Song repertoire size varies with HVC volume and is indicative of male quality in song sparrows (Melospiza melodia)

Complex birdsong is a classic example of a sexually selected ornamental trait. In many species, females prefer males with large song repertoires, possibly because repertoire size is limited by the size of song control nuclei which reflect developmental success. We investigated whether song repertoire size was indicative of brain area and male quality in song sparrows (Melospiza melodia) by determining if repertoire size was related to the volume of song control nucleus HVC, as well as several morphological, immunological and genetic indices of quality. We found that males with large repertoires had larger HVCs and were in better body condition. They also had lower heterophil to lymphocyte ratios, indicating less physiological stress and a robust immune system as measured by the number of lymphocytes per red blood cell. Song repertoire size also tended to increase with neutral-locus genetic diversity, as assessed by mean d2, but was not related to internal relatedness. Our results suggest several mechanisms that might explain the finding of a recent study that song sparrows with large song repertoires have higher lifetime fitness.     

Hatching late in the season requires flexibility in the timing of song learning

Most songbirds learn their songs from adult tutors, who can be their father or other male conspecifics. However, the variables that control song learning in a natural social context are largely unknown. We investigated whether the time of hatching of male domesticated canaries has an impact on their song development and on the neuroendocrine parameters of the song control system. Average age difference between early- and late-hatched males was 50 days with a maximum of 90 days. Song activity of adult tutor males decreased significantly during the breeding season. While early-hatched males were exposed to tutor songs for on average the first 99 days, late-hatched peers heard adult song only during the first 48 days of life. Remarkably, although hatching late in the season negatively affected body condition, no differences between both groups of males were found in song characteristics either in autumn or in the following spring. Similarly, hatching date had no effect on song nucleus size and circulating testosterone levels. Our data suggest that late-hatched males must have undergone accelerated song development. Furthermore, the limited tutor song exposure did not affect adult song organization and song performance.     

Regulation of aromatase, 5α- and 5β-reductase in primary cell cultures of developing zebra finch telencephalon

Sex steroids act on the developing and adult telencephalon of songbirds to organize and activate the neural circuits required for the learning and production of song. Presumably, the availability of active androgens and estrogens to steroid-sensitive neural circuits controlling song is modulated by the local expression of androgen-metabolizing enzymes. Two enzymes, 5α- and 5β-reductase, are expressed widely in the songbird telencephalon, as they are in the telencephalons of other avian species. These enzymes convert circulating testosterone (T) into the active and inactive metabolites, 5α- and 5β-dihydrotestosterone (DHT), respectively. A third enzyme, aromatase, converts T into estradiol (E₂) and is expressed at unusually high levels in several regions of the songbird telencephalon. In many tissues, including the brain, the regulation of expression of one or more of these enzymes can be a critical feature of their ability to control the production of active sex steroids. We have used primary cell cultures to examine factors that might regulate the expression of these enzymes in developing zebra finch telencephalon. Cultures were treated for 0-72 h with sex steroids (T, E₂, 5α-DHT, and 5β-DHT) or with dibutyryl cAMP. Afterward, activities of aromatase, 5α-, and 5β-reductase were determined or total RNA was extracted for Northern analysis. Treatments with cAMP increased both aromatase activity and aromatase mRNA levels by 220%. E₂ significantly reduced aromatase activity by an average of 65%, whereas 5α- and 5β-DHT had no effect on aromatase activity. Compared to untreated controls, E₂ treatment decreased aromatase mRNA levels by 56%. None of these treatments consistently affected either 5α- or 5β-reductase activities. These results suggest that telencephalic E₂ may regulate its own synthesis by repression of aromatase expression, whereas factors that upregulate cAMP in the telencephalon can increase the local concentrations of E₂. © 1998 John Wiley & Sons, Inc. J Neurobiol 36: 30–40, 1998     

Aromatase and 5β-reductase activity in cultures of developing zebra finch brain: An investigation of sex and regional differences

