Maternal androgens in avian brood parasites and their hosts: Responses to parasitism and competition?

Abstract

In the coevolutionary dynamic of avian brood parasites and their hosts, maternal (or transgenerational) effects have rarely been investigated. We examined the potential role of elevated yolk testosterone in eggs of the principal brood parasite in North America, the brown-headed cowbird, and three of its frequent host species. Elevated maternal androgens in eggs are a common maternal effect observed in many avian species when breeding conditions are unfavorable. These steroids accelerate embryo development, shorten incubation period, increase nestling growth rate, and enhance begging vigor, all traits that can increase the survival of offspring. We hypothesized that elevated maternal androgens in host eggs are a defense against brood parasitism. Our second hypothesis was that elevated maternal androgens in cowbird eggs are a defense against intra-specific competition. For host species, we found that elevated yolk testosterone was correlated with parasitized nests of small species, those whose nest success is most reduced by cowbird parasitism. For cowbirds, we found that elevated yolk testosterone was correlated with eggs in multiply-parasitized nests, which indicate intra-specific competition for nests due to high cowbird density. We propose experimental work to further examine the use of maternal effects by cowbirds and their hosts.

Host defences against avian brood parasitism: an endocrine perspective

“…Exposure to brood parasites is predicted to increase T in parasitized host individuals, and populations or species with higher brood parasitism levels should show higher T levels (or higher sensitivity to androgens, the brain areas associated with aggression). Supporting these predictions, Hahn et al [22], in a first study of its kind, showed that eggs from parasitized nests had higher T levels relative to eggs from non-parasitized nests, suggesting that host mothers increase T in response to brood parasite sighting or recognizing parasitic eggs in their nests. The regulation of aggression against brood parasites by sex steroids may have important consequences to host life histories.…”

“…CORT has also been shown to reduce nestling provisioning [79,80]. Host adults that have been naturally parasitized show higher CORT [21] and, probably, higher T levels [22], which may result in a reduction in an associative response to own nests, eggs, of offspring, potentially leading the nest abandonment, increase in egg or nestling rejection probability or/and overall reduction of offspring provisioning.…”

“…As shown by Hahn et al [22], host hormonal responses to brood parasites may depend on the host species morphology or ecology-small hosts that suffer more severe negative fitness consequences of brood parasitism may mount a stronger stress response in response to brood parasites than larger host species, and as a result potentially suffer more negative fitness consequences. Along the same line, hosts that reject parasite eggs may mount a lower stress response than those that do not reject parasite eggs.…”

“…Maternally controlled differences in yolk hormones may underlie differences in the mean behaviour or growth rate of host nestlings across species, or changes in behaviour or growth rate in response to specific parasitism events [22]. At the between-species level, females from frequently parasitized species are predicted to deposit higher levels of T in their eggs compared with less frequently parasitized species, resulting in higher offspring growth rates and higher begging rates; indeed, small cowbird host species that pay a greater cost of competition with the parasitic nestling [82] have higher yolk T levels than do larger host species [25].…”

“…Furthermore, hormones may underlie variation in host defences at the within-individual ( plasticity), betweenindividual and across-species levels, potentially facilitating evolution of plastic defences to fixed responses [19]. A small number of studies have begun to uncover the hormonal responses of avian hosts to brood parasites at the within-species [20][21][22][23] and across-species levels [24][25][26]. To better understand the role that hormones may play in host defence evolution and variation, however, we need an overarching hypotheses-driven framework that generates testable predictions.…”