title: Do group size and laying order influence maternal deposition of testosterone in smooth-billed ani eggs?
abstract: The avian egg contains maternal hormones that affect behavior, growth, morphology, and offspring survival. Evidence to date suggests that patterns of yolk androgen deposition could provide females with a means to manipulate sibling competition and, thereby, increase their fitness. We examined yolk testosterone (T) concentrations in eggs of the smooth-billed ani (Crotophaga ani) to understand patterns of androgen deposition in eggs of this plural-breeding joint-nesting cooperatively breeding species. We tested the hatching asynchrony adjustment hypothesis, which states that increases in yolk androgen levels over the laying sequence function to mitigate the disadvantage of being a later-hatched chick in species without adaptive brood reduction. We also investigated the effect of group size on yolk T deposition to test the hypothesis that females in multi-female groups could give a competitive edge to their own chicks by depositing higher T levels in their eggs. Predictions of the hatching asynchrony adjustment hypothesis were supported in both single- and multi-female groups as yolk testosterone levels increased from early- to late-laid eggs. This suggests that ani females can influence nestling competition and chick survival by within-clutch differential T allocation. Unexpectedly, we did not observe an effect of group size on yolk T deposition. Yolk testosterone concentrations may not be a mere reflection of a female's hormonal status as female plasma circulating levels of T did not vary in the same direction as yolk T levels. Results of this study therefore support the idea that females may adaptively manipulate chick behavior through hormonal deposition in eggs.
authors.affiliation: Department of Biology, McMaster University, 1280 Main Street West, Hamilton, ON, Canada L8S 4K1.
authors.forename: Gregory
authors.initials: G
authors.lastname: Schmaltz
authors.forename: James S
authors.initials: JS
authors.lastname: Quinn
authors.forename: Stephan J
authors.initials: SJ
authors.lastname: Schoech
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D001717:Birds / Q000502:physiology*; D001722:Birth Order; D052284:Clutch Size; D004527:Egg Proteins / Q000378:metabolism; D004530:Egg Yolk / Q000378:metabolism*; D004625:Embryo, Nonmammalian / Q000378:metabolism; D005260:Female; D008425:Maternal Behavior; D009425:Nesting Behavior / Q000502:physiology; D010058:Oviposition / Q000502:physiology*; D011156:Population Density; D012098:Reproduction / Q000502:physiology; D013739:Testosterone / Q000378:metabolism*; D013997:Time Factors
chemical_list: D004527:Egg Proteins; D013739:Testosterone