Data from: Forecasting the future? Differential allocation of maternal hormones in function of experimentally manipulated social contexts in blue tits 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. This variation may be maintained because more
sociable individuals can thrive better in certain conditions, in which
social interactions facilitate information exchange or cooperation, but
not so if social encounters increase aggressive disputes or infection
risk. 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 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
concentration showed a higher network 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.