The competitive side of social living: Maternal hormonal programming and phenotypic outcomes in blue tit offspring.

Social interactions are vital in animal life, providing access to key information about foraging opportunities, mates, and predators. Yet in highly competitive environments, being social can be costly, as increased interaction rates may lead to more frequent aggressive encounters, favouring the development of more competitive phenotypes. Remarkably, these environmental pressures can have transgenerational effects. For example, mothers can respond to their social environment by transferring non-genetic compounds, such as hormones, to their offspring. However, evidence for an effect of social density on maternal hormone allocation and subsequent offspring social phenotypes remains scarce. Here, we experimentally manipulated social density in the blue tit (Cyanistes caeruleus) by rearranging nest-boxes before egg-laying, thus creating areas of high and low density -and likely high and low competition levels. We measured yolk testosterone and assessed nestling behaviour through a social preference test (sociability) and breathing rate (a proxy for proactivity). As predicted, mothers in low-density areas deposited less testosterone than those in high-density areas. Nestlings raised at low density showed higher sociability, though this effect was male-specific. Breathing rate had a non-linear, positive relationship with yolk testosterone concentration but was, unexpectedly, lower in high-density areas. Together, these findings suggest that competitive environments increase maternal testosterone transfer, likely promoting more competitive and less social offspring phenotypes.