Baseline corticosterone does not reflect iridescent plumage traits in female tree swallows

Clutch Abandoning Parasitised Yellow Warblers Have Increased Circulating Corticosterone With No Effect of Past Corticosterone or Differences in Egg Maculation Characteristics

Parental care can be costly to parents’ fitness. As such, abandonment of the current reproductive attempt may benefit potential future opportunities, maximising lifetime reproductive success. Obligate brood parasitism, a reproductive strategy in which parasites lay their eggs in the nests of other species and rely solely on them to raise the parasitic young, is an ideal system to study brood abandonment. Some parasitised host species have evolved anti-parasitic defences, notably clutch abandonment (egg burial and nest desertion), that may mitigate negative consequences of parasitism. Abandonment of clutches due to parasitism is not unlike abandonment of reproduction in times of stress, suggesting that host responses to parasitism could be triggered at least partly by elevated stress hormones that mediate individual decisions. Yet, the mechanistic basis for clutch abandonment remains unclear. Here, we experimentally parasitised clutches of yellow warblers (Setophaga petechia), a common host of the brown-headed cowbird (Molothrus ater), with model cowbird eggs to examine whether host circulating corticosterone (CORT) differed among females that accepted parasitic eggs or rejected them through clutch abandonment. We also assessed whether feather CORT, a measure of past corticosterone exposure, differed between accepters and abandoners. Finally, we investigated whether egg visual signals, specifically differences in maculation characteristics between model cowbird and host eggs, predicted abandonment of experimentally parasitised clutches. Circulating CORT was higher in females who abandoned their parasitised clutches, but not in those who accepted, relative to controls with no egg addition. Past stress and differences in maculation characteristics did not predict whether individuals accepted or abandoned experimentally parasitised clutches. Moreover, differences in maculation characteristics between the host and model cowbird eggs did not predict CORT levels or nest abandonment. Thus, parasitism with subsequent clutch abandonment may be associated with elevated circulating CORT, but neither past stress nor differences in maculation characteristics influenced abandonment. The combination of these results contributes to our understanding of the roles of corticosterone and egg visual signals in the context of clutch abandonment in brood parasitism specifically, and of parental care more broadly.

Glucocorticoids, male sexual signals, and mate choice by females: Implications for sexual selection

We review work relating glucocorticoids (GCs), male sexual signals, and mate choice by females to understand the potential for GCs to modulate the expression of sexually selected traits and how sexual selection potentially feeds back on GC regulation. Our review reveals that the relationship between GC concentrations and the quality of male sexual traits is mixed, regardless of whether studies focused on structural traits (e.g., coloration) or behavioral traits (e.g., vocalizations) or were examined in developmental or activational frameworks. In contrast, the few mate choice experiments that have been done consistently show that females prefer males with low GCs, suggesting that mate choice by females favors males that maintain low levels of GCs. We point out, however, that just as sexual selection can drive the evolution of diverse reproductive strategies, it may also promote diversity in GC regulation. We then shift the focus to females where we highlight evidence indicating that stressors or high GCs can dampen female sexual proceptivity and the strength of preferences for male courtship signals. Hence, even in cases where GCs are tightly coupled with male sexual signals, the strength of sexual selection on aspects of GC physiology can vary depending on the endocrine status of females. Studies examining how GCs relate to sexual selection may shed light on how variation in stress physiology, sexual signals, and mate choice are maintained in natural populations and may be important in understanding context-dependent relationships between GC regulation and fitness.