels.
Yolk androgens reduce offspring survival
Additional studies are necessary in order to determine whether the deposition of yolk androgens is an adaptive form of parental favouritism or an adverse by–product of endocrine processes during egg formation, despite its adaptive significance.


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