Multiple environmental stressors induce an adaptive maternal effect

Evolution of adaptation requires predictability and recurrence of
functional contexts. Yet, organisms live in multi-faceted environments
that are dynamic and ever-changing, making it difficult to understand how
complex adaptations evolve. This problem is particularly apparent in the
evolution of adaptive maternal effects, which are often assumed to require
reliable and discrete cues that predict conditions in the offspring
environment. One resolution to this problem is if adaptive maternal
effects evolve through pre-existing, generalized maternal pathways that
respond to many cues and also influence offspring development. Here, we
assess whether an adaptive maternal effect in western bluebirds is
influenced by maternal stress pathways across multiple challenging
environments. Combining seventeen years of hormone sampling across diverse
environmental contexts with an experimental manipulation of the
competitive environment, we show that multiple environmental factors
influenced maternal corticosterone levels which, in turn, influenced the
maternal effect on aggression of sons in adulthood. Together, these
results support the idea that multiple stressors can induce a known
maternal effect in this system. More generally, it suggests that
activation of general pathways, such as the HPA axis, may simplify and
facilitate the evolution of adaptive maternal effects by integrating
variable environmental conditions into pre-existing maternal physiological
systems.