In many species, embryos are exposed to maternal hormones in utero, in the egg, or in the seed. In birds, mothers deposit substantial testosterone into their eggs, which enhances competitive ability of offspring. These maternal testosterone concentrations vary systematically within clutches in different patterns and may enable mothers to adaptively fine-tune competitive hierarchies within broods. We performed a comparative analysis to investigate this hypothesis using a broad set of avian species. We expected species with small size differences among siblings (arising from small hatching asynchrony or slow growth rates) to aim for survival of the whole brood in good years and therefore compensate last-hatching eggs with relatively more testosterone. We expected species with large size differences among siblings (large hatching asynchrony or fast growth rates) to produce surplus young as insurance against failed offspring and to facilitate elimination of redundant surplus young by bestowing last-hatching eggs with relatively less testosterone. As predicted, we found that maternal testosterone compensation to last-hatching eggs is stronger when size differences among siblings become smaller. Maternal testosterone compensation to last-hatching eggs also correlated negatively with hatching asynchrony and growth rates. These findings provide evidence for correlated evolution of several maternal effects that together support different maternal reproductive strategies.
Sources for yolk testosterone used in this study.
Mean yolk testosterone concentrations in core eggs, marginal eggs and the difference in yolk testosterone between core and marginal eggs. Marginal eggs represent the later-laid, asynchronously-hatching eggs and were identified by dividing the hatching spread by the laying interval between subsequent eggs (app. C). If these calculations produced non-integers, we rounded the values up. Core eggs represent the early-laid, synchronously-hatching eggs and were identified by subtracting the number of marginal eggs from the average clutch size. Yolk testosterone concentrations for core and marginal eggs were averaged to produce mean core T concentrations and mean marginal T concentrations, respectively. In our analyses, we included mean marginal T minus mean core T as a response variable and mean core T as a covariate that corrects for differences in average clutch T values across species as well as assay differences. * indicates the species excluded from analyses containing initial size asymmetries between core and marginal chicks as a predictor (n=28 species included). **indicates the species excluded from analyses containing hatching spread and logistic growth rate constant as a predictor (n=25 species included).
Data1.docx
Data on egg size and their sources.
Data on within-clutch variation in egg size used in this study. Marginal eggs represent the later-laid, asynchronously-hatching eggs and were identified by dividing the hatching spread by the laying interval between subsequent eggs. If these calculations produced non-integers, we rounded the values up. Core eggs represent the early-laid, synchronously-hatching eggs and were identified by subtracting the number of marginal eggs from the average clutch size. Sizes of core eggs and marginal eggs were averaged to produce mean core egg size and mean marginal egg size, respectively. In our statistical models, we included proportional difference in egg size between core and marginal eggs as a covariate. This was calculated by dividing the difference in egg size between core and marginal eggs (mean marginal size minus mean core size) by mean core egg size.
Data2.docx
Data on life history variables and their sources.
Data on life-history variables used in this study (data in bold, reference codes not in bold). Hatching spread represents the number or days between the hatching of the first egg and the last egg of a clutch. Days between eggs represents the time interval between the laying of subsequent eggs. Logistic growth rate constant represents the rate of mass increase in a sigmoid curve fitted to chick growth curves. % size difference was calculated as the mass of the core nestlings (estimated from their growth curves) at the time the last marginal chick hatched (hatching spread), divided by the size of a newly hatched chick. This represents the initial proportional size advantage of core chicks over marginal chicks.
Data3.docx