Does corticosterone regulate the onset of breeding in free-living birds?: The CORT-Flexibility Hypothesis and six potential mechanisms for priming corticosteroid function

Abstract

For many avian species, the decision to initiate breeding is based on information from a variety of environmental cues, including photoperiod, temperature, food availability, and social interactions. There is evidence that the hormone corticosterone may be involved in delaying the onset of breeding in cases where supplemental cues, such as low food availability and inclement weather, indicate that the environment is not suitable. However, not all studies have found the expected relationships between breeding delays and corticosterone titers. In this review, we present the hypothesis that corticosterone physiology mediates flexibility in breeding initiation (the "CORT-Flexibility Hypothesis"), and propose six possible corticosterone-driven mechanisms in pre-breeding birds that may delay breeding initiation: altering hormone titers, negative feedback regulation, plasma binding globulin concentrations, intracellular receptor concentrations, enzyme activity and interacting hormone systems. Based on the length of the breeding season and species-specific natural history, we also predict variation in corticosterone-regulated pre-breeding flexibility. Although few studies thus far have examined mechanisms beyond plasma hormone titers, the CORT-Flexibility Hypothesis is grounded on a solid foundation of research showing seasonal variation in the physiological stress response and knowledge of physiological mechanisms modulating corticosteroid effects. We propose six possible mechanisms as testable and falsifiable predictions to help clarify the extent of HPA axis regulation of the initiation of breeding.

Contrasting seasonal and aseasonal environments across stages of the annual cycle in the rufous‐collared sparrow, Zonotrichia capensis: Differences in endocrine function, proteome and body condition

“…Variations in CORT levels across the year were expected (Ramenofsky et al., ), allowing birds to modulate their breeding season, and improve their reproductive success depending on local conditions (Goutte, Antoine, & Chastel, ; Schoech, Rensel, Bridge, Boughton, & Wilcoxen, ). In particular, our findings in the seasonal FJ site are coincident with higher levels of CORT during breeding season in comparison with other life cycles in nonmigratory species (Lattin, Breuner, & Romero, ). In the aseasonal AR site, we also observed a peak of CORT in October which aligned with the high proportion of birds breeding at this time of the year.…”

“…In particular, our findings in the seasonal FJ site are coincident with higher levels of CORT during breeding season in comparison with other life cycles in nonmigratory species (Lattin, Breuner, & Romero, 2016). In the aseasonal AR site, we also observed a peak of CORT in October which aligned with the high proportion of birds breeding at this time of the year.…”

Dose‐response effects of light at night on the reproductive physiology of great tits (Parus major): Integrating morphological analyses with candidate gene expression

“…During gonadal development in birds, transcript levels of glucocorticoid (nr3c1, also referred to as gr) and mineralocorticoid receptors (nr3c2, also referred to as mr) also increase (Fudickar et al, 2017;Kirby, Geraghty, Ubuka, Bentley, & Kaufer, 2009;Lattin et al, 2012;McGuire, Koh, & Bentley, 2013). This provides a mechanistic basis for the widespread links between adrenal steroids and timing of reproduction and reproductive investment reported in several bird species (Crespi, Williams, Jessop, & Delehanty, 2013;Deviche et al, 2010;Goutte et al, 2010a;Lattin, Breuner, & Michael Romero, 2016;McGuire et al, 2013;Schoech, Rensel, Bridge, Boughton, & Wilcoxen, 2009). Indeed, during stressful periods, sex steroid production and spermatogenesis can be suppressed via several routes, one of which is by increased glucocorticoid levels (Blas, 2015;Hazra et al, 2014;McGuire et al, 2013;Witorsch, 2016).…”

Seasonally sympatric but allochronic: differential expression of hypothalamic genes in a songbird during gonadal development

“…Rivier & Rivest [56] suggested that CORT may bind to GR and MR to temporarily delay reproduction during stressful events. Late-winter storms and other unpredictable perturbations of the environment can inhibit reproduction by elevated glucocorticoids in birds [57,58]. Dark-eyed juncos breed in highly seasonal environments with unpredictable early spring conditions, therefore increased gonadal sensitivity to glucocorticoids just prior to reproduction could suppress reproduction under unfavourable conditions when juncos are in an advanced reproductive state.…”