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.
Highlights
Maternal effects are defined as modifications of offspring phenotype caused by the maternal phenotype or the environment that mothers experience [1]
Brood size on day 3 did not differ among treatments (x22,50 = 3.18, P = 0.20, mean brood size on day 361 SE: 5.4060.50 chick for control clutches, 4.8060.39 chick for T-First clutches, 4.0060.34 chick for T-Last clutches)
Verifying that the position of the eggs in the laying sequence predicted their hatching order, we found that last-laid eggs were significantly more likely to hatch at least 1 day later than the first-laid eggs
Summary
Maternal effects are defined as modifications of offspring phenotype caused by the maternal phenotype or the environment that mothers experience [1]. Its anabolic properties [34] may lead, for instance, to accelerated growth of the neck muscles involved in begging or sucking [35,36,37] Besides these beneficial effects, high levels of maternal T have been shown to have costs [7,38,39,40]. It may underlie variation in the levels of T found in the egg yolk or in the blood of neonates within the clutch or the litter of a given mother [7,52,53] This latter pattern of variation in the levels of maternally-derived T across eggs or neonates is mostly seen in oviparous and ovoviviparous species where mechanisms allowing the mother to adjust the amount of T deposited into each of the eggs are more likely to have evolved [7,14,21,28]
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