Estrogen treatment of hatchling female zebra finches causes the masculine development of singing behavior and of the telencephalic brain regions involved in the control of song. However, early estrogen treatment of males also blocks masculine development of copulatory behavior, presumably controlled by diencephalic regions. In an effort to determine whether the differences in estrogen action are related to sex and regional differences in androgen metabolism (estrogen synthesis or androgen inactivation), we measured aromatase and 5β-reductase activity in dissociated-cell cultures made separately from the telencephalon, diencephalon, and also cerebellum of hatching zebra finches under a variety of conditions. Cultures from all three brain regions express high levels of aromatase and 5β-reductase activity. Comparisons between telencephalic and diencephalic cultures of the activity and kinetics of aromatase suggest that the telencephalic cultures convert androgen to estrogen more efficiently than diencephalic cultures, which might be important in the differential action of estrogen in the two brain regions. However, the activity of neither aromatase nor 5β-reductase was significantly different between the sexes in either telencephalic or diencephalic cultures. Thus, comparisons between the sexes do not support the idea that differences in posthatching aromatase or 5β-reductase activity account for the pattern of sexual differentiation of the song and copulatory systems. © 1995 John Wiley & Sons, Inc.     

Interactions between nerve growth factor binding and estradiol in early development of the zebra finch telencephalon

The zebra finch telencephalon exhibits rapid and substantial development in the first few weeks after hatching. In parallel, the rate of estradiol synthesis is very high in the zebra finch forebrain, and estradiol can have potent neurotrophic effects in specific telencephalic regions, including those that control the learning and production of song. In an attempt to elucidate mechanisms regulating telencephalic development, potentially including a role for the large capacity for estrogen production, ¹²⁵I–nerve growth factor (NGF) binding was measured in homogenates of telencephalon from zebra finches age 3, 15, 30, 60, and 120 days. The highest density of low‐ and high‐affinity ¹²⁵I‐NGF binding sites was observed in 3‐day‐old finches. Using an aromatase inhibitor, Fadrozole, to reduce estradiol levels in 1 to 4‐day‐old zebra finches significantly decreased both high‐ and low‐affinity ¹²⁵I‐NGF binding sites. Conversely, treating adult or 8 to 14‐day‐old hatchlings with estradiol increased high‐affinity ¹²⁵I‐NGF binding sites. These results are consistent with the hypothesis that estradiol influences the level of NGF receptors, and suggest one mechanism through which the steroid could affect brain development. The data also indicate that estradiol and NGF activity may be important for very early development of the telencephalon. © 1999 John Wiley & Sons, Inc. J Neurobiol 40: 149–157, 1999     

Female marsh wrens do not provide evidence of anatomical specializations of song nuclei for perception of male song

In songbirds the forebrain nuclei HVC (high vocal center) and RA (robust nucleus of the archistriatum) are larger in individuals or species that produce larger song repertoires, but the extent to which the size of these nuclei reflects a need for either producing or perceiving large repertoires is unknown. We, therefore, tested the hypothesis that species differences in the size of song nuclei reflect a commitment of “brain space” to the perceptual processing of conspecific song. The two species of marsh wren (Cistothorus palustris western and eastern) provide a good test case. Western males produce larger song repertoires, and have larger HVC and RA than do eastern males. Female marsh wrens do not sing, and if they use their song nuclei to assess conspecific male song repertoires, then we predicted that measurable cellular and nuclear parameters of HVC and RA would be greater in western than eastern female wrens. For males we confirmed that the volumes of HVC and RA, and cellular parameters of HVC, are greater in western than in eastern birds. These nuclei were also considerably larger in males than in conspecific females. Western and eastern female wrens, however, did not differ in any measured parameters of HVC or RA. Females of these wren species thus do not provide any direct evidence of anatomical specializations of song nuclei for the perceptual processing of conspecific male song. 1994 John Wiley & Sons, Inc.