Supplementary material from "Forecasting the future? Differential allocation of maternal hormones under different social contexts in the blue tit Cyanistes caeruleus"

Sociability, i.e., the tendency to interact with other individuals, varies significantly within populations, with some individuals being consistently more sociable than others. Variation may be maintained because the balance between costs (e.g., increase in aggressive disputes, infection risk) and benefits (e.g., information exchange, cooperation) of sociability varies with the environmental context. At the proximate level, apart from genes, mothers transfer non-genetic compounds to their offspring that can influence the development of social skills. In this context, they may adjust their offspring’s sociability to match the social environment they will experience after birth, for example, via prenatal hormones. To test this, we experimentally manipulated the social density as perceived by blue tit females before egg laying. We subsequently measured yolk testosterone concentrations and social interactions among family members post-hatching. Females that were exposed to a simulated high social density transferred less testosterone to their eggs than control females. Network average degree (i.e., the number of social interactions of the brood) was not affected by the social density treatment, but broods with lower yolk testosterone concentrations showed a higher network average degree. This suggests that mothers experiencing an environment with high social density (but not increased resource competition) deposit less yolk testosterone to produce offspring that are likely less aggressive but more sociable.