title: Brood reduction via intra-clutch variation in testosterone--an experimental test in the great tit.
abstract: In birds, yolk androgen concentrations in eggs can increase or decrease over the laying sequence and common hypotheses hold that this serves to favour the competitive ability of either first- or last-hatched chicks depending on the prevailing conditions, and thus promote brood reduction or maintenance of original brood size respectively. Intra-clutch variation of testosterone can shift relative competitive ability of siblings and hence competitive dynamics. In a natural population of great tits, we experimentally investigated the effects and function of maternal testosterone on offspring phenotype in relation to the laying position of the egg in a context of hatching asynchrony. To this end, we created three types of clutches where either the first three or the last three eggs of a clutch were injected with testosterone (T) dissolved in sesame oil, and the remaining eggs with sesame oil only, or where all eggs of a clutch were injected with sesame oil. Increased levels of yolk T in the last-laid eggs resulted in the last-hatched chicks being significantly lighter and smaller than their siblings, while increased levels of T in the first-laid eggs had no direct effect on the first-hatched chicks, but an indirect negative effect on their siblings. Our results suggest that females can potentially adjust offspring phenotype by modulating, over the laying sequence, the amounts of T deposited in the eggs. These results are in contradiction, however, with current hypotheses and previous findings, which suggest that under good conditions higher levels of maternally derived T in the last-laid eggs should mitigate the negative effects of hatching asynchrony.
authors: Institute of Ecology and Evolution, University of Bern, Bern, Switzerland. podlas.kasia@gmail.com
authors: Katarzyna
authors: K
authors: Podlas
authors: Fabrice
authors: F
authors: Helfenstein
authors: Heinz
authors: H
authors: Richner
mesh_terms: D000818:Animals; D001717:Birds / Q000502:physiology*; D004530:Egg Yolk / Q000378:metabolism* / Q000502:physiology; D005260:Female; D010058:Oviposition / Q000502:physiology; D012098:Reproduction / Q000502:physiology*; D013739:Testosterone / Q000378:metabolism* / Q000502:physiology
chemical_list: D013739:Testosterone