DOI: 10.5061/dryad.b87j7
title: Data from: Within-clutch variation in yolk testosterone as an adaptive maternal effect to modulate avian sibling competition: evidence from a comparative study

description: 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.
description: 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.docxData 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.docxData 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