Jason E Davis

Seasonal changes in aromatase and androgen receptor, but not estrogen receptor mRNA expression in the brain of the free-living male song sparrow, Melospiza melodia morphna

Free‐living male song sparrows experience three annually repeating life history stages associated with differential expression of sex steroid‐dependent reproductive and aggressive behavior. In the breeding stage, they display reproductive and aggressive behavior and have elevated circulating testosterone levels. During molt, males show little or no aggression and no reproductive behavior, and have basal levels of circulating testosterone. In the non‐breeding stage, they display high levels of aggression and no reproductive behavior, and have basal levels of circulating testosterone. In order to understand more fully the neural regulation of seasonal aggressive and reproductive behavior, birds were collected during all three life history stages, and levels of neural aromatase, androgen receptor (AR), and estrogen receptor α (ERα) and β (ERβ) mRNA expression were measured. Breeding males had the highest levels of aromatase expression in both the preoptic area (POA) and medial preoptic area/medial bed nucleus of the stria terminalis (mPOA/BSTm), and the highest AR expression levels in the POA, consistent with the well‐established role these regions play in the regulation of male reproductive behavior. Aromatase expression in the ventromedial nucleus of the hypothalamus (VMH) was higher during breeding and non‐breeding compared with molt, suggesting that the VMH may play a role in the estrogen‐dependent regulation of aggression in this species. AR expression also varied in medial HVC and pvMSt, a newly described periventricular region in the medial striatum. ERα and ERβ mRNA expression did not vary seasonally in any brain region examined, suggesting that estrogen‐dependent changes in behavior are mediated by differences in neural estrogen synthesis. J. Comp. Neurol. 518:3819–3835, 2010.

Organism-environment interactions in a changing world: a mechanistic approach

Understanding the interactions of an organism and its environment is essential for us to integrate ultimate and proximate causation on a global scale. Organism–environment interaction includes all organisms including animals, plants, and non-eukaryotes, etc. because all of them are responsive to environmental change including those that are human-induced. A mechanistic approach is important for us to understand why some organisms can cope with change and others cannot. Here, we present three examples of environments (“the three poles”) that are changing rapidly and how avian species typical of these ecosystems are responding. These examples include apparently adaptive responses to change in climate (i.e. the predictable environment) in one species in which a lengthened breeding season now allows multiple breeding attempts. Why other species are unable to respond in a similar way remains unclear. A second example describes how changing weather (i.e. the unpredictable) may have disastrous results for breeding success in a species adapted to an extreme cold environment. Implications for climate change in which weather extremes will become more common again suggest a mechanistic approach will be important to understand how organisms may respond. The third example outlines a scenario in which multiple human-induced rapid changes (a combination of predictable and unpredictable such as development, habitat change, introduction of invasive species and climate change) may influence indigenous species in different ways. Organism–environment interaction is a fundamental concept that may unify ultimate and proximate causation and point the way for future investigations striving to understand coping mechanisms in a world where both predictable and unpredictable components of the environment are changing.

Across time and space: Effects of urbanization on corticosterone and body condition vary over multiple years in song sparrows (Melospiza melodia)

Animals inhabiting urban areas must simultaneously cope with the unique challenges presented by this novel habitat type while exploiting the distinctive opportunities it offers. The costs and benefits of urban living are often assumed to be consistent across time, but may in fact vary depending on the habitat features influencing them. Here we examine the glucocorticoid levels and body condition of song sparrows (Melospiza melodia) resident at urban and rural sites over four consecutive years to determine whether these traits, which may be linked to the relative costs and benefits of these respective habitats, are consistent over time. Glucocorticoid levels and body condition varied by year in both habitat types. While habitat alone did not influence glucocorticoid levels, there was a significant interaction between year and habitat, indicating that glucocorticoids differ between habitats in some years but not others. There was no discernable effect of habitat alone on body condition. Overall, these data suggest that the costs and benefits of inhabiting urban versus rural habitats differ substantially from year to year.

Dehydroepiandrosterone heightens aggression and increases androgen receptor and aromatase mRNA expression in the brain of a male songbird

Dehydroepiandrosterone (DHEA) is a testosterone/oestrogen precursor and known modulator of vertebrate aggression. Male song sparrows (Melospiza melodia morphna) show high aggression during breeding and nonbreeding life‐history stages when circulating DHEA levels are high, and low aggression during molt when DHEA levels are low. We previously showed that androgen receptor and aromatase mRNA expression are higher during breeding and/or nonbreeding in brain regions associated with reproductive and aggressive behaviour, although the potential role of DHEA in mediating these seasonal changes remained unclear. In the present study, nonbreeding male song sparrows were captured and held in the laboratory under short days (8 : 16 h light/dark cycle) and implanted with s.c. DHEA‐filled or empty (control) implants for 14 days. DHEA implants increased aggression in a laboratory‐based simulated territorial intrusion. Brains of DHEA‐implanted birds showed higher aromatase mRNA expression in the preoptic area (POA) and higher androgen receptor mRNA expression in the periventricular nucleus of the medial striatum (pvMSt) and ventromedial nucleus of the hypothalamus. The DHEA‐induced increases in aromatase expression in the POA and androgen receptor expression in the pvMSt are consistent with previously reported seasonal increases in these markers associated with naturally elevated DHEA levels. This suggests that DHEA facilitates seasonal increases in aggression in nonbreeding male song sparrows by up‐regulating steroid signalling/synthesis machinery in a brain region‐specific fashion.